@dircategory GNU programming tools @direntry * Libtool: (libtool). Generic shared library support script.
@dircategory Individual utilities @direntry * libtoolize: (libtool)Invoking libtoolize. Adding libtool support.
Copyright (C) 1996-1998 Free Software Foundation, Inc.
This is the first edition of the GNU Libtool documentation,
and is consistent with GNU Libtool 1.1.
Published by the Free Software Foundation
59 Temple Place, Suite 330,
Boston, MA 02111-1307 USA
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Free Software Foundation.
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 I propose that 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:
ldconfig(8).
I have investigated several different implementations of systems that build shared libraries as part of a free software package. At first, I made notes on the features of each of these packages for comparison purposes.
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 I 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 for me to use (much less modify) without understanding exactly what the implementation does, and they were generally not documented.
I think the main problem is that different vendors have different views of what libraries are, and none of the packages I 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, I don't have the time or power to convince operating system developers to mend their evil ways, and I want to build shared libraries right now, even on buggy, broken, confused operating systems.
For this reason, I have designed libtool 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.
I hope that libtool will be useful to and used by the GNU community, and that the lessons I've learned in writing it will be taken up and implemented by designers of library systems.
At first, libtool was designed to support an arbitrary number of library object types. After porting libtool to more platforms, I discovered a new paradigm 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(3) math library
function, which is usually found in the standalone math library, and not
the C library. 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 ln -s foo.o foo.lo a23$ libtool gcc -g -O -c hello.c gcc -g -O -c hello.c ln -s hello.o hello.lo a23$
Note that libtool creates two object files for each invocation. The `.lo' file is a library object, and the `.o' file is a standard object file. On `a23', these files are identical, 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 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 burger$
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):
burger$ libtool gcc -g -O -o libhello.la foo.o hello.o
libtool: cannot build libtool library `libhello.la' from non-libtool \
objects
burger$
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:(1)
a23$ libtool gcc -g -O -o libhello.la foo.lo hello.lo -lm libtool: you must specify an installation directory with `-rpath' a23$
Argh. Another complication in building shared libraries is that we need
to specify the path to the directory in which they (eventually) will be
installed. So, we try again, with an rpath setting of
`/usr/local/lib':
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$
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 it the libtool wrapper script makes it possible to run the program directly, but unfortunately, it 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 absolutely no warranty for GDB; type "show warranty" for details. GDB 4.16 (i386-unknown-netbsd), Copyright 1996 Free Software Foundation, Inc... "hell": not in executable format: File format not recognized (gdb) quit burger$
Sad. It doesn't work because GDB isn't 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 absolutely no warranty for GDB; type "show warranty" for details. GDB 4.16 (i386-unknown-netbsd), Copyright 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 absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-netbsd), Copyright 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, for informational purposes, and to help libtool with uninstallation (see section Uninstall mode).
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 ldconfig -m /usr/local/lib To link against installed libraries in LIBDIR, users may have to: - add LIBDIR to their `LD_LIBRARY_PATH' environment variable - use the `-LLIBDIR' linker flag 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 "convenience library" that is a collection of related object
files without a really nice interface.
To do this, you should ignore libtool entirely, and just use the old
ar and ranlib commands to create a static library.
If you want to install the library (but you probably don't), then you may use libtool if you want:
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.
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:
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 C source code suffix `.c' with the library object suffix, `.lo'.
If shared libraries are being built, any necessary PIC generation flags are substituted into the compilation command.
Note that the `-o' option is not supported for compile mode, because it cannot be implemented properly for all platforms. It is far easier just to change your Makefiles to create all the output files in the current working directory.
`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:
dlsym(3)
(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.
This mode deletes installed libraries (and other files).
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
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, 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 with 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 has its own equivalent to the configure script,
ltconfig.
ltconfig
ltconfig runs a series of configuration tests, then creates a
system-specific libtool in the current directory. The
ltconfig program has the following synopsis:
ltconfig [option]... ltmain [host]
and accepts the following options:
libtool that only builds static libraries.
libtool that builds only shared libraries if they are
available. If only static libraries can be built, then this flag has
no effect.
config.sub to verify that host is a valid
canonical host system name.
config.guess and config.sub in dir.
ltconfig version information and exit.
libtool to compile and link object files.
ltmain is the ltmain.sh shell script fragment that provides
the basic libtool functionality (see section Including libtool with your package).
host is the canonical host system name, which by default is
guessed by running config.guess.
ltconfig also recognizes the following environment variables:
libtool.
libtool requires one).
ranlib.
ltconfig
Here is a simple example of using ltconfig to configure libtool
on my NetBSD/i386 1.2 system:
burger$ ./ltconfig ltmain.sh checking host system type... i386-unknown-netbsd1.2 checking for ranlib... ranlib checking for gcc... gcc checking whether we are using GNU C... yes checking for gcc option to produce PIC... -fPIC -DPIC checking for gcc option to statically link programs... -static checking if ld is GNU ld... no checking if ld supports shared libraries... yes checking dynamic linker characteristics... netbsd1.2 ld.so checking if libtool supports shared libraries... yes checking whether to build shared libraries... yes creating libtool burger$
This example shows how to configure libtool for cross-compiling
to a i486 GNU/Hurd 0.1 system (assuming compiler tools reside in
`/local/i486-gnu/bin'):
burger$ export PATH=/local/i486-gnu/bin:$PATH burger$ ./ltconfig ltmain.sh i486-gnu0.1 checking host system type... i486-unknown-gnu0.1 checking for ranlib... ranlib checking for gcc... gcc checking whether we are using GNU C... yes checking for gcc option to produce PIC... -fPIC -DPIC checking for gcc option to statically link programs... -static checking if ld is GNU ld... yes checking if GNU ld supports shared libraries... yes checking dynamic linker characteristics... gnu0.1 ld.so checking if libtool supports shared libraries... yes checking whether to build shared libraries... yes creating libtool burger$
AM_PROG_LIBTOOL macro
If you are using GNU Autoconf (or Automake), you should add a call to
AM_PROG_LIBTOOL to your `configure.in' file. This macro
offers seamless integration between the configure script and
ltconfig:
configure flags. Invoke ltconfig with the correct
arguments to configure the package (see section Invoking ltconfig).(4)
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 setting calling either the
AM_DISABLE_SHARED or AM_DISABLE_STATIC macros:
# Turn off shared libraries during beta-testing, since they make the # build process take too long. AM_DISABLE_SHARED AM_PROG_LIBTOOL
The user may specify a modified form of `--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 specifying `--enable-shared' to every package named in the pkgs list, and `--disable-shared' to every other package. The `--enable-static=pkgs' flag behaves similarly, except it translates into `--enable-static' and `--disable-static'.
The package name `default' matches any packages which have not set
their name in the PACKAGE environment variable.
AM_PROG_LIBTOOL to disable
shared libraries. The user may still override this default by
specifying `--enable-shared'.
AM_PROG_LIBTOOL to disable
static libraries. The user may still override this default by
specifying `--enable-static'.
When you invoke the libtoolize program (see section Invoking libtoolize), it will tell you where to find a definition of
AM_PROG_LIBTOOL. If you use Automake, the aclocal program
will automatically add AM_PROG_LIBTOOL support to your
configure script.
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:
AM_PROG_LIBTOOL appears in your
`configure.in'.
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 most recent version of Autoconf (2.12, 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 "$LIBOBJS" | sed 's/\.o/.lo/g'` AC_SUBST(LTLIBOBJS) LTALLOCA=`echo "$ALLOCA" | sed 's/\.o/.lo/g'` 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 AM_PROG_LIBTOOL by using the AM_DISABLE_SHARED
Autoconf macro (see section The AM_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, I 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 very 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 compatibility 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 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 just a symbolic link to `/usr/lib/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 accomodate 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 Linux:
trick$ ls /usr/lib/libbfd* /usr/lib/libbfd-2.9.0.so.0 /usr/lib/libbfd.so /usr/lib/libbfd-2.9.0.so.0.0.0 /usr/lib/libbfd.a trick$
In this case, `/usr/lib/libbfd.so' is a symbolic link to `/usr/lib/libbfd-2.9.0.so.0.0.0'. 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 actually causes a modification of the library name, so do not use it if unless you want to break binary compatibility with any past library releases. In general, you should only use `-release' for libraries 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_DECLS should be used at the beginning of your declarations,
so that C++ compilers don't mangle their names. Use __END_DECLS at
the end of C declarations. */
#undef __BEGIN_DECLS
#undef __END_DECLS
#ifdef __cplusplus
# define __BEGIN_DECLS extern "C" {
# define __END_DECLS }
#else
# define __BEGIN_DECLS /* empty */
# define __END_DECLS /* empty */
#endif
/* __P 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 __P
#if defined (__STDC__) || defined (_AIX) \
|| (defined (__mips) && defined (_SYSTYPE_SVR4)) \
|| defined(WIN32) || defined(__cplusplus)
# define __P(protos) protos
#else
# define __P(protos) ()
#endif
These macros are used in `foo.h' as follows:
#ifndef _FOO_H_ #define _FOO_H_ 1 /* The above macro definitions. */ ... __BEGIN_DECLS int foo __P((void)); int hello __P((void)); __END_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.
Feel free to copy the definitions of __P, __BEGIN_DECLS,
and __END_DECLS into your own headers. Then, you may use them to
create header files that are valid for C++, ANSI, and non-ANSI
compilers.
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 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(3) function, then it has an inter-library dependency
on `libm', the math library that implements cos(3).
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.(6) 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$
In order to link a program against `libhello', you need to specify the same `-l' options, 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 no references to external symbols), and you need to specify the -no-undefined flag to allow a shared library to be built. By default, libtool builds only static libraries on these kinds of platforms.
It can sometimes be confusing to discuss dynamic linking, because the term is used to refer to two different concepts:
dlopen(3),(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 1.1, libtool provides experimental support for dlopened modules, which does not radically simplify the development of dlopening applications. However, this support is designed to be a portable foundation for generic, higher-level dlopen functions.
This chapter discusses the preliminary support that libtool offers, and how you as a dlopen application developer might use libtool to generate dlopen-accessible modules. It is important to remember that these are experimental features, and not to rely on them for easy answers to the problems associated with dlopened modules.
On some operating systems, a program symbol must be specially declared
in order to be dynamically resolved with the dlsym(3) (or
equivalent) function.
Libtool provides the `-export-dynamic' link flag (see section Link mode), which does this declaration. You need to use this flag 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 `-export-dynamic' to the other link flags:
burger$ libtool gcc -export-dynamic -o libhello.la foo.lo \
hello.lo -rpath /usr/local/lib -lm
burger$
Another situation where you would use `-export-dynamic' is if symbols from your executable are needed to satisfy unresolved references in a library you want to dlopen. In this case, 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(3) and dlsym(3)
functions..
Consider the following alternative ways of loading code into your program, in order of increasing "laziness":
Libtool emulates `-export-dynamic' 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, which is &fprintf in
this example.
0.
-1, to indicate
that the application needs to sort and count dld_preloaded_symbols
itself, or search it linearly.
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 dld_preloaded_symbols.
After a library has been linked with `-export-dynamic', 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(3).
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(3) family, which do package-specific tricks when dlopening
is unsupported or not available on a given platform.
dlopen(3)
family of functions. Some platforms do not even use the same function
names (notably HP-UX, with its `shl_load(3)' family).
dlopen(3).
Each of these limitations will be addressed in GNU DLD 4.(9)
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 is a fairly straightforward process, and differs from C code in only two ways:
This second issue is very complex. Basically, 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.
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:
demo-conf.test
demo-exec.test
demo-inst.test
demo-make.test
demo-unst.test
hardcode.test
link.test
link-2.test
suffix.test
test-e.test
test -e construct is never
used in the libtool scripts. Checking for the existence of a file can
only be done in a portable way by using test -f.
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 mailing list @email{<bug-libtool@gnu.org>} 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 ltconfig --version).
Please include the generated libtool script with your bug report,
so that I can see what values ltconfig guessed for your system.
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.
To port libtool to a new system, you'll generally need the following information:
ld(1) and cc(1)
ld.so(8), rtld(8), or equivalent
ldconfig(8), or equivalent
This table describes when libtool was last known to be tested on platforms where it claims to support shared libraries:
--------------------------------------------------------
canonical host name compiler libtool results
release
--------------------------------------------------------
alpha-dec-osf3.2 cc 0.8 ok
alpha-dec-osf3.2 gcc 0.8 ok
alpha-dec-osf4.0 cc 1.0f ok
alpha-dec-osf4.0 gcc 1.0f ok
alpha-unknown-linux gcc 0.9h ok
hppa1.1-hp-hpux9.07 cc 1.0f ok
hppa1.1-hp-hpux9.07 gcc 1.0f ok
hppa1.1-hp-hpux10.10 cc 0.9h ok
hppa1.1-hp-hpux10.10 gcc 0.9h ok
i386-unknown-freebsd2.1.5 gcc 0.5 ok
i386-unknown-gnu0.0 gcc 0.5 ok
i386-unknown-netbsd1.2 gcc 0.9g ok
i586-pc-linux-gnulibc1 gcc 1.0i ok
i586-pc-linux-gnu gcc 1.0i ok
mips-sgi-irix5.2 gcc 1.0i ok
mips-sgi-irix5.3 cc 0.8 ok
mips-sgi-irix5.3 gcc 0.8 ok
mips-sgi-irix6.2 cc 0.9 ok
mips-sgi-irix6.3 cc 1.0f ok
mips-sgi-irix6.3 gcc 1.0i ok
mips-sgi-irix6.3 irix5-gcc 1.0f ok
mipsel-unknown-openbsd2.1 gcc 1.0 ok
powerpc-ibm-aix4.1.4.0 xlc 1.0i ok
powerpc-ibm-aix4.1.4.0 gcc 1.0 ok
rs6000-ibm-aix3.2.5 xlc 1.0i ok
rs6000-ibm-aix3.2.5 gcc 1.0i ok*
sparc-sun-linux2.1.23 gcc 0.9h ok
sparc-sun-sunos4.1.3 gcc 1.0i ok
sparc-sun-sunos4.1.4 cc 1.0f ok
sparc-sun-sunos4.1.4 gcc 1.0f ok
sparc-sun-solaris2.4 cc 1.0a ok
sparc-sun-solaris2.4 gcc 1.0a ok
sparc-sun-solaris2.5 cc 1.0f ok
sparc-sun-solaris2.5 gcc 1.0i ok
sparc-sun-solaris2.6 gcc 1.0i ok
--------------------------------------------------------
* Some versions of GCC's collect2 linker program cannot link trivial
static binaries on AIX 3. For these configurations, libtool's `-static'
flag has no effect.
This section is dedicated to the sanity of the libtool maintainer. 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 is very difficult to understand just by reading it from top to bottom. This section helps show why libtool does things a certain way. After reading it, then reading 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:
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 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.
libtool script contents
The libtool script is generated by ltconfig
(see section Configuring libtool). Ever since libtool version 0.7, this script
simply sets shell variables, then sources the libtool backend,
ltmain.sh.
Here is a listing of each of these variables, and how they are used
within ltmain.sh:
ltconfig script, to
prevent mismatches between the configuration information in
libtool, and how that information is used in ltmain.sh.
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
echo(1) program which does not interpret backslashes as an
escape character.
$ $NM | $global_symbol_pipe symbol1 C-symbol1 symbol2 C-symbol2 symbol3 C-symbol3 ... $
char.
Variables ending in `_cmds' or `_eval' contain a
semicolon-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.
This document was generated on 1 June 1998 using the texi2html translator version 1.51.