libtool
In the past, if a source code package developer wanted to take advantage of the power of shared libraries, he needed to write custom support code for each platform on which his package ran. He also had to design a configuration interface so that the package installer could choose what sort of libraries were built.
GNU Libtool simplifies the developer's job by encapsulating both the platform-specific dependencies, and the user interface, in a single script. GNU Libtool is designed so that the complete functionality of each host type is available via a generic interface, but nasty quirks are hidden from the programmer.
GNU Libtool's consistent interface is reassuring... users don't need
to read obscure documentation in order to have their favorite source
package build shared libraries. They just run your package
configure script (or equivalent), and libtool does all the dirty
work.
There are several examples throughout this document. All assume the same environment: we want to build a library, `libhello', in a generic way.
`libhello' could be a shared library, a static library, or both... whatever is available on the host system, as long as libtool has been ported to it.
This chapter explains the original design philosophy of libtool. Feel free to skip to the next chapter, unless you are interested in history, or want to write code to extend libtool in a consistent way.
Since early 1995, several different GNU developers have recognized the importance of having shared library support for their packages. The primary motivation for such a change is to encourage modularity and reuse of code (both conceptually and physically) in GNU programs.
Such a demand means that the way libraries are built in GNU packages needs to be general, to allow for any library type the package installer might want. The problem is compounded by the absence of a standard procedure for creating shared libraries on different platforms.
The following sections outline the major issues facing shared library support in GNU, and how shared library support could be standardized with libtool.
The following specifications were used in developing and evaluating this system:
The following issues need to be addressed in any reusable shared library system, specifically libtool:
LD_LIBRARY_PATH must be set properly (if
it is supported), or programs fail to run.
LD_LIBRARY_PATH or equivalent),
or run ldconfig.
Even before libtool was developed, many free software packages built and installed their own shared libraries. At first, these packages were examined to avoid reinventing existing features.
Now it is clear that none of these packages have documented the details of shared library systems that libtool requires. So, other packages have been more or less abandoned as influences.
In all fairness, each of the implementations that were examined do the job that they were intended to do, for a number of different host systems. However, none of these solutions seem to function well as a generalized, reusable component.
Most were too complex to use (much less modify) without understanding exactly what the implementation does, and they were generally not documented.
The main difficulty is that different vendors have different views of what libraries are, and none of the packages which were examined seemed to be confident enough to settle on a single paradigm that just works.
Ideally, libtool would be a standard that would be implemented as series of extensions and modifications to existing library systems to make them work consistently. However, it is not an easy task to convince operating system developers to mend their evil ways, and people want to build shared libraries right now, even on buggy, broken, confused operating systems.
For this reason, libtool was designed as an independent shell script. It isolates the problems and inconsistencies in library building that plague `Makefile' writers by wrapping the compiler suite on different platforms with a consistent, powerful interface.
With luck, libtool will be useful to and used by the GNU community, and that the lessons that were learned in writing it will be taken up by designers of future library systems.
At first, libtool was designed to support an arbitrary number of library object types. After libtool was ported to more platforms, a new paradigm gradually developed for describing the relationship between libraries and programs.
In summary, "libraries are programs with multiple entry points, and more formally defined interfaces."
Version 0.7 of libtool was a complete redesign and rewrite of libtool to reflect this new paradigm. So far, it has proved to be successful: libtool is simpler and more useful than before.
The best way to introduce the libtool paradigm is to contrast it with the paradigm of existing library systems, with examples from each. It is a new way of thinking, so it may take a little time to absorb, but when you understand it, the world becomes simpler.
It makes little sense to talk about using libtool in your own packages until you have seen how it makes your life simpler. The examples in this chapter introduce the main features of libtool by comparing the standard library building procedure to libtool's operation on two different platforms:
You can follow these examples on your own platform, using the preconfigured libtool script that was installed with libtool (see section Configuring libtool).
Source files for the following examples are taken from the `demo' subdirectory of the libtool distribution. Assume that we are building a library, `libhello', out of the files `foo.c' and `hello.c'.
Note that the `foo.c' source file uses the cos math library
function, which is usually found in the standalone math library, and not
the C library (see section `Trigonometric Functions' in The GNU C Library Reference Manual). So, we need to add -lm to
the end of the link line whenever we link `foo.o' or `foo.lo'
into an executable or a library (see section Inter-library dependencies).
The same rule applies whenever you use functions that don't appear in the standard C library... you need to add the appropriate -lname flag to the end of the link line when you link against those objects.
After we have built that library, we want to create a program by linking `main.o' against `libhello'.
To create an object file from a source file, the compiler is invoked with the `-c' flag (and any other desired flags):
burger$ gcc -g -O -c main.c burger$
The above compiler command produces an object file, `main.o', from the source file `main.c'.
For most library systems, creating object files that become part of a static library is as simple as creating object files that are linked to form an executable:
burger$ gcc -g -O -c foo.c burger$ gcc -g -O -c hello.c burger$
Shared libraries, however, may only be built from position-independent code (PIC). So, special flags must be passed to the compiler to tell it to generate PIC rather than the standard position-dependent code.
Since this is a library implementation detail, libtool hides the complexity of PIC compiler flags by using separate library object files (which end in `.lo' instead of `.o'). On systems without shared libraries (or without special PIC compiler flags), these library object files are identical to "standard" object files.
To create library object files for `foo.c' and `hello.c', simply invoke libtool with the standard compilation command as arguments (see section Compile mode):
a23$ libtool gcc -g -O -c foo.c gcc -g -O -c foo.c echo timestamp > foo.lo a23$ libtool gcc -g -O -c hello.c gcc -g -O -c hello.c echo timestamp > hello.lo a23$
Note that libtool creates two files for each invocation. The `.lo' file is a library object, which may be built into a shared library, and the `.o' file is a standard object file. On `a23', the library objects are just timestamps, because only static libraries are supported.
On shared library systems, libtool automatically inserts the PIC generation flags into the compilation command, so that the library object and the standard object differ:
burger$ libtool gcc -g -O -c foo.c gcc -g -O -c -fPIC -DPIC foo.c mv -f foo.o foo.lo gcc -g -O -c foo.c >/dev/null 2>&1 burger$ libtool gcc -g -O -c hello.c gcc -g -O -c -fPIC -DPIC hello.c mv -f hello.o hello.lo gcc -g -O -c hello.c >/dev/null 2>&1 burger$
Notice that the second run of GCC has its output discarded. This is done so that compiler warnings aren't annoyingly duplicated.
Without libtool, the programmer would invoke the ar command to
create a static library:
burger$ ar cru libhello.a hello.o foo.o burger$
But of course, that would be too simple, so many systems require that
you run the ranlib command on the resulting library (to give it
better karma, or something):
burger$ ranlib libhello.a burger$
It seems more natural to use the C compiler for this task, given
libtool's "libraries are programs" approach. So, on platforms without
shared libraries, libtool simply acts as a wrapper for the system
ar (and possibly ranlib) commands.
Again, the libtool library name differs from the standard name (it has a `.la' suffix instead of a `.a' suffix). The arguments to libtool are the same ones you would use to produce an executable named `libhello.la' with your compiler (see section Link mode):
a23$ libtool gcc -g -O -o libhello.la foo.o hello.o
libtool: cannot build libtool library `libhello.la' from non-libtool \
objects
a23$
Aha! Libtool caught a common error... trying to build a library from standard objects instead of library objects. This doesn't matter for static libraries, but on shared library systems, it is of great importance.
So, let's try again, this time with the library object files. Remember
also that we need to add -lm to the link command line because
`foo.c' uses the cos math library function (see section Using libtool).
Another complication in building shared libraries is that we need to specify the path to the directory in which they (eventually) will be installed (in this case, `/usr/local/lib')(1):
a23$ libtool gcc -g -O -o libhello.la foo.lo hello.lo \
-rpath /usr/local/lib -lm
mkdir .libs
ar cru .libs/libhello.a foo.o hello.o
ranlib .libs/libhello.a
creating libhello.la
a23$
Now, let's try the same trick on the shared library platform:
burger$ libtool gcc -g -O -o libhello.la foo.lo hello.lo \
-rpath /usr/local/lib -lm
mkdir .libs
ld -Bshareable -o .libs/libhello.so.0.0 foo.lo hello.lo -lm
ar cru .libs/libhello.a foo.o hello.o
ranlib .libs/libhello.a
creating libhello.la
burger$
Now that's significantly cooler... libtool just ran an obscure
ld command to create a shared library, as well as the static
library.
Note how libtool creates extra files in the `.libs' subdirectory, rather than the current directory. This feature is to make it easier to clean up the build directory, and to help ensure that other programs fail horribly if you accidentally forget to use libtool when you should.
If you choose at this point to install the library (put it in a permanent location) before linking executables against it, then you don't need to use libtool to do the linking. Simply use the appropriate `-L' and `-l' flags to specify the library's location.
Some system linkers insist on encoding the full directory name of each shared library in the resulting executable. Libtool has to work around this misfeature by special magic to ensure that only permanent directory names are put into installed executables.
The importance of this bug must not be overlooked: it won't cause programs to crash in obvious ways. It creates a security hole, and possibly even worse, if you are modifying the library source code after you have installed the package, you will change the behaviour of the installed programs!
So, if you want to link programs against the library before you install it, you must use libtool to do the linking.
Here's the old way of linking against an uninstalled library:
burger$ gcc -g -O -o hell.old main.o libhello.a -lm burger$
Libtool's way is almost the same(2) (see section Link mode):
a23$ libtool gcc -g -O -o hell main.o libhello.la -lm gcc -g -O -o hell main.o ./.libs/libhello.a -lm a23$
That looks too simple to be true. All libtool did was transform `libhello.la' to `./.libs/libhello.a', but remember that `a23' has no shared libraries.
On `burger' the situation is different:
burger$ libtool gcc -g -O -o hell main.o libhello.la -lm gcc -g -O -o .libs/hell main.o -L./.libs -R/usr/local/lib -lhello -lm creating hell burger$
Now assume `libhello.la' had already been installed, and you want to link a new program with it. You could figure out where it lives by yourself, then run:
burger$ gcc -g -O -o test test.o -L/usr/local/lib -lhello
However, unless `/usr/local/lib' is in the standard library search
path, you won't be able to run test. However, if you use libtool
to link the already-installed libtool library, it will do The Right
Thing (TM) for you:
burger$ libtool gcc -g -O -o test test.o /usr/local/lib/libhello.la gcc -g -O -o .libs/test test.o -Wl,--rpath -Wl,/usr/local/lib /usr/local/lib/libhello.a -lm creating test burger$
Note that libtool added the necessary run-time path flag, as well as `-lm', the library libhello.la depended upon. Nice, huh?
Since libtool created a wrapper script, you should use libtool to install it and debug it too. However, since the program does not depend on any uninstalled libtool library, it is probably usable even without the wrapper script. Libtool could probably be made smarter to avoid the creation of the wrapper script in this case, but this is left as an exercise for the reader.
Notice that the executable, hell, was actually created in the
`.libs' subdirectory. Then, a wrapper script was created
in the current directory.
On NetBSD 1.2, libtool encodes the installation directory of `libhello', by using the `-R/usr/local/lib' compiler flag. Then, the wrapper script guarantees that the executable finds the correct shared library (the one in `./.libs') until it is properly installed.
Let's compare the two different programs:
burger$ time ./hell.old
Welcome to GNU Hell!
** This is not GNU Hello. There is no built-in mail reader. **
0.21 real 0.02 user 0.08 sys
burger$ time ./hell
Welcome to GNU Hell!
** This is not GNU Hello. There is no built-in mail reader. **
0.63 real 0.09 user 0.59 sys
burger$
The wrapper script takes significantly longer to execute, but at least the results are correct, even though the shared library hasn't been installed yet.
So, what about all the space savings that shared libraries are supposed to yield?
burger$ ls -l hell.old libhello.a -rwxr-xr-x 1 gord gord 15481 Nov 14 12:11 hell.old -rw-r--r-- 1 gord gord 4274 Nov 13 18:02 libhello.a burger$ ls -l .libs/hell .libs/libhello.* -rwxr-xr-x 1 gord gord 11647 Nov 14 12:10 .libs/hell -rw-r--r-- 1 gord gord 4274 Nov 13 18:44 .libs/libhello.a -rwxr-xr-x 1 gord gord 12205 Nov 13 18:44 .libs/libhello.so.0.0 burger$
Well, that sucks. Maybe I should just scrap this project and take up basket weaving.
Actually, it just proves an important point: shared libraries incur overhead because of their (relative) complexity. In this situation, the price of being dynamic is eight kilobytes, and the payoff is about four kilobytes. So, having a shared `libhello' won't be an advantage until we link it against at least a few more programs.
If `hell' was a complicated program, you would certainly want to test and debug it before installing it on your system. In the above section, you saw how the libtool wrapper script makes it possible to run the program directly, but unfortunately, this mechanism interferes with the debugger:
burger$ gdb hell GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is no warranty for GDB; type "show warranty" for details. GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc. "hell": not in executable format: File format not recognized (gdb) quit burger$
Sad. It doesn't work because GDB doesn't know where the executable lives. So, let's try again, by invoking GDB directly on the executable:
burger$ gdb .libs/hell trick:/home/src/libtool/demo$ gdb .libs/hell GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is no warranty for GDB; type "show warranty" for details. GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc. (gdb) break main Breakpoint 1 at 0x8048547: file main.c, line 29. (gdb) run Starting program: /home/src/libtool/demo/.libs/hell /home/src/libtool/demo/.libs/hell: can't load library 'libhello.so.2' Program exited with code 020. (gdb) quit burger$
Argh. Now GDB complains because it cannot find the shared library that `hell' is linked against. So, we must use libtool in order to properly set the library path and run the debugger. Fortunately, we can forget all about the `.libs' directory, and just run it on the executable wrapper (see section Execute mode):
burger$ libtool gdb hell
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc.
(gdb) break main
Breakpoint 1 at 0x8048547: file main.c, line 29.
(gdb) run
Starting program: /home/src/libtool/demo/.libs/hell
Breakpoint 1, main (argc=1, argv=0xbffffc40) at main.c:29
29 printf ("Welcome to GNU Hell!\n");
(gdb) quit
The program is running. Quit anyway (and kill it)? (y or n) y
burger$
Installing libraries on a non-libtool system is quite straightforward... just copy them into place:(3)
burger$ su Password: ******** burger# cp libhello.a /usr/local/lib/libhello.a burger#
Oops, don't forget the ranlib command:
burger# ranlib /usr/local/lib/libhello.a burger#
Libtool installation is quite simple, as well. Just use the
install or cp command that you normally would
(see section Install mode):
a23# libtool cp libhello.la /usr/local/lib/libhello.la cp libhello.la /usr/local/lib/libhello.la cp .libs/libhello.a /usr/local/lib/libhello.a ranlib /usr/local/lib/libhello.a a23#
Note that the libtool library `libhello.la' is also installed, to help libtool with uninstallation (see section Uninstall mode) and linking (see section Linking executables) and to help programs with dlopening (see section Dlopened modules).
Here is the shared library example:
burger# libtool install -c libhello.la /usr/local/lib/libhello.la install -c .libs/libhello.so.0.0 /usr/local/lib/libhello.so.0.0 install -c libhello.la /usr/local/lib/libhello.la install -c .libs/libhello.a /usr/local/lib/libhello.a ranlib /usr/local/lib/libhello.a burger#
It is safe to specify the `-s' (strip symbols) flag if you use a BSD-compatible install program when installing libraries. Libtool will either ignore the `-s' flag, or will run a program that will strip only debugging and compiler symbols from the library.
Once the libraries have been put in place, there may be some additional configuration that you need to do before using them. First, you must make sure that where the library is installed actually agrees with the `-rpath' flag you used to build it.
Then, running `libtool -n --finish libdir' can give you further hints on what to do (see section Finish mode):
burger# libtool -n --finish /usr/local/lib
PATH="$PATH:/sbin" ldconfig -m /usr/local/lib
-----------------------------------------------------------------
Libraries have been installed in:
/usr/local/lib
To link against installed libraries in a given directory, LIBDIR,
you must use the `-LLIBDIR' flag during linking.
You will also need to do one of the following:
- add LIBDIR to the `LD_LIBRARY_PATH' environment variable
during execution
- add LIBDIR to the `LD_RUN_PATH' environment variable
during linking
- use the `-RLIBDIR' linker flag
See any operating system documentation about shared libraries for
more information, such as the ld and ld.so manual pages.
-----------------------------------------------------------------
burger#
After you have completed these steps, you can go on to begin using the installed libraries. You may also install any executables that depend on libraries you created.
If you used libtool to link any executables against uninstalled libtool libraries (see section Linking executables), you need to use libtool to install the executables after the libraries have been installed (see section Installing libraries).
So, for our Ultrix example, we would run:
a23# libtool install -c hell /usr/local/bin/hell install -c hell /usr/local/bin/hell a23#
On shared library systems, libtool just ignores the wrapper script and installs the correct binary:
burger# libtool install -c hell /usr/local/bin/hell install -c .libs/hell /usr/local/bin/hell burger#
Why return to ar and ranlib silliness when you've had a
taste of libtool? Well, sometimes it is desirable to create a static
archive that can never be shared. The most frequent case is when you
have a set of object files that you use to build several different
programs. You can create a "convenience library" out of those
objects, and link programs with the library, instead of listing all
object files for every program. This technique is often used to
overcome GNU automake's lack of support for linking object files built
from sources in other directories, because it supports linking with
libraries from other directories. This limitation applies to GNU
automake up to release 1.4; newer releases should support sources in
other directories.
If you just want to link this convenience library into programs, then
you could just ignore libtool entirely, and use the old ar and
ranlib commands (or the corresponding GNU automake
`_LIBRARIES' rules). You can even install a convenience library
(but you probably don't want to) using libtool:
burger$ libtool ./install-sh -c libhello.a /local/lib/libhello.a ./install-sh -c libhello.a /local/lib/libhello.a ranlib /local/lib/libhello.a burger$
Using libtool for static library installation protects your library from
being accidentally stripped (if the installer used the `-s' flag),
as well as automatically running the correct ranlib command.
But libtool libraries are more than just collections of object files: they can also carry library dependency information, which old archives do not. If you want to create a libtool static convenience library, you can omit the `-rpath' flag and use `-static' to indicate that you're only interested in a static library. When you link a program with such a library, libtool will actually link all object files and dependency libraries into the program.
If you omit both `-rpath' and `-static', libtool will create a convenience library that can be used to create other libtool libraries, even shared ones. Just like in the static case, the library behaves as an alias to a set of object files and dependency libraries, but in this case the object files are suitable for inclusion in shared libraries. But be careful not to link a single convenience library, directly or indirectly, into a single program or library, otherwise you may get errors about symbol redefinitions.
When GNU automake is used, you should use noinst_LTLIBRARIES
instead of lib_LTLIBRARIES for convenience libraries, so that
the `-rpath' option is not passed when they are linked.
As a rule of thumb, link a libtool convenience library into at most one libtool library, and never into a program, and link libtool static convenience libraries only into programs, and only if you need to carry library dependency information to the user of the static convenience library.
Another common situation where static linking is desirable is in creating a standalone binary. Use libtool to do the linking and add the `-all-static' flag.
libtool
The libtool program has the following synopsis:
libtool [option]... [mode-arg]...
and accepts the following options:
less (or
more) or redirect to a file.
The mode-args are a variable number of arguments, depending on the selected operation mode. In general, each mode-arg is interpreted by programs libtool invokes, rather than libtool itself.
For compile mode, mode-args is a compiler command to be used in creating a `standard' object file. These arguments should begin with the name of the C compiler, and contain the `-c' compiler flag so that only an object file is created.
Libtool determines the name of the output file by removing the directory component from the source file name, then substituting the source code suffix (e.g. `.c' for C source code) with the library object suffix, `.lo'.
If shared libraries are being built, any necessary PIC generation flags are substituted into the compilation command. You can pass compiler and linker specific flags using `-Wc,flag' and `-Xcompiler flag' or `-Wl,flag' and `-Xlinker flag', respectively.
If the `-static' option is given, then a `.o' file is built, even if libtool was configured with `--disable-static'.
Note that the `-o' option is now fully supported. It is emulated on the platforms that don't support it (by locking and moving the objects), so it is really easy to use libtool, just with minor modifications to your Makefiles. Typing for example
libtool gcc -c foo/x.c -o foo/x.lo
will do what you expect.
Note, however, that, if the compiler does not support `-c' and `-o', it is impossible to compile `foo/x.c' without overwriting an existing `./x.o'. Therefore, if you do have a source file `./x.c', make sure you introduce dependencies in your `Makefile' to make sure `./x.o' (or `./x.lo') is re-created after any sub-directory's `x.lo':
x.o x.lo: foo/x.lo bar/x.lo
This will also ensure that make won't try to use a temporarily corrupted `x.o' to create a program or library. It may cause needless recompilation on platforms that support `-c' and `-o' together, but it's the only way to make it safe for those that don't.
Link mode links together object files (including library objects) to form another library or to create an executable program.
mode-args consist of a command using the C compiler to create an output file (with the `-o' flag) from several object files.
The following components of mode-args are treated specially:
self libtool will make
sure that the program can dlopen itself, either by enabling
-export-dynamic or by falling back to `-dlpreopen self'.
self, the symbols of the program itself will be added to
lt_preloaded_symbols.
If file is force libtool will make sure that
lt_preloaded_symbols is always defined, regardless of whether
it's empty or not.
dlsym
(see section Dlopened modules).
If the output-file ends in `.la', then a libtool library is created, which must be built only from library objects (`.lo' files). The `-rpath' option is required. In the current implementation, libtool libraries may not depend on other uninstalled libtool libraries (see section Inter-library dependencies).
If the output-file ends in `.a', then a standard library is
created using ar and possibly ranlib.
If output-file ends in `.o' or `.lo', then a reloadable object file is created from the input files (generally using `ld -r'). This method is often called partial linking.
Otherwise, an executable program is created.
For execute mode, the library path is automatically set, then a program is executed.
The first of the mode-args is treated as a program name, with the rest as arguments to that program.
The following components of mode-args are treated specially:
This mode sets the library path environment variable according to any `-dlopen' flags.
If any of the args are libtool executable wrappers, then they are translated into the name of their corresponding uninstalled binary, and any of their required library directories are added to the library path.
In install mode, libtool interprets mode-args as an
installation command beginning with cp, or a BSD-compatible
install program.
The rest of the mode-args are interpreted as arguments to that command.
The command is run, and any necessary unprivileged post-installation commands are also completed.
Finish mode helps system administrators install libtool libraries so that they can be located and linked into user programs.
Each mode-arg is interpreted as the name of a library directory. Running this command may require superuser privileges, so the `--dry-run' option may be useful.
Uninstall mode deletes installed libraries, executables and objects.
The first mode-arg is the name of the program to use to delete files (typically `/bin/rm').
The remaining mode-args are either flags for the deletion program (beginning with a `-'), or the names of files to delete.
Clean mode deletes uninstalled libraries, executables, objects and libtool's temporary files associated with them.
The first mode-arg is the name of the program to use to delete files (typically `/bin/rm').
The remaining mode-args are either flags for the deletion program (beginning with a `-'), or the names of files to delete.
This chapter describes how to integrate libtool with your packages so that your users can install hassle-free shared libraries.
Libtool is fully integrated with Automake (see section `Introduction' in The Automake Manual), starting with Automake version 1.2.
If you want to use libtool in a regular `Makefile' (or `Makefile.in'), you are on your own. If you're not using Automake 1.2, and you don't know how to incorporate libtool into your package you need to do one of the following:
Libtool library support is implemented under the `LTLIBRARIES' primary.
Here are some samples from the Automake `Makefile.am' in the libtool distribution's `demo' subdirectory.
First, to link a program against a libtool library, just use the `program_LDADD' variable:
bin_PROGRAMS = hell hell.debug # Build hell from main.c and libhello.la hell_SOURCES = main.c hell_LDADD = libhello.la # Create an easier-to-debug version of hell. hell_debug_SOURCES = main.c hell_debug_LDADD = libhello.la hell_debug_LDFLAGS = -static
The flags `-dlopen' or `-dlpreopen' (see section Link mode) would fit better in the program_LDADD variable. Unfortunately, GNU automake, up to release 1.4, doesn't accept these flags in a program_LDADD variable, so you have the following alternatives:
program_LDADD = "-dlopen" libfoo.la program_DEPENDENCIES = libfoo.la
You may use the `program_LDFLAGS' variable to stuff in any flags you want to pass to libtool while linking `program' (such as `-static' to avoid linking uninstalled shared libtool libraries).
Building a libtool library is almost as trivial... note the use of `libhello_la_LDFLAGS' to pass the `-version-info' (see section Library interface versions) option to libtool:
# Build a libtool library, libhello.la for installation in libdir. lib_LTLIBRARIES = libhello.la libhello_la_SOURCES = hello.c foo.c libhello_la_LDFLAGS = -version-info 3:12:1
The `-rpath' option is passed automatically by Automake (except for
libraries listed as noinst_LTLIBRARIES), so you
should not specify it.
See section `The Automake Manual' in The Automake Manual, for more information.
Libtool requires intimate knowledge of your compiler suite and operating system in order to be able to create shared libraries and link against them properly. When you install the libtool distribution, a system-specific libtool script is installed into your binary directory.
However, when you distribute libtool with your own packages (see section Including libtool in your package), you do not always know which compiler suite and operating system are used to compile your package.
For this reason, libtool must be configured before it can be
used. This idea should be familiar to anybody who has used a GNU
configure script. configure runs a number of tests for
system features, then generates the `Makefiles' (and possibly a
`config.h' header file), after which you can run make and
build the package.
Libtool adds its own tests to your configure script in order to
generate a libtool script for the installer's host machine.
AC_PROG_LIBTOOL macro
If you are using GNU Autoconf (or Automake), you should add a call to
AC_PROG_LIBTOOL to your `configure.in' file. This macro
adds many new tests to the configure script so that the generated
libtool script will understand the characteristics of the host:
configure flags.(4) AM_PROG_LIBTOOL was the
old name for this macro, and although supported at the moment is
deprecated.
By default, this macro turns on shared libraries if they are available,
and also enables static libraries if they don't conflict with the shared
libraries. You can modify these defaults by calling either the
AC_DISABLE_SHARED or AC_DISABLE_STATIC macros:
# Turn off shared libraries during beta-testing, since they # make the build process take too long. AC_DISABLE_SHARED AC_PROG_LIBTOOL
The user may specify modified forms of the configure flags
`--enable-shared' and `--enable-static' to choose whether
shared or static libraries are built based on the name of the package.
For example, to have shared `bfd' and `gdb' libraries built,
but not shared `libg++', you can run all three configure
scripts as follows:
trick$ ./configure --enable-shared=bfd,gdb
In general, specifying `--enable-shared=pkgs' is the same as configuring with `--enable-shared' every package named in the comma-separated pkgs list, and every other package with `--disable-shared'. The `--enable-static=pkgs' flag behaves similarly, but it uses `--enable-static' and `--disable-static'. The same applies to the `--enable-fast-install=pkgs' flag, which uses `--enable-fast-install' and `--disable-fast-install'.
The package name `default' matches any packages which have not set
their name in the PACKAGE environment variable.
This macro also sets the shell variable LIBTOOL_DEPS, that you can use to automatically update the libtool script if it becomes out-of-date. In order to do that, add to your `configure.in':
AC_PROG_LIBTOOL AC_SUBST(LIBTOOL_DEPS)
and, to `Makefile.in' or `Makefile.am':
LIBTOOL_DEPS = @LIBTOOL_DEPS@
libtool: $(LIBTOOL_DEPS)
$(SHELL) ./config.status --recheck
If you are using GNU automake, you can omit the assignment, as automake will take care of it. You'll obviously have to create some dependency on `libtool'.
AC_PROG_LIBTOOL.
__declspec(dllexport) and imported with
__declspec(dllimport). If this macro is not used, libtool will
assume that the package libraries are not dll clean and will build only
static libraries on win32 hosts.
This macro must be called before AC_PROG_LIBTOOL, and
provision must be made to pass `-no-undefined' to libtool
in link mode from the package Makefile. Naturally, if you pass
`-no-undefined', you must ensure that all the library symbols
really are defined at link time!
AC_PROG_LIBTOOL to disable
optimization for fast installation. The user may still override this
default, depending on platform support, by specifying
`--enable-fast-install'.
AC_PROG_LIBTOOL to disable
shared libraries. The user may still override this default by
specifying `--enable-shared'.
AC_PROG_LIBTOOL to disable
static libraries. The user may still override this default by
specifying `--enable-static'.
The tests in AC_PROG_LIBTOOL also recognize the following
environment variables:
libtool. If
this is not set, AC_PROG_LIBTOOL will look for gcc or
cc.
AC_PROG_LIBTOOL will not use any such flags. It affects
only the way AC_PROG_LIBTOOL runs tests, not the produced
libtool.
AC_PROG_LIBTOOL will
not use any such flags. It affects only the way AC_PROG_LIBTOOL
runs tests, not the produced libtool.
libtool requires one).
If this is not set, AC_PROG_LIBTOOL will try to find out what is
the linker used by CC.
libtool when it links a program. If
this is not set, AC_PROG_LIBTOOL will not use any such flags. It
affects only the way AC_PROG_LIBTOOL runs tests, not the produced
libtool.
AC_PROG_LIBTOOL when it links a
program. If this is not set, AC_PROG_LIBTOOL will not use any
such flags. It affects only the way AC_PROG_LIBTOOL runs tests,
not the produced libtool.
nm.
ranlib.
AC_PROG_LIBTOOL will check for a suitable
program if this variable is not set.
dlltool. Only meaningful
for Cygwin/MS-Windows.
objdump. Only meaningful
for Cygwin/MS-Windows.
as. Only used on
Cygwin/MS-Windows at the moment.
When you invoke the libtoolize program (see section Invoking libtoolize), it will tell you where to find a definition of
AC_PROG_LIBTOOL. If you use Automake, the aclocal program
will automatically add AC_PROG_LIBTOOL support to your
configure script.
Nevertheless, it is advisable to include a copy of `libtool.m4' in
`acinclude.m4', so that, even if `aclocal.m4' and
`configure' are rebuilt for any reason, the appropriate libtool
macros will be used. The alternative is to hope the user will have a
compatible version of `libtool.m4' installed and accessible for
aclocal. This may lead to weird errors when versions don't
match.
In order to use libtool, you need to include the following files with your package:
Note that the libtool script itself should not be included with your package. See section Configuring libtool.
You should use the libtoolize program, rather than manually
copying these files into your package.
libtoolize
The libtoolize program provides a standard way to add libtool
support to your package. In the future, it may implement better usage
checking, or other features to make libtool even easier to use.
The libtoolize program has the following synopsis:
libtoolize [option]...
and accepts the following options:
AC_PROG_LIBTOOL appears in your
`configure.in'.
less (or
more) or redirect to a file.
libtoolize won't
overwrite existing files.
libtoolize version information and exit.
If libtoolize detects an explicit call to
AC_CONFIG_AUX_DIR (see section `The Autoconf Manual' in The Autoconf Manual) in your `configure.in', it
will put the files in the specified directory.
libtoolize displays hints for adding libtool support to your
package, as well.
The Autoconf package comes with a few macros that run tests, then set a variable corresponding to the name of an object file. Sometimes it is necessary to use corresponding names for libtool objects.
Here are the names of variables that list libtool objects:
AC_FUNC_ALLOCA (see section `The Autoconf Manual' in The Autoconf Manual). Is either empty, or contains `alloca.lo'.
AC_REPLACE_FUNCS (see section `The Autoconf Manual' in The Autoconf Manual), and a few other functions.
Unfortunately, the stable release of Autoconf (2.13, at the time of
this writing) does not have any way for libtool to provide support for
these variables. So, if you depend on them, use the following code
immediately before the call to AC_OUTPUT in your
`configure.in':
LTLIBOBJS=`echo "$LIBOBJS" | sed 's/\.[^.]* /.lo /g;s/\.[^.]*$/.lo/'` AC_SUBST(LTLIBOBJS) LTALLOCA=`echo "$ALLOCA" | sed 's/\.[^.]* /.lo /g;s/\.[^.]*$/.lo/'` AC_SUBST(LTALLOCA) AC_OUTPUT(...)
When you are developing a package, it is often worthwhile to configure
your package with the `--disable-shared' flag, or to override the
defaults for AC_PROG_LIBTOOL by using the
AC_DISABLE_SHARED Autoconf macro (see section The AC_PROG_LIBTOOL macro). This prevents libtool from building
shared libraries, which has several advantages:
You may want to put a small note in your package `README' to let other developers know that `--disable-shared' can save them time. The following example note is taken from the GIMP(5) distribution `README':
The GIMP uses GNU Libtool in order to build shared libraries on a variety of systems. While this is very nice for making usable binaries, it can be a pain when trying to debug a program. For that reason, compilation of shared libraries can be turned off by specifying the `--disable-shared' option to `configure'.
The most difficult issue introduced by shared libraries is that of
creating and resolving runtime dependencies. Dependencies on programs
and libraries are often described in terms of a single name, such as
sed. So, one may say "libtool depends on sed," and that is
good enough for most purposes.
However, when an interface changes regularly, we need to be more specific: "Gnus 5.1 requires Emacs 19.28 or above." Here, the description of an interface consists of a name, and a "version number."
Even that sort of description is not accurate enough for some purposes. What if Emacs 20 changes enough to break Gnus 5.1?
The same problem exists in shared libraries: we require a formal version system to describe the sorts of dependencies that programs have on shared libraries, so that the dynamic linker can guarantee that programs are linked only against libraries that provide the interface they require.
Interfaces for libraries may be any of the following (and more):
Note that static functions do not count as interfaces, because they are not directly available to the user of the library.
Libtool has its own formal versioning system. It is not as flexible as some, but it is definitely the simplest of the more powerful versioning systems.
Think of a library as exporting several sets of interfaces, arbitrarily represented by integers. When a program is linked against a library, it may use any subset of those interfaces.
Libtool's description of the interfaces that a program uses is simple: it encodes the least and the greatest interface numbers in the resulting binary (first-interface, last-interface).
The dynamic linker is guaranteed that if a library supports every interface number between first-interface and last-interface, then the program can be relinked against that library.
Note that this can cause problems because libtool's compatibility requirements are actually stricter than is necessary.
Say `libhello' supports interfaces 5, 16, 17, 18, and 19, and that libtool is used to link `test' against `libhello'.
Libtool encodes the numbers 5 and 19 in `test', and the dynamic linker will only link `test' against libraries that support every interface between 5 and 19. So, the dynamic linker refuses to link `test' against `libhello'!
In order to eliminate this problem, libtool only allows libraries to declare consecutive interface numbers. So, `libhello' can declare at most that it supports interfaces 16 through 19. Then, the dynamic linker will link `test' against `libhello'.
So, libtool library versions are described by three integers:
current -
age to current.
If two libraries have identical current and age numbers, then the dynamic linker chooses the library with the greater revision number.
If you want to use libtool's versioning system, then you must specify the version information to libtool using the `-version-info' flag during link mode (see section Link mode).
This flag accepts an argument of the form `current[:revision[:age]]'. So, passing `-version-info 3:12:1' sets current to 3, revision to 12, and age to 1.
If either revision or age are omitted, they default to 0. Also note that age must be less than or equal to the current interface number.
Here are a set of rules to help you update your library version information:
Never try to set the interface numbers so that they correspond to the release number of your package. This is an abuse that only fosters misunderstanding of the purpose of library versions. Instead, use the `-release' flag (see section Managing release information), but be warned that every release of your package will not be binary compatible with any other release.
Often, people want to encode the name of the package release into the shared library so that it is obvious to the user which package their programs are linked against. This convention is used especially on GNU/Linux:
trick$ ls /usr/lib/libbfd* /usr/lib/libbfd.a /usr/lib/libbfd.so.2.7.0.2 /usr/lib/libbfd.so trick$
On `trick', `/usr/lib/libbfd.so' is a symbolic link to `libbfd.so.2.7.0.2', which was distributed as a part of `binutils-2.7.0.2'.
Unfortunately, this convention conflicts directly with libtool's idea of library interface versions, because the library interface rarely changes at the same time that the release number does, and the library suffix is never the same across all platforms.
So, in order to accommodate both views, you can use the `-release' flag in order to set release information for libraries which you do not want to use `-version-info'. For the `libbfd' example, the next release which uses libtool should be built with `-release 2.9.0', which will produce the following files on GNU/Linux:
trick$ ls /usr/lib/libbfd* /usr/lib/libbfd-2.9.0.so /usr/lib/libbfd.a /usr/lib/libbfd.so trick$
In this case, `/usr/lib/libbfd.so' is a symbolic link to `libbfd-2.9.0.so'. This makes it obvious that the user is dealing with `binutils-2.9.0', without compromising libtool's idea of interface versions.
Note that this option causes a modification of the library name, so do not use it unless you want to break binary compatibility with any past library releases. In general, you should only use `-release' for package-internal libraries or for ones whose interfaces change very frequently.
Writing a good library interface takes a lot of practice and thorough understanding of the problem that the library is intended to solve.
If you design a good interface, it won't have to change often, you won't have to keep updating documentation, and users won't have to keep relearning how to use the library.
Here is a brief list of tips for library interface design, which may help you in your exploits:
static keyword (or equivalent) whenever possible
Writing portable C header files can be difficult, since they may be read by different types of compilers:
extern "C" directive, so that the
names aren't mangled. See section Writing libraries for C++, for other issues relevant
to using C++ with libtool.
#included.
These complications mean that your library interface headers must use some C preprocessor magic in order to be usable by each of the above compilers.
`foo.h' in the `demo' subdirectory of the libtool distribution serves as an example for how to write a header file that can be safely installed in a system directory.
Here are the relevant portions of that file:
/* BEGIN_C_DECLS should be used at the beginning of your declarations,
so that C++ compilers don't mangle their names. Use END_C_DECLS at
the end of C declarations. */
#undef BEGIN_C_DECLS
#undef END_C_DECLS
#ifdef __cplusplus
# define BEGIN_C_DECLS extern "C" {
# define END_C_DECLS }
#else
# define BEGIN_C_DECLS /* empty */
# define END_C_DECLS /* empty */
#endif
/* PARAMS is a macro used to wrap function prototypes, so that
compilers that don't understand ANSI C prototypes still work,
and ANSI C compilers can issue warnings about type mismatches. */
#undef PARAMS
#if defined (__STDC__) || defined (_AIX) \
|| (defined (__mips) && defined (_SYSTYPE_SVR4)) \
|| defined(WIN32) || defined(__cplusplus)
# define PARAMS(protos) protos
#else
# define PARAMS(protos) ()
#endif
These macros are used in `foo.h' as follows:
#ifndef FOO_H #define FOO_H 1 /* The above macro definitions. */ #include "..." BEGIN_C_DECLS int foo PARAMS((void)); int hello PARAMS((void)); END_C_DECLS #endif /* !FOO_H */
Note that the `#ifndef FOO_H' prevents the body of `foo.h' from being read more than once in a given compilation.
Also the only thing that must go outside the
BEGIN_C_DECLS/END_C_DECLS pair are #include lines.
Strictly speaking it is only C symbol names that need to be protected,
but your header files will be more maintainable if you have a single
pair of of these macros around the majority of the header contents.
You should use these definitions of PARAMS, BEGIN_C_DECLS,
and END_C_DECLS into your own headers. Then, you may use them to
create header files that are valid for C++, ANSI, and non-ANSI
compilers(6).
Do not be naive about writing portable code. Following the tips given above will help you miss the most obvious problems, but there are definitely other subtle portability issues. You may need to cope with some of the following issues:
void * generic
pointer type, and so need to use char * in its place.
const, inline and signed keywords are not
supported by some compilers, especially pre-ANSI compilers.
long double type is not supported by many compilers.
By definition, every shared library system provides a way for executables to depend on libraries, so that symbol resolution is deferred until runtime.
An inter-library dependency is one in which a library depends on
other libraries. For example, if the libtool library `libhello'
uses the cos function, then it has an inter-library dependency
on `libm', the math library that implements cos.
Some shared library systems provide this feature in an internally-consistent way: these systems allow chains of dependencies of potentially infinite length.
However, most shared library systems are restricted in that they only allow a single level of dependencies. In these systems, programs may depend on shared libraries, but shared libraries may not depend on other shared libraries.
In any event, libtool provides a simple mechanism for you to declare
inter-library dependencies: for every library `libname' that
your own library depends on, simply add a corresponding
-lname option to the link line when you create your
library. To make an example of our
`libhello' that depends on `libm':
burger$ libtool gcc -g -O -o libhello.la foo.lo hello.lo \
-rpath /usr/local/lib -lm
burger$
When you link a program against `libhello', you don't need to specify the same `-l' options again: libtool will do that for you, in order to guarantee that all the required libraries are found. This restriction is only necessary to preserve compatibility with static library systems and simple dynamic library systems.
Some platforms, such as AIX, do not even allow you this flexibility. In order to build a shared library, it must be entirely self-contained (that is, have references only to symbols that are found in the `.lo' files or the specified `-l' libraries), and you need to specify the -no-undefined flag. By default, libtool builds only static libraries on these kinds of platforms.
The simple-minded inter-library dependency tracking code of libtool releases prior to 1.2 was disabled because it was not clear when it was possible to link one library with another, and complex failures would occur. A more complex implementation of this concept was re-introduced before release 1.3, but it has not been ported to all platforms that libtool supports. The default, conservative behavior is to avoid linking one library with another, introducing their inter-dependencies only when a program is linked with them.
It can sometimes be confusing to discuss dynamic linking, because the term is used to refer to two different concepts:
dlopen,(7) which load
arbitrary, user-specified modules at runtime. This type of dynamic
linking is explicitly controlled by the application.
To mitigate confusion, this manual refers to the second type of dynamic linking as dlopening a module.
The main benefit to dlopening object modules is the ability to access compiled object code to extend your program, rather than using an interpreted language. In fact, dlopen calls are frequently used in language interpreters to provide an efficient way to extend the language.
As of version 1.4.2, libtool provides support for dlopened modules. However, you should indicate that your package is willing to use such support, by using the macro `AC_LIBTOOL_DLOPEN' in `configure.in'. If this macro is not used (or it is used after `AC_PROG_LIBTOOL'), libtool will assume no dlopening mechanism is available, and will try to simulate it.
This chapter discusses how you as a dlopen application developer might use libtool to generate dlopen-accessible modules.
On some operating systems, a program symbol must be specially declared
in order to be dynamically resolved with the dlsym (or
equivalent) function.
Libtool provides the `-export-dynamic' and `-module' link flags (see section Link mode), which do this declaration. You need to use these flags if you are linking an application program that dlopens other modules or a libtool library that will also be dlopened.
For example, if we wanted to build a shared library, `libhello', that would later be dlopened by an application, we would add `-module' to the other link flags:
burger$ libtool gcc -module -o libhello.la foo.lo \
hello.lo -rpath /usr/local/lib -lm
burger$
If symbols from your executable are needed to satisfy unresolved references in a library you want to dlopen you will have to use the flag `-export-dynamic'. You should use `-export-dynamic' while linking the executable that calls dlopen:
burger$ libtool gcc -export-dynamic -o hell-dlopener main.o burger$
Libtool provides special support for dlopening libtool object and
libtool library files, so that their symbols can be resolved even
on platforms without any dlopen and dlsym
functions.
Consider the following alternative ways of loading code into your program, in order of increasing "laziness":
Libtool emulates `-dlopen' on static platforms by linking objects into the program at compile time, and creating data structures that represent the program's symbol table.
In order to use this feature, you must declare the objects you want your application to dlopen by using the `-dlopen' or `-dlpreopen' flags when you link your program (see section Link mode).
"fprintf". The address attribute is a
generic pointer to the appropriate object, such as &fprintf.
0, followed by all symbols exported from this file.
For the executable itself the special name @PROGRAM@ is used.
The last element has a name and address of 0.
Some compilers may allow identifiers which are not valid in ANSI C, such as dollar signs. Libtool only recognizes valid ANSI C symbols (an initial ASCII letter or underscore, followed by zero or more ASCII letters, digits, and underscores), so non-ANSI symbols will not appear in lt_preloaded_symbols.
After a library has been linked with `-module', it can be dlopened. Unfortunately, because of the variation in library names, your package needs to determine the correct file to dlopen.
The most straightforward and flexible implementation is to determine the name at runtime, by finding the installed `.la' file, and searching it for the following lines:
# The name that we can dlopen.
dlname='dlname'
If dlname is empty, then the library cannot be dlopened. Otherwise, it gives the dlname of the library. So, if the library was installed as `/usr/local/lib/libhello.la', and the dlname was `libhello.so.3', then `/usr/local/lib/libhello.so.3' should be dlopened.
If your program uses this approach, then it should search the
directories listed in the LD_LIBRARY_PATH(8) environment variable, as well as
the directory where libraries will eventually be installed. Searching
this variable (or equivalent) will guarantee that your program can find
its dlopened modules, even before installation, provided you have linked
them using libtool.
The following problems are not solved by using libtool's dlopen support:
dlopen family, which do package-specific tricks when dlopening
is unsupported or not available on a given platform.
dlopen
family of functions. Some platforms do not even use the same function
names (notably HP-UX, with its shl_load family).
dlopen.
Libtool provides a small library, called `libltdl', that aims at hiding the various difficulties of dlopening libraries from programmers. It consists of a header-file and a small C source file that can be distributed with applications that need dlopening functionality. On some platforms, whose dynamic linkers are too limited for a simple implementation of `libltdl' services, it requires GNU DLD, or it will only emulate dynamic linking with libtool's dlpreopening mechanism.
libltdl supports currently the following dynamic linking mechanisms:
dlopen (Solaris, Linux and various BSD flavors)
shl_load (HP-UX)
LoadLibrary (Win16 and Win32)
load_add_on (BeOS)
libltdl is licensed under the terms of the GNU Library General Public License, with the following exception:
As a special exception to the GNU Lesser General Public License, if you distribute this file as part of a program or library that is built using GNU libtool, you may include it under the same distribution terms that you use for the rest of that program.
The libltdl API is similar to the dlopen interface of Solaris and Linux, which is very simple but powerful.
To use libltdl in your program you have to include the header file `ltdl.h':
#include <ltdl.h>
The last release of libltdl used some symbols that violated the POSIX namespace conventions. These symbols are now deprecated, and have been replaced by those described here. If you have code that relies on the old deprecated symbol names, defining `LT_NON_POSIX_NAMESPACE' before you include `ltdl.h' provides conversion macros. Whichever set of symbols you use, the new api is not binary compatible with the last, so you will need to recompile your application in order to use this version of libltdl.
Note that libltdl is not threadsafe, i.e. a multithreaded application
has to use a mutex for libltdl. It was reported that GNU/Linux's glibc
2.0's dlopen with `RTLD_LAZY' (which libltdl uses by
default) is not thread-safe, but this problem is supposed to be fixed in
glibc 2.1. On the other hand, `RTLD_NOW' was reported to introduce
problems in multi-threaded applications on FreeBSD. Working around
these problems is left as an exercise for the reader; contributions are
certainly welcome.
The following types are defined in `ltdl.h':
lt_ptr is a generic pointer.
lt_dlhandle is a module "handle".
Every lt_dlopened module has a handle associated with it.
lt_dlsymlist is a symbol list for dlpreopened modules.
This structure is described in see section Dlpreopening.
libltdl provides the following functions:
lt_dlinit has been successfully called.
Return 0 on success, otherwise the number of errors.
lt_dlopen is able to open libtool dynamic
modules, preloaded static modules, the program itself and
native dynamic libraries.
Unresolved symbols in the module are resolved using its dependency
libraries (not implemented yet) and previously dlopened modules. If the
executable using this module was linked with the -export-dynamic
flag, then the global symbols in the executable will also be used to
resolve references in the module.
If filename is NULL and the program was linked with
-export-dynamic or -dlopen self, lt_dlopen will
return a handle for the program itself, which can be used to access its
symbols.
If libltdl cannot find the library and the file name filename does not have a directory component it will additionally search in the following search paths for the module (in the order as follows):
lt_dlsetsearchpath and lt_dladdsearchdir.
Each search path must be a colon-separated list of absolute directories,
for example, "/usr/lib/mypkg:/lib/foo".
If the same module is loaded several times, the same handle is returned.
If lt_dlopen fails for any reason, it returns NULL.
lt_dlopen, except that it tries to append
different file name extensions to the file name.
If the file with the file name filename cannot be found
libltdl tries to append the following extensions:
This lookup strategy was designed to allow programs that don't
have knowledge about native dynamic libraries naming conventions
to be able to dlopen such libraries as well as libtool modules
transparently.
NULL is returned.
NULL if no errors have occurred since initialization
or since it was last called.
NULL, then all previously registered
symbol lists, except the list set by lt_dlpreload_default,
are deleted. Return 0 on success.
lt_dlpreload. Note that this function does
not require libltdl to be initialized using lt_dlinit and
can be used in the program to register the default preloaded modules.
Instead of calling this function directly, most programs will use the
macro LTDL_SET_PRELOADED_SYMBOLS.
Return 0 on success.
#include <ltdl.h>
int main() {
/* ... */
LTDL_SET_PRELOADED_SYMBOLS();
/* ... */
}
If you use `lt_dlopen (NULL)' to get a handle for the running binary, that handle will always be marked as resident, and consequently cannot be successfully `lt_dlclose'd.
lt_dlerror.
malloc and free, by default,
but you can set them to any other functions that provides equivalent
functionality. However, you must not modify their values after calling
any libltdl function other than lt_dlpreopen_default or the macro
LTDL_SET_PRELOADED_SYMBOLS.
dlopenedLibtool modules are like normal libtool libraries with a few exceptions:
You have to link the module with libtool's `-module' switch, and you should link any program that is intended to dlopen the module with `-dlopen modulename.la' so that libtool can dlpreopen the module on platforms which don't support dlopening. If the module depends on any other libraries, make sure you specify them either when you link the module or when you link programs that dlopen it. If you want to disable see section Library interface versions for a specific module you should link it with the `-avoid-version' switch. Note that libtool modules don't need to have a "lib" prefix. However, automake 1.4 or higher is required to build such modules.
Usually a set of modules provide the same interface, i.e, exports the same symbols, so that a program can dlopen them without having to know more about their internals. In order to avoid symbol conflicts all exported symbols must be prefixed with "modulename_LTX_" (`modulename' is the name of the module). Internal symbols must be named in such a way that they won't conflict with other modules, for example, by prefixing them with "_modulename_". Although some platforms support having the same symbols defined more than once it is generally not portable and it makes it impossible to dlpreopen such modules. libltdl will automatically cut the prefix off to get the real name of the symbol. Additionally, it supports modules which don't use a prefix so that you can also dlopen non-libtool modules.
`foo1.c' gives an example of a portable libtool module. Exported symbols are prefixed with "foo1_LTX_", internal symbols with "_foo1_". Aliases are defined at the beginning so that the code is more readable.
/* aliases for the exported symbols */
#define foo foo1_LTX_foo
#define bar foo1_LTX_bar
/* a global variable definition */
int bar = 1;
/* a private function */
int _foo1_helper() {
return bar;
}
/* an exported function */
int foo() {
return _foo1_helper();
}
The `Makefile.am' contains the necessary rules to build the module `foo1.la':
... lib_LTLIBRARIES = foo1.la foo1_la_SOURCES = foo1.c foo1_la_LDFLAGS = -module ...
Using the lt_dlmutex_register() function, and by providing some
appropriate callback function definitions, libltdl can be used in a
multi-threaded environment.
Because libltdl is inherantly recursive, it is important that the locking mechanism employed by these callback functions are reentrant, or else strange problems will occur.
lt_dlerror().
A function of this type must be registered with the library in order for it to work in a multi-threaded context. The function should store any error message passed in thread local storage.
When regeistered correctly this function will be used by
lt_dlerror()) from all threads to retrieve error messages for the
client.
NULL function addresses, or else all
NULL to return to single threaded operation.
Some of the internal information about each loaded module that is maintained by libltdl is available to the user, in the form of this structure:
lt_dlinfo is used to store information about a module.
The filename attribute is a null-terminated character string of
the real module file name. If the module is a libtool module then
name is its module name (e.g. "libfoo" for
"dir/libfoo.la"), otherwise it is set to NULL. The
ref_count attribute is a reference counter that describes how
often the same module is currently loaded.
The following function will return a pointer to libltdl's internal copy of this structure for the given handle:
NULL on failure.
Furthermore, in order to save you from having to keep a list of the handles of all the modules you have loaded, these functions allow you to iterate over libltdl's list of loaded modules:
lt_dlforeach.
As soon as func returns a non-zero value for one of the handles,
lt_dlforeach will stop calling func and immediately return 1.
Otherwise 0 is returned.
NULL, and the next one on subsequent calls.
If place is the last element in the list of loaded modules, this
function returns NULL.
Of course, you would still need to maintain your own list of loaded module handles to parallel the list maintained by libltdl if there are any other data that you need to associate with each handle for the purposes of your application. However, if you use the following API calls to associate your application data with individual module handles as they are loaded there is actually no need to do that. You must first obtain a unique caller id from libltdl which you subsequently use to retrieve the data you stored earlier. This allows for different libraries that each wish to store their own data against loaded modules to do so without interfering with one another's data.
lt_dlerror().
For example, to correctly remove some associated data:
lt_ptr stale = lt_dlcaller_set_data (key, handle, 0);
if (stale == NULL)
{
char *error_msg = lt_dlerror ();
if (error_msg != NULL)
{
my_error_handler (error_msg);
return STATUS_FAILED;
}
}
else
{
free (stale);
}
NULL if there is none.
The preceding functions can be combined with lt_dlforeach to
implement search and apply operations without the need for your
application to track the modules that have been loaded and unloaded:
int
my_dlcaller_callback (lt_dlhandle handle, lt_ptr key_ptr)
{
struct my_module_data *my_data;
my_data = lt_dlcaller_get_data (handle, (lt_dlcaller_id) *key_ptr);
return process (my_data);
}
int
my_dlcaller_foreach (lt_dlcaller_id key)
{
lt_dlforeach (my_dlcaller_callback, (lt_ptr) &key);
}
Sometimes libltdl's many ways of gaining access to modules are not
sufficient for the purposes of a project. You can write your own
loader, and register it with libltdl so that lt_dlopen will be
able to use it.
Writing a loader involves writing at least three functions which can be
called by lt_dlopen, lt_dlsym and lt_dlclose.
Optionally, you can provide a finalisation function to perform any
cleanup operations when lt_dlexit executes, and a symbol prefix
string which will be prepended to any symbols passed to lt_dlsym.
These functions must match the function pointer types below, after
which they can be allocated to an instance of lt_user_dlloader
and registered.
Registering the loader requires that you choose a name for it, so that it
can be recognised by lt_dlloader_find and removed with
lt_dlloader_remove. The name you choose must be unique, and not
already in use by libltdl's builtin loaders:
lt_dlopening of preloaded static modules.
The prefix "dl" is reserved for loaders supplied with future versions of libltdl, so you should not use that for your own loader names.
The following types are defined in `ltdl.h':
lt_module is a dlloader dependent module.
The dynamic module loader extensions communicate using these low
level types.
lt_dlloader is a handle for module loader types.
lt_dlloader_data is used for specifying loader instance data.
lt_dlopen API use it, you need to instantiate one of these
structures and pass it to lt_dlloader_add. You can pass whatever
you like in the dlloader_data field, and it will be passed back as
the value of the first parameter to each of the functions specified in
the function pointer fields.
lt_dlloader module
loader. The value set in the dlloader_data field of the struct
lt_user_dlloader structure will be passed into this function in the
loader_data parameter. Implementation of such a function should
attempt to load the named module, and return an lt_module
suitable for passing in to the associated lt_module_close and
lt_sym_find function pointers. If the function fails it should
return NULL, and set the error message with lt_dlseterror.
lt_dlseterror and return non-zero.
lt_dlseterror and return NULL if lookup fails.
dlloader_data field of the lt_user_dlloader. If non-NULL,
the function will be called by lt_dlexit, and
lt_dlloader_remove.
For example:
int
register_myloader (void)
{
lt_user_dlloader dlloader;
/* User modules are responsible for their own initialisation. */
if (myloader_init () != 0)
return MYLOADER_INIT_ERROR;
dlloader.sym_prefix = NULL;
dlloader.module_open = myloader_open;
dlloader.module_close = myloader_close;
dlloader.find_sym = myloader_find_sym.
dlloader.dlloader_exit = myloader_exit;
dlloader.dlloader_data = (lt_user_data)myloader_function;
/* Add my loader as the default module loader. */
if (lt_dlloader_add (lt_dlloader_next (NULL), &dlloader, "myloader") != 0)
return ERROR;
return OK;
}
Note that if there is any initialisation required for the loader, it must be performed manually before the loader is registered -- libltdl doesn't handle user loader initialisation.
Finalisation is handled by libltdl however, and it is important
to ensure the dlloader_exit callback releases any resources claimed
during the initialisation phase.
libltdl provides the following functions for writing your own module loaders:
NULL), else immediately before the
loader passed as place. loader_name will be returned by
lt_dlloader_name if it is subsequently passed a newly
registered loader. These loader_names must be unique, or
lt_dlloader_remove and lt_dlloader_find cannot
work. Returns 0 for success.
{
/* Make myloader be the last one. */
if (lt_dlloader_add (NULL, myloader) != 0)
perror (lt_dlerror ());
}
lt_dlerror.
{
/* Remove myloader. */
if (lt_dlloader_remove ("myloader") != 0)
perror (lt_dlerror ());
}
NULL, and the next one on subsequent calls. The handle is for use with
lt_dlloader_add.
{
/* Make myloader be the first one. */
if (lt_dlloader_add (lt_dlloader_next (NULL), myloader) != 0)
return ERROR;
}
NULL, if the identifier is not found.
The identifiers which may be used by libltdl itself, if the host architecture supports them are dlopen(9), dld and dlpreload.
{
/* Add a user loader as the next module loader to be tried if
the standard dlopen loader were to fail when lt_dlopening. */
if (lt_dlloader_add (lt_dlloader_find ("dlopen"), myloader) != 0)
return ERROR;
}
lt_dlloader_next or lt_dlloader_find. If this function fails,
it will return NULL and set an error for retrieval with
lt_dlerror.
dlloader_data of PLACE, as
obtained from lt_dlloader_next or lt_dlloader_find. If
this function fails, it will return NULL and set an error for
retrieval with lt_dlerror.
lt_dlerror. Pass in a suitable diagnostic message for return by
lt_dlerror, and an error identifier for use with
lt_dlseterror is returned.
If the allocation of an identifier fails, this function returns -1.
int myerror = lt_dladderror ("Doh!");
if (myerror < 0)
perror (lt_dlerror ());
lt_dlerror
interface. All of the standard errors used by libltdl are declared in
`ltdl.h', or you can add more of your own with
lt_dladderror. This function returns 0 on success.
if (lt_dlseterror (LTDL_ERROR_NO_MEMORY) != 0) perror (lt_dlerror ());
Even though libltdl is installed together with libtool, you may wish to
include libltdl in the distribution of your package, for the convenience
of users of your package that don't have libtool or libltdl installed.
In this case, you must decide whether to manually add the ltdl
objects to your package, or else which flavor of libltdl you want to use:
a convenience library or an installable libtool library.
The most simplistic way to add libltdl to your package is to copy
the source files, `ltdl.c' and `ltdl.h', to a source directory
withing your package and to build and link them along with the rest of
your sources. To help you do this, the m4 macros for autoconf are
available in `ltdl.m4'. You must ensure that they are available in
`aclocal.m4' before you run autoconf -- by appending the contents
of `ltdl.m4' to `acinclude.m4', if you are using automake, or
to `aclocal.m4' if you are not. Having made the macros available,
you must add a call to the `AC_LIB_LTDL' macro to your package's
`configure.in' to perform the configure time checks required to
build `ltdl.o' correctly. This method has problems if you then try
to link the package binaries with an installed libltdl, or a library
which depends on libltdl: you may have problems with duplicate symbol
definitions.
One advantage of the convenience library is that it is not installed, so the fact that you use libltdl will not be apparent to the user, and it will not overwrite a pre-installed version of libltdl a user might have. On the other hand, if you want to upgrade libltdl for any reason (e.g. a bugfix) you'll have to recompile your package instead of just replacing an installed version of libltdl. However, if your programs or libraries are linked with other libraries that use such a pre-installed version of libltdl, you may get linker errors or run-time crashes. Another problem is that you cannot link the convenience library into more than one libtool library, then link a single program with these libraries, because you may get duplicate symbols. In general you can safely use the convenience library in programs which don't depend on other libraries that might use libltdl too. In order to enable this flavor of libltdl, you should add the line `AC_LIBLTDL_CONVENIENCE' to your `configure.in', before `AC_PROG_LIBTOOL'.
In order to select the installable version of libltdl, you should add a call of the macro `AC_LIBLTDL_INSTALLABLE' to your `configure.in' before `AC_PROG_LIBTOOL'. This macro will check whether libltdl is already installed and, if not, request the libltdl embedded in your package to be built and installed. Note, however, that no version checking is performed. The user may override the test and determine that the libltdl embedded must be installed, regardless of the existence of another version, using the configure switch `--enable-ltdl-install'.
In order to embed libltdl into your package, just add `--ltdl' to
the libtoolize command line. It will copy the libltdl sources
to a subdirectory `libltdl' in your package.
Both macros accept an optional argument to specify the location
of the `libltdl' directory. By the default both macros assume that it
is `${top_srcdir}/libltdl'.
Whatever macro you use, it is up to you to ensure that your `configure.in' will configure libltdl, using `AC_CONFIG_SUBDIRS', and that your `Makefile's will start sub-makes within libltdl's directory, using automake's SUBDIRS, for example. Both macros define the shell variables LIBLTDL, to the link flag that you should use to link with libltdl, and INCLTDL, to the preprocessor flag that you should use to compile with programs that include `ltdl.h'. It is up to you to use `AC_SUBST' to ensure that this variable will be available in `Makefile's, or add them to variables that are `AC_SUBST'ed by default, such as LIBS and CPPFLAGS.
If you're using the convenience libltdl, LIBLTDL will be the pathname for the convenience version of libltdl and INCLTDL will be `-I' followed by the directory that contains libltdl, both starting with `${top_builddir}/' or `${top_srcdir}/', respectively.
If you request an installed version of libltdl and one is found(10), LIBLTDL will be set to `-lltdl' and INCLTDL will be empty (which is just a blind assumption that `ltdl.h' is somewhere in the include path if libltdl is in the library path). If an installable version of libltdl must be built, its pathname, starting with `${top_builddir}/', will be stored in LIBLTDL, and INCLTDL will be set just like in the case of convenience library.
So, when you want to link a program with libltdl, be it a convenience, installed or installable library, just compile with `$(INCLTDL)' and link it with `$(LIBLTDL)', using libtool.
You should probably also add `AC_LIBTOOL_DLOPEN' to your `configure.in' before `AC_PROG_LIBTOOL', otherwise libtool will assume no dlopening mechanism is supported, and revert to dlpreopening, which is probably not what you want.
Avoid using the -static or -all-static switches when
linking programs with libltdl. This will not work on all platforms,
because the dlopening functions may not be available for static linking.
The following example shows you how to embed the convenience libltdl in your package. In order to use the installable variant just replace `AC_LIBLTDL_CONVENIENCE' with `AC_LIBLTDL_INSTALLABLE'. We assume that libltdl was embedded using `libtoolize --ltdl'.
configure.in:
... dnl Enable building of the convenience library dnl and set LIBLTDL accordingly AC_LIBLTDL_CONVENIENCE dnl Substitute INCLTDL and LIBLTDL in the Makefiles AC_SUBST(INCLTDL) AC_SUBST(LIBLTDL) dnl Check for dlopen support AC_LIBTOOL_DLOPEN dnl Configure libtool AC_PROG_LIBTOOL dnl Configure libltdl AC_CONFIG_SUBDIRS(libltdl) ...
Makefile.am:
... SUBDIRS = libltdl INCLUDES = $(INCLTDL) myprog_LDFLAGS = -export-dynamic # The quotes around -dlopen below fool automake <= 1.4 into accepting it myprog_LDADD = $(LIBLTDL) "-dlopen" self "-dlopen" foo1.la myprog_DEPENDENCIES = $(LIBLTDL) foo1.la ...
Libtool was first implemented in order to add support for writing shared libraries in the C language. However, over time, libtool is being integrated with other languages, so that programmers are free to reap the benefits of shared libraries in their favorite programming language.
This chapter describes how libtool interacts with other languages, and what special considerations you need to make if you do not use C.
Creating libraries of C++ code should be a fairly straightforward process, because its object files differ from C ones in only three ways:
The conclusion is that libtool is not ready for general use for C++ libraries. You should avoid any global or static variable initializations that would cause an "initializer element is not constant" error if you compiled them with a standard C compiler.
There are other ways of working around this problem, but they are beyond the scope of this manual.
Furthermore, you'd better find out, at configure time, what are the C++ Standard libraries that the C++ compiler will link in by default, and explicitly list them in the link command line. Hopefully, in the future, libtool will be able to do this job by itself.
Libtool is under constant development, changing to remain up-to-date with modern operating systems. If libtool doesn't work the way you think it should on your platform, you should read this chapter to help determine what the problem is, and how to resolve it.
Libtool comes with its own set of programs that test its capabilities, and report obvious bugs in the libtool program. These tests, too, are constantly evolving, based on past problems with libtool, and known deficiencies in other operating systems.
As described in the `INSTALL' file, you may run make check after you have built libtool (possibly before you install it) in order to make sure that it meets basic functional requirements.
Here is a list of the current programs in the test suite, and what they test for:
deplibs_check_method to prevent such cases.
This tests checks whether libtool's deplibs_check_method
works properly.
--dry-run mode works properly.
Each of the above tests are designed to produce no output when they are run via make check. The exit status of each program tells the `Makefile' whether or not the test succeeded.
If a test fails, it means that there is either a programming error in libtool, or in the test program itself.
To investigate a particular test, you may run it directly, as you would a normal program. When the test is invoked in this way, it produces output which may be useful in determining what the problem is.
Another way to have the test programs produce output is to set the VERBOSE environment variable to `yes' before running them. For example, env VERBOSE=yes make check runs all the tests, and has each of them display debugging information.
If you think you have discovered a bug in libtool, you should think twice: the libtool maintainer is notorious for passing the buck (or maybe that should be "passing the bug"). Libtool was invented to fix known deficiencies in shared library implementations, so, in a way, most of the bugs in libtool are actually bugs in other operating systems. However, the libtool maintainer would definitely be happy to add support for somebody else's buggy operating system. [I wish there was a good way to do winking smiley-faces in Texinfo.]
Genuine bugs in libtool include problems with shell script portability, documentation errors, and failures in the test suite (see section The libtool test suite).
First, check the documentation and help screens to make sure that the behaviour you think is a problem is not already mentioned as a feature.
Then, you should read the Emacs guide to reporting bugs (see section `Reporting Bugs' in The Emacs Manual). Some of the details listed there are specific to Emacs, but the principle behind them is a general one.
Finally, send a bug report to the libtool bug reporting address [email protected] with any appropriate facts, such as test suite output (see section When tests fail), all the details needed to reproduce the bug, and a brief description of why you think the behaviour is a bug. Be sure to include the word "libtool" in the subject line, as well as the version number you are using (which can be found by typing libtool --version).
This chapter contains information that the libtool maintainer finds important. It will be of no use to you unless you are considering porting libtool to new systems, or writing your own libtool.
Before you embark on porting libtool to an unsupported system, it is worthwhile to send e-mail to the libtool mailing list [email protected], to make sure that you are not duplicating existing work.
If you find that any porting documentation is missing, please complain! Complaints with patches and improvements to the documentation, or to libtool itself, are more than welcome.
Once it is clear that a new port is necessary, you'll generally need the following information:
config.guess for this system, so that you
can make changes to the libtool configuration process without affecting
other systems.
ld and cc
ld.so, rtld, or equivalent
ldconfig, or equivalent
If you know how to program the Bourne shell, then you can complete the port yourself; otherwise, you'll have to find somebody with the relevant skills who will do the work. People on the libtool mailing list are usually willing to volunteer to help you with new ports, so you can send the information to them.
To do the port yourself, you'll definitely need to modify the
libtool.m4 macros in order to make platform-specific changes to
the configuration process. You should search that file for the
PORTME keyword, which will give you some hints on what you'll
need to change. In general, all that is involved is modifying the
appropriate configuration variables (see section libtool script contents).
Your best bet is to find an already-supported system that is similar to
yours, and make your changes based on that. In some cases, however,
your system will differ significantly from every other supported system,
and it may be necessary to add new configuration variables, and modify
the ltmain.in script accordingly. Be sure to write to the
mailing list before you make changes to ltmain.in, since they may
have advice on the most effective way of accomplishing what you want.
Since version 1.2c, libtool has re-introduced the ability to do inter-library dependency on some platforms, thanks to a patch by Toshio Kuratomi [email protected]. Here's a shortened version of the message that contained his patch:
The basic architecture is this: in `libtool.m4', the person who writes libtool makes sure `$deplibs' is included in `$archive_cmds' somewhere and also sets the variable `$deplibs_check_method', and maybe `$file_magic_cmd' when `deplibs_check_method' is file_magic.
`deplibs_check_method' can be one of five things:
egrep. When
file_magic_test_file is set by `libtool.m4', it is used as an
argument to `$file_magic_cmd' in order to verify whether the
regular expression matches its output, and warn the user otherwise.
ldd. It is currently unused, and will probably be dropped in the
future.
Then in `ltmain.in' we have the real workhorse: a little initialization and postprocessing (to setup/release variables for use with eval echo libname_spec etc.) and a case statement that decides which method is being used. This is the real code... I wish I could condense it a little more, but I don't think I can without function calls. I've mostly optimized it (moved things out of loops, etc) but there is probably some fat left. I thought I should stop while I was ahead, work on whatever bugs you discover, etc before thinking about more than obvious optimizations.
This table describes when libtool was last known to be tested on platforms where it claims to support shared libraries:
Note: The vendor-distributed HP-UX sed(1) programs are horribly
broken, and cannot handle libtool's requirements, so users may report
unusual problems. There is no workaround except to install a working
sed (such as GNU sed) on these systems.
Note: The vendor-distributed NCR MP-RAS cc programs emits
copyright on standard error that confuse tests on size of
`conftest.err'. The workaround is to specify CC
when run configure with CC='cc -Hnocopyr'.
This section is dedicated to the sanity of the libtool maintainers. It describes the programs that libtool uses, how they vary from system to system, and how to test for them.
Because libtool is a shell script, it can be difficult to understand just by reading it from top to bottom. This section helps show why libtool does things a certain way. Combined with the scripts themselves, you should have a better sense of how to improve libtool, or write your own.
The following is a list of valuable documentation references:
http://techpubs.sgi.com/cgi-bin/infosrch.cgi?cmd=browse&db=man.
http://www.sun.com/service/online/free.html) and documentation
server (http://docs.sun.com/).
http://tru64unix.compaq.com/faqs/publications/pub_page/doc_list.html)
with C++ documentation at
(http://tru64unix.compaq.com/cplus/docs/index.htm).
http://docs.hp.com/index.html).
http://www.rs6000.ibm.com/resource/aix_resource/Pubs/).
The only compiler characteristics that affect libtool are the flags needed (if any) to generate PIC objects. In general, if a C compiler supports certain PIC flags, then any derivative compilers support the same flags. Until there are some noteworthy exceptions to this rule, this section will document only C compilers.
The following C compilers have standard command line options, regardless of the platform:
gcc
The rest of this subsection lists compilers by the operating system that they are bundled with:
aix3*
aix4*
hpux10*
osf3*
solaris2*
sunos4*
On all known systems, a reloadable object can be created by running ld -r -o output.o input1.o input2.o. This reloadable object may be treated as exactly equivalent to other objects.
On most modern platforms the order that dependent libraries are listed has no effect on object generation. In theory, there are platforms which require libraries which provide missing symbols to other libraries to listed after those libraries whose symbols they provide.
Particularly, if a pair of static archives each resolve some of the other's symbols, it might be necessary to list one of those archives both before and after the other one. Libtool does not currently cope with this situation well, since dupicate libraries are removed from thr link line.
If you find yourself developing on a host that requires you to list libraries multiple times in order for it to generate correctly linked objects, you can defeat libtool's removal algorithm like this:
$ libtool ... -lfoo -lbar -Wl,-lfoo
On all known systems, building a static library can be accomplished by running ar cru libname.a obj1.o obj2.o ..., where the `.a' file is the output library, and each `.o' file is an object file.
On all known systems, if there is a program named ranlib, then it
must be used to "bless" the created library before linking against it,
with the ranlib libname.a command. Some systems, like Irix,
use the ar ts command, instead.
libtool script contents
Since version 1.4, the libtool script is generated by
configure (see section Configuring libtool). In earlier versions,
configure achieved this by calling a helper script called
`ltconfig'. From libtool version 0.7 to 1.0, this script
simply set shell variables, then sourced the libtool backend,
ltmain.sh. ltconfig from libtool version 1.1 through 1.3
inlined the contents of ltmain.sh into the generated
libtool, which improved performance on many systems. The tests
that `ltconfig' used to perform are now kept in `libtool.m4'
where thay can be written using Autoconf. This has the runtime
performance benefits of inlined ltmain.sh, and improves
the build time a little while considerably easing the amount of raw
shell code that used to need maintaining.
The convention used for naming variables which hold shell commands for
delayed evaluation, is to use the suffix _cmd where a single
line of valid shell script is needed, and the suffix _cmds where
multiple lines of shell script may be delayed for later
evaluation. By convention, _cmds variables delimit the
evaluation units with the ~ character where necessary.
Here is a listing of each of the configuration variables, and how they
are used within ltmain.sh (see section Configuring libtool):
nm program, which produces listings
of global symbols in one the following formats:
address C global-variable-name address D global-variable-name address T global-function-name
-c and -o options
simultaneously. Set to `yes' or `no'.
dlopen is supported on the platform.
Set to `yes' or `no'.
dlopen the executable itself.
Set to `yes' or `no'.
dlopen the executable itself, when it
is linked statically (`-all-static'). Set to `yes' or
`no'.
echo program which does not interpret backslashes as an
escape character.
$ eval "$NM progname | $global_symbol_pipe" D symbol1 C-symbol1 T symbol2 C-symbol2 C symbol3 C-symbol3 ... $
The first column contains the symbol type (used to tell data from code on some platforms), but its meaning is system dependent.
dlopen and to
link against a library without 'lib' prefix,
i.e. it requires hardcode_direct to be `yes'.
char.
fast_install. The default value is `unknown', which is
equivalent to `no'.
striplib) or static (old_striplib)
library, respectively. If these variables are empty, the strip flag
in the install mode will be ignored for libraries (see section Install mode).
-L also augment the search path.
${wl}some-flag.
Variables ending in `_cmds' or `_eval' contain a
`~'-separated list of commands that are evaled one after
another. If any of the commands return a nonzero exit status, libtool
generally exits with an error message.
Variables ending in `_spec' are evaled before being used by
libtool.
Here are a few tricks that you can use in order to make maintainership easier:
ltmain.in, I keep a permanent libtool script in my
PATH, which sources ltmain.in directly.
The following steps describe how to create such a script, where
/home/src/libtool is the directory containing the libtool source
tree, /home/src/libtool/libtool is a libtool script that has been
configured for your platform, and ~/bin is a directory in your
PATH:
trick$ cd ~/bin trick$ sed '/^# ltmain\.sh/q' /home/src/libtool/libtool > libtool trick$ echo '. /home/src/libtool/ltmain.in' >> libtool trick$ chmod +x libtool trick$ libtool --version ltmain.sh (GNU @PACKAGE@) @VERSION@@TIMESTAMP@ trick$
The output of the final `libtool --version' command shows that the
ltmain.in script is being used directly. Now, modify
~/bin/libtool or /home/src/libtool/ltmain.in directly in
order to test new changes without having to rerun configure.
Version 1.1, March 2000
Copyright (C) 2000 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other written document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
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To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (c) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. A copy of the license is included in the section entitled "GNU Free Documentation License".
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If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
If you don't
specify an rpath, then libtool builds a libtool convenience
archive, not a shared library (see section Linking static libraries).
However, you should avoid using `-L' or `-l' flags to link against an uninstalled libtool library. Just specify the relative path to the `.la' file, such as `../intl/libintl.la'. This is a design decision to eliminate any ambiguity when linking against uninstalled shared libraries.
Don't accidentally strip the libraries, though, or they will be unusable.
AC_PROG_LIBTOOL requires that
you define the `Makefile' variable top_builddir in your
`Makefile.in'. Automake does this automatically, but Autoconf
users should set it to the relative path to the top of your build
directory (`../..', for example).
GNU Image
Manipulation Program, for those who haven't taken the plunge. See
http://www.gimp.org/.
We used to recommend __P,
__BEGIN_DECLS and __END_DECLS. This was bad advice since
symbols (even preprocessor macro names) that begin with an underscore
are reserved for the use of the compiler.
HP-UX,
to be different, uses a function named shl_load.
LIBPATH
on AIX, and SHLIB_PATH on HP-UX.
This is used for
the host dependent module loading API -- shl_load and
LoadLibrary for example
Even if libltdl is installed, `AC_LIBLTDL_INSTALLABLE' may fail to detect it, if libltdl depends on symbols provided by libraries other than the C library. In this case, it will needlessly build and install libltdl.
All code compiled for the PowerPC
and RS/6000 chips (powerpc-*-*, powerpcle-*-*, and
rs6000-*-*) is position-independent, regardless of the operating
system or compiler suite. So, "regular objects" can be used to build
shared libraries on these systems and no special PIC compiler flags are
required.
This document was generated on 7 October 2001 using the texi2html translator version 1.54.