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Building Programs and Libraries

A large part of Automake's functionality is dedicated to making it easy to build programs and libraries.

Building a program

Introductory blathering

In a directory containing source that gets built into a program (as opposed to a library), the `PROGRAMS' primary is used. Programs can be installed in bindir, sbindir, libexecdir, pkglibdir, or not at all (`noinst'). They can also be built only for make check, in which case the prefix is `check'.

For instance:

bin_PROGRAMS = hello

In this simple case, the resulting `' will contain code to generate a program named hello.

Associated with each program are several assisting variables which are named after the program. These variables are all optional, and have reasonable defaults. Each variable, its use, and default is spelled out below; we use the "hello" example throughout.

The variable hello_SOURCES is used to specify which source files get built into an executable:

hello_SOURCES = hello.c version.c getopt.c getopt1.c getopt.h system.h

This causes each mentioned `.c' file to be compiled into the corresponding `.o'. Then all are linked to produce `hello'.

If `hello_SOURCES' is not specified, then it defaults to the single file `hello.c'; that is, the default is to compile a single C file whose base name is the name of the program itself. (This is a terrible default but we are stuck with it for historical reasons.)

Multiple programs can be built in a single directory. Multiple programs can share a single source file, which must be listed in each `_SOURCES' definition.

Header files listed in a `_SOURCES' definition will be included in the distribution but otherwise ignored. In case it isn't obvious, you should not include the header file generated by `configure' in a `_SOURCES' variable; this file should not be distributed. Lex (`.l') and Yacc (`.y') files can also be listed; see section Yacc and Lex support.

Conditional compilations

You can't put a configure substitution (e.g., `@FOO@') into a `_SOURCES' variable. The reason for this is a bit hard to explain, but suffice to say that it simply won't work. Automake will give an error if you try to do this.

Automake must know all the source files that could possibly go into a program, even if not all the files are built in every circumstance. Any files which are only conditionally built should be listed in the appropriate `EXTRA_' variable. For instance, if `hello-linux.c' were conditionally included in hello, the `' would contain:

EXTRA_hello_SOURCES = hello-linux.c

In this case, `hello-linux.o' would be added, via a `configure' substitution, to hello_LDADD in order to cause it to be built and linked in.

An often simpler way to compile source files conditionally is to use Automake conditionals. For instance, you could use this construct to conditionally use `hello-linux.c' or `hello-generic.c' as the basis for your program `hello':

hello_SOURCES = hello-linux.c
hello_SOURCES = hello-generic.c

When using conditionals like this you don't need to use the `EXTRA_' variable, because Automake will examine the contents of each variable to construct the complete list of source files.

Sometimes it is useful to determine the programs that are to be built at configure time. For instance, GNU cpio only builds mt and rmt under special circumstances.

In this case, you must notify Automake of all the programs that can possibly be built, but at the same time cause the generated `' to use the programs specified by configure. This is done by having configure substitute values into each `_PROGRAMS' definition, while listing all optionally built programs in EXTRA_PROGRAMS.

Of course you can use Automake conditionals to determine the programs to be built.

Linking the program

If you need to link against libraries that are not found by configure, you can use LDADD to do so. This variable actually can be used to add any options to the linker command line.

Sometimes, multiple programs are built in one directory but do not share the same link-time requirements. In this case, you can use the `prog_LDADD' variable (where prog is the name of the program as it appears in some `_PROGRAMS' variable, and usually written in lowercase) to override the global LDADD. If this variable exists for a given program, then that program is not linked using LDADD.

For instance, in GNU cpio, pax, cpio and mt are linked against the library `libcpio.a'. However, rmt is built in the same directory, and has no such link requirement. Also, mt and rmt are only built on certain architectures. Here is what cpio's `src/' looks like (abridged):

bin_PROGRAMS = cpio pax @MT@
libexec_PROGRAMS = @RMT@

LDADD = ../lib/libcpio.a @INTLLIBS@
rmt_LDADD =

cpio_SOURCES = ...
pax_SOURCES = ...
mt_SOURCES = ...
rmt_SOURCES = ...

`prog_LDADD' is inappropriate for passing program-specific linker flags (except for `-l', `-L', `-dlopen' and `-dlpreopen'). So, use the `prog_LDFLAGS' variable for this purpose.

It is also occasionally useful to have a program depend on some other target which is not actually part of that program. This can be done using the `prog_DEPENDENCIES' variable. Each program depends on the contents of such a variable, but no further interpretation is done.

If `prog_DEPENDENCIES' is not supplied, it is computed by Automake. The automatically-assigned value is the contents of `prog_LDADD', with most configure substitutions, `-l', `-L', `-dlopen' and `-dlpreopen' options removed. The configure substitutions that are left in are only `@LIBOBJS@' and `@ALLOCA@'; these are left because it is known that they will not cause an invalid value for `prog_DEPENDENCIES' to be generated.

Building a library

Building a library is much like building a program. In this case, the name of the primary is `LIBRARIES'. Libraries can be installed in libdir or pkglibdir.

See section Building a Shared Library, for information on how to build shared libraries using Libtool and the `LTLIBRARIES' primary.

Each `_LIBRARIES' variable is a list of the libraries to be built. For instance to create a library named `libcpio.a', but not install it, you would write:

noinst_LIBRARIES = libcpio.a

The sources that go into a library are determined exactly as they are for programs, via the `_SOURCES' variables. Note that the library name is canonicalized (see section How derived variables are named), so the `_SOURCES' variable corresponding to `liblob.a' is `liblob_a_SOURCES', not `liblob.a_SOURCES'.

Extra objects can be added to a library using the `library_LIBADD' variable. This should be used for objects determined by configure. Again from cpio:


In addition, sources for extra objects that will not exist until configure-time must be added to the BUILT_SOURCES variable (see section Built sources).

Building a Shared Library

Building shared libraries is a relatively complex matter. For this reason, GNU Libtool (see section `Introduction' in The Libtool Manual) was created to help build shared libraries in a platform-independent way.

Automake uses Libtool to build libraries declared with the `LTLIBRARIES' primary. Each `_LTLIBRARIES' variable is a list of shared libraries to build. For instance, to create a library named `libgettext.a' and its corresponding shared libraries, and install them in `libdir', write:


Note that shared libraries must be installed, so check_LTLIBRARIES is not allowed. However, noinst_LTLIBRARIES is allowed. This feature should be used for libtool "convenience libraries".

For each library, the `library_LIBADD' variable contains the names of extra libtool objects (`.lo' files) to add to the shared library. The `library_LDFLAGS' variable contains any additional libtool flags, such as `-version-info' or `-static'.

Where an ordinary library might include @LIBOBJS@, a libtool library must use @LTLIBOBJS@. This is required because the object files that libtool operates on do not necessarily end in `.o'. The libtool manual contains more details on this topic.

For libraries installed in some directory, Automake will automatically supply the appropriate `-rpath' option. However, for libraries determined at configure time (and thus mentioned in EXTRA_LTLIBRARIES), Automake does not know the eventual installation directory; for such libraries you must add the `-rpath' option to the appropriate `_LDFLAGS' variable by hand.

Ordinarily, Automake requires that a shared library's name start with `lib'. However, if you are building a dynamically loadable module then you might wish to use a "nonstandard" name. In this case, put -module into the `_LDFLAGS' variable.

See section `The Libtool Manual' in The Libtool Manual, for more information.

Program and Library Variables

Associated with each program are a collection of variables which can be used to modify how that program is built. There is a similar list of such variables for each library. The canonical name of the program (or library) is used as a base for naming these variables.

In the list below, we use the name "maude" to refer to the program or library. In your `' you would replace this with the canonical name of your program. This list also refers to "maude" as a program, but in general the same rules apply for both static and dynamic libraries; the documentation below notes situations where programs and libraries differ.

This variable, if it exists, lists all the source files which are compiled to build the program. These files are added to the distribution by default. When building the program, Automake will cause each source file to be compiled to a single `.o' file (or `.lo' when using libtool). Normally these object files are named after the source file, but other factors can change this. If a file in the `_SOURCES' variable has an unrecognized extension, Automake will do one of two things with it. If a suffix rule exists for turning files with the unrecognized extension into `.o' files, then automake will treat this file as it will any other source file (see section Support for Other Languages). Otherwise, the file will be ignored as though it were a header file. The prefixes `dist_' and `nodist_' can be used to control whether files listed in a `_SOURCES' variable are distributed. `dist_' is redundant, as sources are distributed by default, but it can be specified for clarity if desired. It is possible to have both `dist_' and `nodist_' variants of a given `_SOURCES' variable at once; this lets you easily distribute some files and not others, for instance:
nodist_maude_SOURCES = nodist.c
dist_maude_SOURCES = dist-me.c
By default the output file (on Unix systems, the `.o' file) will be put into the current build directory. However, if the option subdir-objects is in effect in the current directory then the `.o' file will be put into the subdirectory named after the source file. For instance, with subdir-objects enabled, `sub/dir/file.c' will be compiled to `sub/dir/file.o'. Some people prefer this mode of operation. You can specify subdir-objects in AUTOMAKE_OPTIONS (see section Changing Automake's Behavior).
Automake needs to know the list of files you intend to compile statically. For one thing, this is the only way Automake has of knowing what sort of language support a given `' requires. (3) This means that, for example, you can't put a configure substitution like `@my_sources@' into a `_SOURCES' variable. If you intend to conditionally compile source files and use `configure' to substitute the appropriate object names into, e.g., `_LDADD' (see below), then you should list the corresponding source files in the `EXTRA_' variable. This variable also supports `dist_' and `nodist_' prefixes, e.g., `nodist_EXTRA_maude_SOURCES'.
A static library is created by default by invoking $(AR) cru followed by the name of the library and then the objects being put into the library. You can override this by setting the `_AR' variable. This is usually used with C++; some C++ compilers require a special invocation in order to instantiate all the templates which should go into a library. For instance, the SGI C++ compiler likes this macro set like so:
libmaude_a_AR = $(CXX) -ar -o
Extra objects can be added to a static library using the `_LIBADD' variable. This should be used for objects determined by configure. Note that `_LIBADD' is not used for shared libraries; there you must use `_LDADD'.
Extra objects can be added to a shared library or a program by listing them in the `_LDADD' variable. This should be used for objects determined by configure. `_LDADD' is inappropriate for passing program-specific linker flags (except for `-l', `-L', `-dlopen' and `-dlpreopen'). Use the `_LDFLAGS' variable for this purpose. For instance, if your `' uses AC_PATH_XTRA, you could link your program against the X libraries like so:
This variable is used to pass extra flags to the link step of a program or a shared library.
You can override the linker on a per-program basis. By default the linker is chosen according to the languages used by the program. For instance, a program that includes C++ source code would use the C++ compiler to link. The `_LINK' variable must hold the name of a command which can be passed all the `.o' file names as arguments. Note that the name of the underlying program is not passed to `_LINK'; typically one uses `$@':
maude_LINK = $(CCLD) -magic -o $@
Automake allows you to set compilation flags on a per-program (or per-library) basis. A single source file can be included in several programs, and it will potentially be compiled with different flags for each program. This works for any language directly supported by Automake. The flags are `_CFLAGS', `_CXXFLAGS', `_OBJCFLAGS', `_YFLAGS', `_ASFLAGS', `_FFLAGS', `_RFLAGS', and `_GCJFLAGS'. When using a per-program compilation flag, Automake will choose a different name for the intermediate object files. Ordinarily a file like `sample.c' will be compiled to produce `sample.o'. However, if the program's `_CFLAGS' variable is set, then the object file will be named, for instance, `maude-sample.o'. In compilations with per-program flags, the ordinary `AM_' form of the flags variable is not automatically included in the compilation (however, the user form of the variable is included). So for instance, if you want the hypothetical `maude' compilations to also use the value of `AM_CFLAGS', you would need to write:
maude_CFLAGS = ... your flags ... $(AM_CFLAGS)
It is also occasionally useful to have a program depend on some other target which is not actually part of that program. This can be done using the `_DEPENDENCIES' variable. Each program depends on the contents of such a variable, but no further interpretation is done. If `_DEPENDENCIES' is not supplied, it is computed by Automake. The automatically-assigned value is the contents of `_LDADD', with most configure substitutions, `-l', `-L', `-dlopen' and `-dlpreopen' options removed. The configure substitutions that are left in are only `@LIBOBJS@' and `@ALLOCA@'; these are left because it is known that they will not cause an invalid value for `_DEPENDENCIES' to be generated.
On some platforms the allowable file names are very short. In order to support these systems and per-program compilation flags at the same time, Automake allows you to set a "short name" which will influence how intermediate object files are named. For instance, if you set `maude_SHORTNAME' to `m', then in the above per-program compilation flag example the object file would be named `m-sample.o' rather than `maude-sample.o'. This facility is rarely needed in practice, and we recommend avoiding it until you find it is required.

Special handling for LIBOBJS and ALLOCA

Automake explicitly recognizes the use of @LIBOBJS@ and @ALLOCA@, and uses this information, plus the list of LIBOBJS files derived from `' to automatically include the appropriate source files in the distribution (see section What Goes in a Distribution). These source files are also automatically handled in the dependency-tracking scheme; see See section Automatic dependency tracking.

@LIBOBJS@ and @ALLOCA@ are specially recognized in any `_LDADD' or `_LIBADD' variable.

Variables used when building a program

Occasionally it is useful to know which `Makefile' variables Automake uses for compilations; for instance you might need to do your own compilation in some special cases.

Some variables are inherited from Autoconf; these are CC, CFLAGS, CPPFLAGS, DEFS, LDFLAGS, and LIBS.

There are some additional variables which Automake itself defines:

The contents of this macro are passed to every compilation which invokes the C preprocessor; it is a list of arguments to the preprocessor. For instance, `-I' and `-D' options should be listed here. Automake already provides some `-I' options automatically. In particular it generates `-I$(srcdir)', `-I.', and a `-I' pointing to the directory holding `config.h' (if you've used AC_CONFIG_HEADER or AM_CONFIG_HEADER). You can disable the default `-I' options using the `nostdinc' option.
This does the same job as `AM_CPPFLAGS'. It is an older name for the same functionality. This macro is deprecated; we suggest using `AM_CPPFLAGS' instead.
This is the variable which the `' author can use to pass in additional C compiler flags. It is more fully documented elsewhere. In some situations, this is not used, in preference to the per-executable (or per-library) CFLAGS.
This is the command used to actually compile a C source file. The filename is appended to form the complete command line.
This is the command used to actually link a C program. It already includes `-o $@' and the usual variable references (for instance, CFLAGS); it takes as "arguments" the names of the object files and libraries to link in.

Yacc and Lex support

Automake has somewhat idiosyncratic support for Yacc and Lex.

Automake assumes that the `.c' file generated by yacc (or lex) should be named using the basename of the input file. That is, for a yacc source file `foo.y', Automake will cause the intermediate file to be named `foo.c' (as opposed to `', which is more traditional).

The extension of a yacc source file is used to determine the extension of the resulting `C' or `C++' file. Files with the extension `.y' will be turned into `.c' files; likewise, `.yy' will become `.cc'; `.y++', `c++'; and `.yxx', `.cxx'.

Likewise, lex source files can be used to generate `C' or `C++'; the extensions `.l', `.ll', `.l++', and `.lxx' are recognized.

You should never explicitly mention the intermediate (`C' or `C++') file in any `SOURCES' variable; only list the source file.

The intermediate files generated by yacc (or lex) will be included in any distribution that is made. That way the user doesn't need to have yacc or lex.

If a yacc source file is seen, then your `' must define the variable `YACC'. This is most easily done by invoking the macro `AC_PROG_YACC' (see section `Particular Program Checks' in The Autoconf Manual).

When yacc is invoked, it is passed `YFLAGS' and `AM_YFLAGS'. The former is a user variable and the latter is intended for the `' author.

Similarly, if a lex source file is seen, then your `' must define the variable `LEX'. You can use `AC_PROG_LEX' to do this (see section `Particular Program Checks' in The Autoconf Manual). Automake's lex support also requires that you use the `AC_DECL_YYTEXT' macro--automake needs to know the value of `LEX_OUTPUT_ROOT'. This is all handled for you if you use the AM_PROG_LEX macro (see section Autoconf macros supplied with Automake).

When yacc is invoked, it is passed `LFLAGS' and `AM_LFLAGS'. The former is a user variable and the latter is intended for the `' author.

Automake makes it possible to include multiple yacc (or lex) source files in a single program. Automake uses a small program called ylwrap to run yacc (or lex) in a subdirectory. This is necessary because yacc's output filename is fixed, and a parallel make could conceivably invoke more than one instance of yacc simultaneously. The ylwrap program is distributed with Automake. It should appear in the directory specified by `AC_CONFIG_AUX_DIR' (see section `Finding `configure' Input' in The Autoconf Manual), or the current directory if that macro is not used in `'.

For yacc, simply managing locking is insufficient. The output of yacc always uses the same symbol names internally, so it isn't possible to link two yacc parsers into the same executable.

We recommend using the following renaming hack used in gdb:

#define	yymaxdepth c_maxdepth
#define	yyparse	c_parse
#define	yylex	c_lex
#define	yyerror	c_error
#define	yylval	c_lval
#define	yychar	c_char
#define	yydebug	c_debug
#define	yypact	c_pact
#define	yyr1	c_r1
#define	yyr2	c_r2
#define	yydef	c_def
#define	yychk	c_chk
#define	yypgo	c_pgo
#define	yyact	c_act
#define	yyexca	c_exca
#define yyerrflag c_errflag
#define yynerrs	c_nerrs
#define	yyps	c_ps
#define	yypv	c_pv
#define	yys	c_s
#define	yy_yys	c_yys
#define	yystate	c_state
#define	yytmp	c_tmp
#define	yyv	c_v
#define	yy_yyv	c_yyv
#define	yyval	c_val
#define	yylloc	c_lloc
#define yyreds	c_reds
#define yytoks	c_toks
#define yylhs	c_yylhs
#define yylen	c_yylen
#define yydefred c_yydefred
#define yydgoto	c_yydgoto
#define yysindex c_yysindex
#define yyrindex c_yyrindex
#define yygindex c_yygindex
#define yytable	 c_yytable
#define yycheck	 c_yycheck
#define yyname   c_yyname
#define yyrule   c_yyrule

For each define, replace the `c_' prefix with whatever you like. These defines work for bison, byacc, and traditional yaccs. If you find a parser generator that uses a symbol not covered here, please report the new name so it can be added to the list.

C++ Support

Automake includes full support for C++.

Any package including C++ code must define the output variable `CXX' in `'; the simplest way to do this is to use the AC_PROG_CXX macro (see section `Particular Program Checks' in The Autoconf Manual).

A few additional variables are defined when a C++ source file is seen:

The name of the C++ compiler.
Any flags to pass to the C++ compiler.
The maintainer's variant of CXXFLAGS.
The command used to actually compile a C++ source file. The file name is appended to form the complete command line.
The command used to actually link a C++ program.

Assembly Support

Automake includes some support for assembly code.

The variable AS holds the name of the compiler used to build assembly code. This compiler must work a bit like a C compiler; in particular it must accept `-c' and `-o'. The value of ASFLAGS is passed to the compilation.

You are required to set AS and ASFLAGS via `'. The autoconf macro AM_PROG_AS will do this for you. Unless they are already set, it simply sets AS to the C compiler and ASFLAGS to the C compiler flags.

Fortran 77 Support

Automake includes full support for Fortran 77.

Any package including Fortran 77 code must define the output variable `F77' in `'; the simplest way to do this is to use the AC_PROG_F77 macro (see section `Particular Program Checks' in The Autoconf Manual). See section Fortran 77 and Autoconf.

A few additional variables are defined when a Fortran 77 source file is seen:

The name of the Fortran 77 compiler.
Any flags to pass to the Fortran 77 compiler.
The maintainer's variant of FFLAGS.
Any flags to pass to the Ratfor compiler.
The maintainer's variant of RFLAGS.
The command used to actually compile a Fortran 77 source file. The file name is appended to form the complete command line.
The command used to actually link a pure Fortran 77 program or shared library.

Automake can handle preprocessing Fortran 77 and Ratfor source files in addition to compiling them(4). Automake also contains some support for creating programs and shared libraries that are a mixture of Fortran 77 and other languages (see section Mixing Fortran 77 With C and C++).

These issues are covered in the following sections.

Preprocessing Fortran 77

`N.f' is made automatically from `N.F' or `N.r'. This rule runs just the preprocessor to convert a preprocessable Fortran 77 or Ratfor source file into a strict Fortran 77 source file. The precise command used is as follows:


Compiling Fortran 77 Files

`N.o' is made automatically from `N.f', `N.F' or `N.r' by running the Fortran 77 compiler. The precise command used is as follows:

$(F77) -c $(AM_FFLAGS) $(FFLAGS)

Mixing Fortran 77 With C and C++

Automake currently provides limited support for creating programs and shared libraries that are a mixture of Fortran 77 and C and/or C++. However, there are many other issues related to mixing Fortran 77 with other languages that are not (currently) handled by Automake, but that are handled by other packages(5).

Automake can help in two ways:

  1. Automatic selection of the linker depending on which combinations of source code.
  2. Automatic selection of the appropriate linker flags (e.g. `-L' and `-l') to pass to the automatically selected linker in order to link in the appropriate Fortran 77 intrinsic and run-time libraries. These extra Fortran 77 linker flags are supplied in the output variable FLIBS by the AC_F77_LIBRARY_LDFLAGS Autoconf macro supplied with newer versions of Autoconf (Autoconf version 2.13 and later). See section `Fortran 77 Compiler Characteristics' in The Autoconf.

If Automake detects that a program or shared library (as mentioned in some _PROGRAMS or _LTLIBRARIES primary) contains source code that is a mixture of Fortran 77 and C and/or C++, then it requires that the macro AC_F77_LIBRARY_LDFLAGS be called in `', and that either $(FLIBS) or @FLIBS@ appear in the appropriate _LDADD (for programs) or _LIBADD (for shared libraries) variables. It is the responsibility of the person writing the `' to make sure that $(FLIBS) or @FLIBS@ appears in the appropriate _LDADD or _LIBADD variable.

For example, consider the following `':

bin_PROGRAMS = foo
foo_SOURCES  = foo.f
foo_LDADD    = @FLIBS@

libfoo_la_SOURCES  = bar.f baz.c
libfoo_la_LIBADD   = $(FLIBS)

In this case, Automake will insist that AC_F77_LIBRARY_LDFLAGS is mentioned in `'. Also, if @FLIBS@ hadn't been mentioned in foo_LDADD and libfoo_la_LIBADD, then Automake would have issued a warning.

How the Linker is Chosen

The following diagram demonstrates under what conditions a particular linker is chosen by Automake.

For example, if Fortran 77, C and C++ source code were to be compiled into a program, then the C++ linker will be used. In this case, if the C or Fortran 77 linkers required any special libraries that weren't included by the C++ linker, then they must be manually added to an _LDADD or _LIBADD variable by the user writing the `'.

                     \              Linker
          source      \
           code        \     C        C++     Fortran
     -----------------  +---------+---------+---------+
                        |         |         |         |
     C                  |    x    |         |         |
                        |         |         |         |
                        |         |         |         |
         C++            |         |    x    |         |
                        |         |         |         |
                        |         |         |         |
               Fortran  |         |         |    x    |
                        |         |         |         |
                        |         |         |         |
     C + C++            |         |    x    |         |
                        |         |         |         |
                        |         |         |         |
     C +       Fortran  |         |         |    x    |
                        |         |         |         |
                        |         |         |         |
         C++ + Fortran  |         |    x    |         |
                        |         |         |         |
                        |         |         |         |
     C + C++ + Fortran  |         |    x    |         |
                        |         |         |         |

Fortran 77 and Autoconf

The current Automake support for Fortran 77 requires a recent enough version Autoconf that also includes support for Fortran 77. Full Fortran 77 support was added to Autoconf 2.13, so you will want to use that version of Autoconf or later.

Java Support

Automake includes support for compiled Java, using gcj, the Java front end to the GNU Compiler Collection.

Any package including Java code to be compiled must define the output variable `GCJ' in `'; the variable `GCJFLAGS' must also be defined somehow (either in `' or `'). The simplest way to do this is to use the AM_PROG_GCJ macro.

By default, programs including Java source files are linked with gcj.

As always, the contents of `AM_GCJFLAGS' are passed to every compilation invoking gcj (in its role as an ahead-of-time compiler -- when invoking it to create `.class' files, `AM_JAVACFLAGS' is used instead). If it is necessary to pass options to gcj from `', this macro, and not the user macro `GCJFLAGS', should be used.

gcj can be used to compile `.java', `.class', `.zip', or `.jar' files.

Support for Other Languages

Automake currently only includes full support for C, C++ (see section C++ Support), Fortran 77 (see section Fortran 77 Support), and Java (see section Java Support). There is only rudimentary support for other languages, support for which will be improved based on user demand.

Some limited support for adding your own languages is available via the suffix rule handling; see section Handling new file extensions.

Automatic de-ANSI-fication

Although the GNU standards allow the use of ANSI C, this can have the effect of limiting portability of a package to some older compilers (notably the SunOS C compiler).

Automake allows you to work around this problem on such machines by de-ANSI-fying each source file before the actual compilation takes place.

If the `' variable AUTOMAKE_OPTIONS (see section Changing Automake's Behavior) contains the option ansi2knr then code to handle de-ANSI-fication is inserted into the generated `'.

This causes each C source file in the directory to be treated as ANSI C. If an ANSI C compiler is available, it is used. If no ANSI C compiler is available, the ansi2knr program is used to convert the source files into K&R C, which is then compiled.

The ansi2knr program is simple-minded. It assumes the source code will be formatted in a particular way; see the ansi2knr man page for details.

Support for de-ANSI-fication requires the source files `ansi2knr.c' and `ansi2knr.1' to be in the same package as the ANSI C source; these files are distributed with Automake. Also, the package `' must call the macro AM_C_PROTOTYPES (see section Autoconf macros supplied with Automake).

Automake also handles finding the ansi2knr support files in some other directory in the current package. This is done by prepending the relative path to the appropriate directory to the ansi2knr option. For instance, suppose the package has ANSI C code in the `src' and `lib' subdirs. The files `ansi2knr.c' and `ansi2knr.1' appear in `lib'. Then this could appear in `src/':

AUTOMAKE_OPTIONS = ../lib/ansi2knr

If no directory prefix is given, the files are assumed to be in the current directory.

Files mentioned in LIBOBJS which need de-ANSI-fication will not be automatically handled. That's because configure will generate an object name like `regex.o', while make will be looking for `regex_.o' (when de-ANSI-fying). Eventually this problem will be fixed via autoconf magic, but for now you must put this code into your `', just before the AC_OUTPUT call:

# This is necessary so that .o files in LIBOBJS are also built via
# the ANSI2KNR-filtering rules.
LIBOBJS=`echo $LIBOBJS|sed 's/\.o /\$U.o /g;s/\.o$/\$U.o/'`

Note that automatic de-ANSI-fication will not work when the package is being built for a different host architecture. That is because automake currently has no way to build ansi2knr for the build machine.

Automatic dependency tracking

As a developer it is often painful to continually update the `' whenever the include-file dependencies change in a project. Automake supplies a way to automatically track dependency changes.

Automake always uses complete dependencies for a compilation, including system headers. Automake's model is that dependency computation should be a side effect of the build. To this end, dependencies are computed by running all compilations through a special wrapper program called depcomp. depcomp understands how to coax many different C and C++ compilers into generating dependency information in the format it requires. automake -a will install depcomp into your source tree for you. If depcomp can't figure out how to properly invoke your compiler, dependency tracking will simply be disabled for your build.

Experience with earlier versions of Automake (6) taught us that it is not reliable to generate dependencies only on the maintainer's system, as configurations vary too much. So instead Automake implements dependency tracking at build time.

Automatic dependency tracking can be suppressed by putting no-dependencies in the variable AUTOMAKE_OPTIONS. Or, you can invoke automake with the -i option. Dependency tracking is enabled by default.

The person building your package also can choose to disable dependency tracking by configuring with --disable-dependency-tracking.

Support for executable extensions

On some platforms, such as Windows, executables are expected to have an extension such as `.exe'. On these platforms, some compilers (GCC among them) will automatically generate `foo.exe' when asked to generate `foo'.

Automake provides mostly-transparent support for this. Unfortunately the support isn't completely transparent; if you want your package to support these platforms then you must assist.

One thing you must be aware of is that, internally, Automake rewrites something like this:

bin_PROGRAMS = liver

to this:

bin_PROGRAMS = liver$(EXEEXT)

The targets Automake generates are likewise given the `$(EXEEXT)' extension. EXEEXT

However, Automake cannot apply this rewriting to configure substitutions. This means that if you are conditionally building a program using such a substitution, then your `' must take care to add `$(EXEEXT)' when constructing the output variable.

With Autoconf 2.13 and earlier, you must explicitly use AC_EXEEXT to get this support. With Autoconf 2.50, AC_EXEEXT is run automatically if you configure a compiler (say, through AC_PROG_CC).

Sometimes maintainers like to write an explicit link rule for their program. Without executable extension support, this is easy--you simply write a target with the same name as the program. However, when executable extension support is enabled, you must instead add the `$(EXEEXT)' suffix.

Unfortunately, due to the change in Autoconf 2.50, this means you must always add this extension. However, this is a problem for maintainers who know their package will never run on a platform that has executable extensions. For those maintainers, the no-exeext option (see section Changing Automake's Behavior) will disable this feature. This works in a fairly ugly way; if no-exeext is seen, then the presence of a target named foo in `' will override an automake-generated target of the form foo$(EXEEXT). Without the no-exeext option, this use will give an error.

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