This is libtool.info, produced by makeinfo version 3.12k from libtool.texi. INFO-DIR-SECTION GNU programming tools START-INFO-DIR-ENTRY * Libtool: (libtool). Generic shared library support script. END-INFO-DIR-ENTRY INFO-DIR-SECTION Individual utilities START-INFO-DIR-ENTRY * libtoolize: (libtool)Invoking libtoolize. Adding libtool support. END-INFO-DIR-ENTRY This file documents GNU Libtool 1.3.4 Copyright (C) 1996-1999 Free Software Foundation, Inc. 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 Foundation.  File: libtool.info, Node: Using Automake, Next: Configuring, Prev: Makefile rules, Up: Integrating libtool Using Automake with libtool =========================== 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' (*note 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: * add them to PROGRAM_LDFLAGS, and list the libraries in PROGRAM_DEPENDENCIES, then wait for a release of GNU automake that accepts these flags where they belong; * surround the flags between quotes, but then you must set PROGRAM_DEPENDENCIES too: program_LDADD = "-dlopen" libfoo.la program_DEPENDENCIES = libfoo.la * set and `AC_SUBST' variables DLOPEN and DLPREOPEN in `configure.in' and use `@DLOPEN@' and `@DLPREOPEN@' as replacements for the explicit flags `-dlopen' and `-dlpreopen' in `program_LDADD'. Automake will discard `AC_SUBST'ed variables from dependencies, so it will behave exactly as we expect it to behave when it accepts these flags in `program_LDADD'. But hey!, this is ugly! 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' (*note Versioning::) 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. *Note Building a Shared Library: (automake)A Shared Library, for more information.  File: libtool.info, Node: Configuring, Next: Distributing, Prev: Using Automake, Up: Integrating libtool Configuring libtool =================== 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 (*note Distributing::), 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'. * Menu: * Invoking ltconfig:: `ltconfig' command line options. * ltconfig example:: Manually configuring a `libtool'. * AM_PROG_LIBTOOL:: Configuring `libtool' in `configure.in'.  File: libtool.info, Node: Invoking ltconfig, Next: ltconfig example, Up: Configuring Invoking `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: `--debug' Dump a trace of shell script execution to standard output. This produces a lot of output, so you may wish to pipe it to `less' (or `more') or redirect to a file. `--disable-shared' Create a `libtool' that only builds static libraries. `--disable-static' Create a `libtool' that builds only shared libraries if they are available. If only static libraries can be built, then this flag has no effect. `--disable-fast-install' On platforms in which installable executables, that are created by default, are not suitable for execution in the build directory, create a `libtool' that links executables that search for uninstalled libraries by default, and relinks them at install time. It is ignored on platforms in which a single executable is enough. `--enable-dlopen' Test whether some dlopening mechanism is supported. If this flag is not given, or no working dlopening mechanism is found, create a `libtool' that performs dlpreopening of all dlopened modules. `--help' Display a help message and exit. `--no-verify' Do not use `config.sub' to verify that HOST is a valid canonical host system name. `--output=FILE' `-o FILE' Instead of creating a libtool script called `libtool', create one called FILE. This can be useful if you want to create libtool scripts for cross-compilers, or you want to have more than one libtool in the same directory. `--quiet' `--silent' Do not print informational messages when running configuration tests. `--srcdir=DIR' Look for `config.guess' and `config.sub' in DIR. `--version' Print `ltconfig' version information and exit. `--with-gcc' Assume that the GNU C compiler will be used when invoking the created `libtool' to compile and link object files. `--with-gnu-ld' Assume that the C compiler uses the GNU linker. `--disable-lock' Create a `libtool' that does not perform locking to ensure proper parallel compilation if the C compiler does not support `-c' and `-o' together. `--cache-file=FILE' Use this FILE as a cache for results of a few tests. This is usually `config.cache' used by `configure'. By default, no cache file is used. LTMAIN is the `ltmain.sh' shell script fragment that provides the basic libtool functionality (*note Distributing::). HOST is the canonical host system name, which by default is guessed by running `config.guess'. `ltconfig' also recognizes the following environment variables: - Variable: CC The C compiler that will be used by the generated `libtool'. If this is not set, `ltconfig' will look for `gcc' or `cc'. - Variable: CFLAGS Compiler flags used to generate standard object files. If this is not set, `ltconfig' will not use any such flags. It affects only the way `ltconfig' runs tests, not the produced `libtool'. - Variable: CPPFLAGS C preprocessor flags. If this is not set, `ltconfig' will not use any such flags. It affects only the way `ltconfig' runs tests, not the produced `libtool'. - Variable: LD The system linker to use (if the generated `libtool' requires one). If this is not set, `ltconfig' will try to find out what is the linker used by CC. - Variable: LDFLAGS The flags to be used by `ltconfig' when it links a program. If this is not set, `ltconfig' will not use any such flags. It affects only the way `ltconfig' runs tests, not the produced `libtool'. - Variable: LIBS The libraries to be used by `ltconfig' when it links a program. If this is not set, `ltconfig' will not use any such flags. It affects only the way `ltconfig' runs tests, not the produced `libtool'. - Variable: NM Program to use rather than checking for `nm'. - Variable: RANLIB Program to use rather than checking for `ranlib'. - Variable: LN_S A command that creates a link of a program, a soft-link if possible, a hard-link otherwise. - Variable: DLLTOOL Program to use rather than checking for `dlltool'. Only meaningful for Cygwin/MS-Windows. - Variable: OBJDUMP Program to use rather than checking for `objdump'. Only meaningful for Cygwin/MS-Windows. - Variable: AS Program to use rather than checking for `as'. Only meaningful for Cygwin/MS-Windows.  File: libtool.info, Node: ltconfig example, Next: AM_PROG_LIBTOOL, Prev: Invoking ltconfig, Up: Configuring Using `ltconfig' ---------------- Here is a simple example of using `ltconfig' to configure libtool on a 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$  File: libtool.info, Node: AM_PROG_LIBTOOL, Prev: ltconfig example, Up: Configuring The `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': - Macro: AM_PROG_LIBTOOL Add support for the `--enable-shared' and `--disable-shared' `configure' flags. Invoke `ltconfig' with the correct arguments to configure the package (*note Invoking ltconfig::).(1) 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 AM_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': AM_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'. - Macro: AC_LIBTOOL_DLOPEN Enable checking for dlopen support. This macro should be used if the package makes use of the `-dlopen' and `-dlpreopen' flags, otherwise libtool will assume that the system does not support dlopening. The macro must be called *before* `AM_PROG_LIBTOOL'. - Macro: AC_LIBTOOL_WIN32_DLL This macro should be used if the package has been ported to build clean dlls on win32 platforms. Usually this means that any library data items are exported with `__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. `AM_PROG_LIBTOOL' must be called *after* this macro, and provision must be made to pass `-no-undefined' to `libtool' in link mode from the package `Makefile'. Naturally, passing `-no-undefined' means that all the library symbols *really are* defined at link time! - Macro: AC_DISABLE_FAST_INSTALL Change the default behaviour for `AM_PROG_LIBTOOL' to disable optimization for fast installation. The user may still override this default, depending on platform support, by specifying `--enable-fast-install'. - Macro: AC_DISABLE_SHARED - Macro: AM_DISABLE_SHARED Change the default behaviour for `AM_PROG_LIBTOOL' to disable shared libraries. The user may still override this default by specifying `--enable-shared'. - Macro: AC_DISABLE_STATIC - Macro: AM_DISABLE_STATIC Change the default behaviour for `AM_PROG_LIBTOOL' to disable static libraries. The user may still override this default by specifying `--enable-static'. When you invoke the `libtoolize' program (*note 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. 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. ---------- Footnotes ---------- (1) `AM_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).  File: libtool.info, Node: Distributing, Next: Static-only libraries, Prev: Configuring, Up: Integrating libtool Including libtool in your package ================================= In order to use libtool, you need to include the following files with your package: `config.guess' Attempt to guess a canonical system name. `config.sub' Canonical system name validation subroutine script. `ltconfig' Generate a libtool script for a given system. `ltmain.sh' A generic script implementing basic libtool functionality. Note that the libtool script itself should _not_ be included with your package. *Note Configuring::. You should use the `libtoolize' program, rather than manually copying these files into your package. * Menu: * Invoking libtoolize:: `libtoolize' command line options. * Autoconf .o macros:: Autoconf macros that set object file names.  File: libtool.info, Node: Invoking libtoolize, Next: Autoconf .o macros, Up: Distributing Invoking `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: `--automake' Work silently, and assume that Automake libtool support is used. `libtoolize --automake' is used by Automake to add libtool files to your package, when `AM_PROG_LIBTOOL' appears in your `configure.in'. `--copy' `-c' Copy files from the libtool data directory rather than creating symlinks. `--debug' Dump a trace of shell script execution to standard output. This produces a lot of output, so you may wish to pipe it to `less' (or `more') or redirect to a file. `--dry-run' `-n' Don't run any commands that modify the file system, just print them out. `--force' `-f' Replace existing libtool files. By default, `libtoolize' won't overwrite existing files. `--help' Display a help message and exit. `--ltdl' Install libltdl in a subdirectory of your package. `--ltdl-tar' Add the file libltdl.tar.gz to your package. `--version' Print `libtoolize' version information and exit. If `libtoolize' detects an explicit call to `AC_CONFIG_AUX_DIR' (*note The Autoconf Manual: (autoconf)Input) 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.  File: libtool.info, Node: Autoconf .o macros, Prev: Invoking libtoolize, Up: Distributing Autoconf `.o' macros -------------------- 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: - Variable: LTALLOCA Substituted by `AC_FUNC_ALLOCA' (*note Particular Function Checks: (autoconf)Particular Functions). Is either empty, or contains `alloca.lo'. - Variable: LTLIBOBJS Substituted by `AC_REPLACE_FUNCS' (*note Generic Function Checks: (autoconf)Generic Functions), 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(...)  File: libtool.info, Node: Static-only libraries, Prev: Distributing, Up: Integrating libtool Static-only libraries ===================== 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 (*note The `AM_PROG_LIBTOOL' macro: AM_PROG_LIBTOOL). This prevents libtool from building shared libraries, which has several advantages: * compilation is twice as fast, which can speed up your development cycle, * debugging is easier because you don't need to deal with any complexities added by shared libraries, and * you can see how libtool behaves on static-only platforms. 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(1) 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'. ---------- Footnotes ---------- (1) GNU Image Manipulation Program, for those who haven't taken the plunge. See .  File: libtool.info, Node: Versioning, Next: Library tips, Prev: Integrating libtool, Up: Top Library interface versions ************************** 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. * Menu: * Interfaces:: What are library interfaces? * Libtool versioning:: Libtool's versioning system. * Updating version info:: Changing version information before releases. * Release numbers:: Breaking binary compatibility for aesthetics.  File: libtool.info, Node: Interfaces, Next: Libtool versioning, Up: Versioning What are library interfaces? ============================ Interfaces for libraries may be any of the following (and more): * global variables: both names and types * global functions: argument types and number, return types, and function names * standard input, standard output, standard error, and file formats * sockets, pipes, and other inter-process communication protocol formats Note that static functions do not count as interfaces, because they are not directly available to the user of the library.  File: libtool.info, Node: Libtool versioning, Next: Updating version info, Prev: Interfaces, Up: Versioning Libtool's versioning system =========================== 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 The most recent interface number that this library implements. REVISION The implementation number of the CURRENT interface. AGE The difference between the newest and oldest interfaces that this library implements. In other words, the library implements all the interface numbers in the range from number `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.  File: libtool.info, Node: Updating version info, Next: Release numbers, Prev: Libtool versioning, Up: Versioning Updating library version information ==================================== 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 (*note 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: 1. Start with version information of `0:0:0' for each libtool library. 2. Update the version information only immediately before a public release of your software. More frequent updates are unnecessary, and only guarantee that the current interface number gets larger faster. 3. If the library source code has changed at all since the last update, then increment REVISION (`C:R:A' becomes `C:r+1:A'). 4. If any interfaces have been added, removed, or changed since the last update, increment CURRENT, and set REVISION to 0. 5. If any interfaces have been added since the last public release, then increment AGE. 6. If any interfaces have been removed since the last public release, then set AGE to 0. *_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 (*note Release numbers::), but be warned that every release of your package will not be binary compatible with any other release.  File: libtool.info, Node: Release numbers, Prev: Updating version info, Up: Versioning Managing release information ============================ 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.  File: libtool.info, Node: Library tips, Next: Inter-library dependencies, Prev: Versioning, Up: Top Tips for interface design ************************* 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: Plan ahead Try to make every interface truly minimal, so that you won't need to delete entry points very often. Avoid interface changes Some people love redesigning and changing entry points just for the heck of it (note: _renaming_ a function is considered changing an entry point). Don't be one of those people. If you must redesign an interface, then try to leave compatibility functions behind so that users don't need to rewrite their existing code. Use opaque data types The fewer data type definitions a library user has access to, the better. If possible, design your functions to accept a generic pointer (which you can cast to an internal data type), and provide access functions rather than allowing the library user to directly manipulate the data. That way, you have the freedom to change the data structures without changing the interface. This is essentially the same thing as using abstract data types and inheritance in an object-oriented system. Use header files If you are careful to document each of your library's global functions and variables in header files, and include them in your library source files, then the compiler will let you know if you make any interface changes by accident (*note C header files::). Use the `static' keyword (or equivalent) whenever possible The fewer global functions your library has, the more flexibility you'll have in changing them. Static functions and variables may change forms as often as you like... your users cannot access them, so they aren't interface changes. * Menu: * C header files:: How to write portable include files.  File: libtool.info, Node: C header files, Up: Library tips Writing C header files ====================== Writing portable C header files can be difficult, since they may be read by different types of compilers: C++ compilers C++ compilers require that functions be declared with full prototypes, since C++ is more strongly typed than C. C functions and variables also need to be declared with the `extern "C"' directive, so that the names aren't mangled. *Note C++ libraries::, for other issues relevant to using C++ with libtool. ANSI C compilers ANSI C compilers are not as strict as C++ compilers, but functions should be prototyped to avoid unnecessary warnings when the header file is `#include'd. non-ANSI C compilers Non-ANSI compilers will report errors if functions are prototyped. 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: * Pre-ANSI compilers do not always support the `void *' generic pointer type, and so need to use `char *' in its place. * The `const' and `signed' keywords are not supported by some compilers, especially pre-ANSI compilers. * The `long double' type is not supported by many compilers.  File: libtool.info, Node: Inter-library dependencies, Next: Dlopened modules, Prev: Library tips, Up: Top Inter-library dependencies ************************** 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.(1) 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. ---------- Footnotes ---------- (1) Unfortunately, as of libtool version 1.3.4, there is no way to specify inter-library dependencies on libtool libraries that have not yet been installed. Libtool 1.4 will support this feature.  File: libtool.info, Node: Dlopened modules, Next: Using libltdl, Prev: Inter-library dependencies, Up: Top Dlopened modules **************** It can sometimes be confusing to discuss "dynamic linking", because the term is used to refer to two different concepts: 1. Compiling and linking a program against a shared library, which is resolved automatically at run time by the dynamic linker. In this process, dynamic linking is transparent to the application. 2. The application calling functions such as `dlopen',(1) 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 1.3.4, 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_ `AM_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. * Menu: * Building modules:: Creating dlopenable objects and libraries. * Dlpreopening:: Dlopening that works on static platforms. * Finding the dlname:: Choosing the right file to `dlopen'. * Dlopen issues:: Unresolved problems that need your attention. ---------- Footnotes ---------- (1) HP-UX, to be different, uses a function named `shl_load'.  File: libtool.info, Node: Building modules, Next: Dlpreopening, Up: Dlopened modules Building modules to dlopen ========================== 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 (*note 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$  File: libtool.info, Node: Dlpreopening, Next: Finding the dlname, Prev: Building modules, Up: Dlopened modules Dlpreopening ============ 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": 1. Linking against object files that become part of the program executable, whether or not they are referenced. If an object file cannot be found, then the linker refuses to create the executable. 2. Declaring a static library to the linker, so that it is searched at link time in order to satisfy any undefined references in the above object files. If the static library cannot be found, then the linker refuses to link the executable. 3. Declaring a shared library to the runtime linker, so that it is searched at runtime in order to satisfy any undefined references in the above files. If the shared library cannot be found, then the dynamic linker aborts the program before it runs. 4. Dlopening a module, so that the application can resolve its own, dynamically-computed references. If there is an error opening the module, or the module is not found, then the application can recover without crashing. 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 (*note Link mode::). - Structure: struct lt_dlsymlist { const char *NAME; lt_ptr_t ADDRESS; } The NAME attribute is a null-terminated character string of the symbol name, such as `"fprintf"'. The ADDRESS attribute is a generic pointer to the appropriate object, such as `&fprintf'. - Variable: const lt_dlsymlist * lt_preloaded_symbols An array of LT_SYMBOL structures, representing all the preloaded symbols linked into the program. For each `-dlpreloaded' file there is an element with the NAME of the file and a ADDRESS of `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.  File: libtool.info, Node: Finding the dlname, Next: Dlopen issues, Prev: Dlpreopening, Up: Dlopened modules Finding the correct name to dlopen ================================== 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'(1) 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. ---------- Footnotes ---------- (1) `LIBPATH' on AIX, and `SHLIB_PATH' on HP-UX.  File: libtool.info, Node: Dlopen issues, Prev: Finding the dlname, Up: Dlopened modules Unresolved dlopen issues ======================== The following problems are not solved by using libtool's dlopen support: * Dlopen functions are generally only available on shared library platforms. If you want your package to be portable to static platforms, you have to use either libltdl (*note Using libltdl::) or develop your own alternatives to dlopening dynamic code. Most reasonable solutions involve writing wrapper functions for the `dlopen' family, which do package-specific tricks when dlopening is unsupported or not available on a given platform. * There are major differences in implementations of the `dlopen' family of functions. Some platforms do not even use the same function names (notably HP-UX, with its `shl_load' family). * The application developer must write a custom search function in order to discover the correct module filename to supply to `dlopen'.  File: libtool.info, Node: Using libltdl, Next: Other languages, Prev: Dlopened modules, Up: Top Using libltdl ************* 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) * GNU DLD (emulates dynamic linking for static libraries) * libtool's dlpreopen (see *note Dlpreopening::) libltdl is licensed under the terms of the GNU Library General Public License, with the following exception: As a special exception to the GNU Library General Public License, if you distribute this file as part of a program that uses GNU libtool to create libraries and programs, you may include it under the same distribution terms that you use for the rest of that program. * Menu: * Libltdl interface:: How to use libltdl in your programs. * Modules for libltdl:: Creating modules that can be `dlopen'ed. * Distributing libltdl:: How to distribute libltdl with your package.