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Obsolete Constructs

Autoconf changes, and throughout the years some constructs are obsoleted. Most of the changes involve the macros, but the tools themselves, or even some concepts, are now considered obsolete.

You may completely skip this chapter if you are new to Autoconf, its intention is mainly to help maintainers updating their packages by understanding how to move to more modern constructs.

Obsolete `config.status' Invocation

`config.status' now supports arguments to specify the files to instantiate, see section Recreating a Configuration, for more details. Before, environment variables had to be used.

Variable: CONFIG_COMMANDS
The tags of the commands to execute. The default is the arguments given to AC_OUTPUT and AC_CONFIG_COMMANDS in `configure.ac'.

Variable: CONFIG_FILES
The files in which to perform `@variable@' substitutions. The default is the arguments given to AC_OUTPUT and AC_CONFIG_FILES in `configure.ac'.

Variable: CONFIG_HEADERS
The files in which to substitute C #define statements. The default is the arguments given to AC_CONFIG_HEADERS; if that macro was not called, `config.status' ignores this variable.

Variable: CONFIG_LINKS
The symbolic links to establish. The default is the arguments given to AC_CONFIG_LINKS; if that macro was not called, `config.status' ignores this variable.

In section Recreating a Configuration, using this old interface, the example would be:

config.h: stamp-h
stamp-h: config.h.in config.status
        CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_FILES= \
          CONFIG_HEADERS=config.h ./config.status
        echo > stamp-h

Makefile: Makefile.in config.status
        CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_HEADERS= \
          CONFIG_FILES=Makefile ./config.status

(If `configure.ac' does not call AC_CONFIG_HEADERS, there is no need to set CONFIG_HEADERS in the make rules, equally for CONFIG_COMMANDS etc.)

`acconfig.h'

In order to produce `config.h.in', @command{autoheader} needs to build or to find templates for each symbol. Modern releases of Autoconf use AH_VERBATIM and AH_TEMPLATE (see section Autoheader Macros), but in older releases a file, `acconfig.h', contained the list of needed templates. autoheader copies comments and #define and #undef statements from `acconfig.h' in the current directory, if present. This file used to be mandatory if you AC_DEFINE any additional symbols.

Modern releases of Autoconf also provide AH_TOP and AH_BOTTOM if you need to prepend/append some information to `config.h.in'. Ancient versions of Autoconf had a similar feature: if `./acconfig.h' contains the string `@TOP@', autoheader copies the lines before the line containing `@TOP@' into the top of the file that it generates. Similarly, if `./acconfig.h' contains the string `@BOTTOM@', autoheader copies the lines after that line to the end of the file it generates. Either or both of those strings may be omitted. An even older alternate way to produce the same effect in jurasik versions of Autoconf is to create the files `file.top' (typically `config.h.top') and/or `file.bot' in the current directory. If they exist, autoheader copies them to the beginning and end, respectively, of its output.

In former versions of Autoconf, the files used in preparing a software package for distribution were:

configure.ac --.   .------> autoconf* -----> configure
               +---+
[aclocal.m4] --+   `---.
[acsite.m4] ---'       |
                       +--> [autoheader*] -> [config.h.in]
[acconfig.h] ----.     |
                 +-----'
[config.h.top] --+
[config.h.bot] --'

Use only the AH_ macros, `configure.ac' should be self-contained, and should not depend upon `acconfig.h' etc.

Using autoupdate to Modernize `configure.ac'

The autoupdate program updates a `configure.ac' file that calls Autoconf macros by their old names to use the current macro names. In version 2 of Autoconf, most of the macros were renamed to use a more uniform and descriptive naming scheme. See section Macro Names, for a description of the new scheme. Although the old names still work (see section Obsolete Macros, for a list of the old macros and the corresponding new names), you can make your `configure.ac' files more readable and make it easier to use the current Autoconf documentation if you update them to use the new macro names.

If given no arguments, autoupdate updates `configure.ac', backing up the original version with the suffix `~' (or the value of the environment variable SIMPLE_BACKUP_SUFFIX, if that is set). If you give autoupdate an argument, it reads that file instead of `configure.ac' and writes the updated file to the standard output.

autoupdate accepts the following options:

@option{--help}
@option{-h}
Print a summary of the command line options and exit.
@option{--version}
@option{-V}
Print the version number of Autoconf and exit.
@option{--verbose}
@option{-v}
Report processing steps.
@option{--debug}
@option{-d}
Don't remove the temporary files.
@option{--autoconf-dir=dir}
@option{-A dir}
Override the location where the installed Autoconf data files are looked for. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable. This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.
@option{--localdir=dir}
@option{-l dir}
Look for the package file `aclocal.m4' in directory dir instead of in the current directory.

Obsolete Macros

Several macros are obsoleted in Autoconf, for various reasons (typically they failed to quote properly, couldn't be extended for more recent issues etc.). They are still supported, but deprecated: their use should be avoided.

During the jump from Autoconf version 1 to version 2, most of the macros were renamed to use a more uniform and descriptive naming scheme, but their signature did not change. See section Macro Names, for a description of the new naming scheme. Below, there is just the mapping from old names to new names for these macros, the reader is invited to refer to the definition of the new macro for the signature and the description.

Macro: AC_ALLOCA
AC_FUNC_ALLOCA

Macro: AC_ARG_ARRAY
removed because of limited usefulness

Macro: AC_C_CROSS
This macro is obsolete; it does nothing.

Macro: AC_CANONICAL_SYSTEM
Determine the system type and set output variables to the names of the canonical system types. See section Getting the Canonical System Type, for details about the variables this macro sets.

The user is encouraged to use either AC_CANONICAL_BUILD, or AC_CANONICAL_HOST, or AC_CANONICAL_TARGET, depending on the needs. Using AC_CANONICAL_TARGET is enough to run the two other macros.

Macro: AC_CHAR_UNSIGNED
AC_C_CHAR_UNSIGNED

Macro: AC_CHECK_TYPE (type, default)
Autoconf, up to 2.13, used to provide this version of AC_CHECK_TYPE, deprecated because of its flaws. Firstly, although it is a member of the CHECK clan, singular sub-family, it does more than just checking. Second, missing types are not typedef'd, they are #define'd, which can lead to incompatible code in the case of pointer types.

This use of AC_CHECK_TYPE is obsolete and discouraged, see section Generic Type Checks, for the description of the current macro.

If the type type is not defined, define it to be the C (or C++) builtin type default; e.g., `short' or `unsigned'.

This macro is equivalent to:

AC_CHECK_TYPE([type],
              [AC_DEFINE([type], [default],
                         [Define to `default' if <sys/types.h>
                          does not define.])])

In order to keep backward compatibility, the two versions of AC_CHECK_TYPE are implemented, selected by a simple heuristics:

  1. If there are three or four arguments, the modern version is used.
  2. If the second argument appears to be a C or C++ type, then the obsolete version is used. This happens if the argument is a C or C++ builtin type or a C identifier ending in `_t', optionally followed by one of `[(* ' and then by a string of zero or more characters taken from the set `[]()* _a-zA-Z0-9'.
  3. If the second argument is spelled with the alphabet of valid C and C++ types, the user is warned and the modern version is used.
  4. Otherwise, the modern version is used.

You are encouraged either to use a valid builtin type, or to use the equivalent modern code (see above), or better yet, to use AC_CHECK_TYPES together with

#if !HAVE_LOFF_T
typedef loff_t off_t;
#endif

Macro: AC_CHECKING (feature-description)
Same as `AC_MSG_NOTICE([checking feature-description...]'.

Macro: AC_COMPILE_CHECK (echo-text, includes, function-body, action-if-found, @ovar{action-if-not-found})
This is an obsolete version of AC_TRY_LINK (see section Examining Libraries), with the addition that it prints `checking for echo-text' to the standard output first, if echo-text is non-empty. Use AC_MSG_CHECKING and AC_MSG_RESULT instead to print messages (see section Printing Messages).

Macro: AC_CONST
AC_C_CONST

Macro: AC_CROSS_CHECK
Same as AC_C_CROSS, which is obsolete too, and does nothing :-).

Macro: AC_CYGWIN
Check for the Cygwin environment in which case the shell variable CYGWIN is set to `yes'. Don't use this macro, the dignified means to check the nature of the host is using AC_CANONICAL_HOST. As a matter of fact this macro is defined as:

AC_REQUIRE([AC_CANONICAL_HOST])[]dnl
case $host_os in
  *cygwin* ) CYGWIN=yes;;
         * ) CYGWIN=no;;
esac

Beware that the variable CYGWIN has a very special meaning when running CygWin32, and should not be changed. That's yet another reason not to use this macro.

Macro: AC_DECL_YYTEXT
Does nothing, now integrated in AC_PROG_LEX.

Macro: AC_DIR_HEADER
Like calling AC_FUNC_CLOSEDIR_VOID andAC_HEADER_DIRENT, but defines a different set of C preprocessor macros to indicate which header file is found:

Header `dirent.h'`sys/ndir.h'`sys/dir.h'`ndir.h'
Old Symbol New Symbol
DIRENT HAVE_DIRENT_H
SYSNDIR HAVE_SYS_NDIR_H
SYSDIR HAVE_SYS_DIR_H
NDIR HAVE_NDIR_H
Macro: AC_DYNIX_SEQ
If on Dynix/PTX (Sequent UNIX), add @option{-lseq} to output variable LIBS. This macro used to be defined as
AC_CHECK_LIB(seq, getmntent, LIBS="-lseq $LIBS")
now it is just AC_FUNC_GETMNTENT.
Macro: AC_EXEEXT
Defined the output variable EXEEXT based on the output of the compiler, which is now done automatically. Typically set to empty string if Unix and `.exe' if Win32 or OS/2.
Macro: AC_EMXOS2
Similar to AC_CYGWIN but checks for the EMX environment on OS/2 and sets EMXOS2.
Macro: AC_ERROR
AC_MSG_ERROR
Macro: AC_FIND_X
AC_PATH_X
Macro: AC_FIND_XTRA
AC_PATH_XTRA
Macro: AC_FUNC_CHECK
AC_CHECK_FUNC
Macro: AC_FUNC_WAIT3
If wait3 is found and fills in the contents of its third argument (a `struct rusage *'), which HP-UX does not do, define HAVE_WAIT3. These days portable programs should use waitpid, not wait3, as wait3 is being removed from the Open Group standards, and will not appear in the next revision of POSIX.
Macro: AC_GCC_TRADITIONAL
AC_PROG_GCC_TRADITIONAL
Macro: AC_GETGROUPS_T
AC_TYPE_GETGROUPS
Macro: AC_GETLOADAVG
AC_FUNC_GETLOADAVG
Macro: AC_HAVE_FUNCS
AC_CHECK_FUNCS
Macro: AC_HAVE_HEADERS
AC_CHECK_HEADERS
Macro: AC_HAVE_LIBRARY (library, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries})
This macro is equivalent to calling AC_CHECK_LIB with a function argument of main. In addition, library can be written as any of `foo', @option{-lfoo}, or `libfoo.a'. In all of those cases, the compiler is passed @option{-lfoo}. However, library cannot be a shell variable; it must be a literal name.
Macro: AC_HAVE_POUNDBANG
AC_SYS_INTERPRETER (different calling convention)
Macro: AC_HEADER_CHECK
AC_CHECK_HEADER
Macro: AC_HEADER_EGREP
AC_EGREP_HEADER
Macro: AC_INIT (unique-file-in-source-dir)
Formerly AC_INIT used to have a single argument, and was equivalent to:
AC_INIT
AC_CONFIG_SRCDIR(unique-file-in-source-dir)
Macro: AC_INLINE
AC_C_INLINE
Macro: AC_INT_16_BITS
If the C type int is 16 bits wide, define INT_16_BITS. Use `AC_CHECK_SIZEOF(int)' instead.
Macro: AC_IRIX_SUN
If on IRIX (Silicon Graphics UNIX), add @option{-lsun} to output LIBS. If you were using it to get getmntent, use AC_FUNC_GETMNTENT instead. If you used it for the NIS versions of the password and group functions, use `AC_CHECK_LIB(sun, getpwnam)'. Up to Autoconf 2.13, it used to be
AC_CHECK_LIB(sun, getmntent, LIBS="-lsun $LIBS")
now it is defined as
AC_FUNC_GETMNTENT
AC_CHECK_LIB(sun, getpwnam)
Macro: AC_LANG_C
Same as `AC_LANG(C)'.
Macro: AC_LANG_CPLUSPLUS
Same as `AC_LANG(C++)'.
Macro: AC_LANG_FORTRAN77
Same as `AC_LANG(Fortran 77)'.
Macro: AC_LANG_RESTORE
Select the language that is saved on the top of the stack, as set by AC_LANG_SAVE, remove it from the stack, and call AC_LANG(language).
Macro: AC_LANG_SAVE
Remember the current language (as set by AC_LANG) on a stack. The current language does not change. AC_LANG_PUSH is preferred.
Macro: AC_LINK_FILES (source..., dest...)
This is an obsolete version of AC_CONFIG_LINKS. An updated version of:
AC_LINK_FILES(config/$machine.h config/$obj_format.h,
              host.h            object.h)
is:
AC_CONFIG_LINKS(host.h:config/$machine.h
                object.h:config/$obj_format.h)
Macro: AC_LN_S
AC_PROG_LN_S
Macro: AC_LONG_64_BITS
Define LONG_64_BITS if the C type long int is 64 bits wide. Use the generic macro `AC_CHECK_SIZEOF([long int])' instead.
Macro: AC_LONG_DOUBLE
AC_C_LONG_DOUBLE
Macro: AC_LONG_FILE_NAMES
AC_SYS_LONG_FILE_NAMES
Macro: AC_MAJOR_HEADER
AC_HEADER_MAJOR
Macro: AC_MEMORY_H
Used to define NEED_MEMORY_H if the mem functions were defined in `memory.h'. Today it is equivalent to `AC_CHECK_HEADERS(memory.h)'. Adjust your code to depend upon HAVE_MEMORY_H, not NEED_MEMORY_H, see See section Standard Symbols.
Macro: AC_MINGW32
Similar to AC_CYGWIN but checks for the MingW32 compiler environment and sets MINGW32.
Macro: AC_MINUS_C_MINUS_O
AC_PROG_CC_C_O
Macro: AC_MMAP
AC_FUNC_MMAP
Macro: AC_MODE_T
AC_TYPE_MODE_T
Macro: AC_OBJEXT
Defined the output variable OBJEXT based on the output of the compiler, after .c files have been excluded. Typically set to `o' if Unix, `obj' if Win32. Now the compiler checking macros handle this automatically.
Macro: AC_OBSOLETE (this-macro-name, @ovar{suggestion})
Make m4 print a message to the standard error output warning that this-macro-name is obsolete, and giving the file and line number where it was called. this-macro-name should be the name of the macro that is calling AC_OBSOLETE. If suggestion is given, it is printed at the end of the warning message; for example, it can be a suggestion for what to use instead of this-macro-name. For instance
AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl
You are encouraged to use AU_DEFUN instead, since it gives better services to the user.
Macro: AC_OFF_T
AC_TYPE_OFF_T
Macro: AC_OUTPUT (@ovar{file}..., @ovar{extra-cmds}, @ovar{init-cmds})
The use of AC_OUTPUT with argument is deprecated, this obsoleted interface is equivalent to:
AC_CONFIG_FILES(file...)
AC_CONFIG_COMMANDS([default],
                   extra-cmds, init-cmds)
AC_OUTPUT
Macro: AC_OUTPUT_COMMANDS (extra-cmds, @ovar{init-cmds})
Specify additional shell commands to run at the end of `config.status', and shell commands to initialize any variables from configure. This macro may be called multiple times. It is obsolete, replaced by AC_CONFIG_COMMANDS. Here is an unrealistic example:
fubar=27
AC_OUTPUT_COMMANDS([echo this is extra $fubar, and so on.],
                   fubar=$fubar)
AC_OUTPUT_COMMANDS([echo this is another, extra, bit],
                   [echo init bit])
Aside from the fact that AC_CONFIG_COMMANDS requires an additional key, an important difference is that AC_OUTPUT_COMMANDS is quoting its arguments twice, while AC_CONFIG_COMMANDS. This means that AC_CONFIG_COMMANDS can safely be given macro calls as arguments:
AC_CONFIG_COMMANDS(foo, [my_FOO()])
conversely, where one level of quoting was enough for literal strings with AC_OUTPUT_COMMANDS, you need two with AC_CONFIG_COMMANDS. The following lines are equivalent:
AC_OUTPUT_COMMANDS([echo "Square brackets: []"])
AC_CONFIG_COMMANDS(default, [[echo "Square brackets: []"]])
Macro: AC_PID_T
AC_TYPE_PID_T
Macro: AC_PREFIX
AC_PREFIX_PROGRAM
Macro: AC_PROGRAMS_CHECK
AC_CHECK_PROGS
Macro: AC_PROGRAMS_PATH
AC_PATH_PROGS
Macro: AC_PROGRAM_CHECK
AC_CHECK_PROG
Macro: AC_PROGRAM_EGREP
AC_EGREP_CPP
Macro: AC_PROGRAM_PATH
AC_PATH_PROG
Macro: AC_REMOTE_TAPE
removed because of limited usefulness
Macro: AC_RESTARTABLE_SYSCALLS
AC_SYS_RESTARTABLE_SYSCALLS
Macro: AC_RETSIGTYPE
AC_TYPE_SIGNAL
Macro: AC_RSH
Removed because of limited usefulness.
Macro: AC_SCO_INTL
If on SCO UNIX, add @option{-lintl} to output variable LIBS. This macro used to
AC_CHECK_LIB(intl, strftime, LIBS="-lintl $LIBS")
now it just calls AC_FUNC_STRFTIME instead.
Macro: AC_SETVBUF_REVERSED
AC_FUNC_SETVBUF_REVERSED
Macro: AC_SET_MAKE
AC_PROG_MAKE_SET
Macro: AC_SIZEOF_TYPE
AC_CHECK_SIZEOF
Macro: AC_SIZE_T
AC_TYPE_SIZE_T
Macro: AC_STAT_MACROS_BROKEN
AC_HEADER_STAT
Macro: AC_STDC_HEADERS
AC_HEADER_STDC
Macro: AC_STRCOLL
AC_FUNC_STRCOLL
Macro: AC_ST_BLKSIZE
AC_STRUCT_ST_BLKSIZE
Macro: AC_ST_BLOCKS
AC_STRUCT_ST_BLOCKS
Macro: AC_ST_RDEV
AC_STRUCT_ST_RDEV
Macro: AC_SYS_RESTARTABLE_SYSCALLS
If the system automatically restarts a system call that is interrupted by a signal, define HAVE_RESTARTABLE_SYSCALLS. This macro does not check if system calls are restarted in general--it tests whether a signal handler installed with signal (but not sigaction) causes system calls to be restarted. It does not test if system calls can be restarted when interrupted by signals that have no handler. These days portable programs should use sigaction with SA_RESTART if they want restartable system calls. They should not rely on HAVE_RESTARTABLE_SYSCALLS, since nowadays whether a system call is restartable is a dynamic issue, not a configuration-time issue.
Macro: AC_SYS_SIGLIST_DECLARED
AC_DECL_SYS_SIGLIST
Macro: AC_TEST_CPP
AC_TRY_CPP
Macro: AC_TEST_PROGRAM
AC_TRY_RUN
Macro: AC_TIMEZONE
AC_STRUCT_TIMEZONE
Macro: AC_TIME_WITH_SYS_TIME
AC_HEADER_TIME
Macro: AC_UID_T
AC_TYPE_UID_T
Macro: AC_UNISTD_H
Same as `AC_CHECK_HEADERS(unistd.h)'.
Macro: AC_USG
Define USG if the BSD string functions are defined in `strings.h'. You should no longer depend upon USG, but on HAVE_STRING_H, see See section Standard Symbols.
Macro: AC_UTIME_NULL
AC_FUNC_UTIME_NULL
Macro: AC_VALIDATE_CACHED_SYSTEM_TUPLE (@ovar{cmd})
If the cache file is inconsistent with the current host, target and build system types, it used to execute cmd or print a default error message. This is now handled by default.
Macro: AC_VERBOSE (result-description)
AC_MSG_RESULT.
Macro: AC_VFORK
AC_FUNC_VFORK
Macro: AC_VPRINTF
AC_FUNC_VPRINTF
Macro: AC_WAIT3
AC_FUNC_WAIT3
Macro: AC_WARN
AC_MSG_WARN
Macro: AC_WORDS_BIGENDIAN
AC_C_BIGENDIAN
Macro: AC_XENIX_DIR
This macro used to add @option{-lx} to output variable LIBS if on Xenix. Also, if `dirent.h' is being checked for, added @option{-ldir} to LIBS. Now it is merely an alias of AC_HEADER_DIRENT instead, plus some code to detect whether running XENIX on which you should not depend:
AC_MSG_CHECKING([for Xenix])
AC_EGREP_CPP(yes,
[#if defined M_XENIX && !defined M_UNIX
  yes
#endif],
             [AC_MSG_RESULT([yes]); XENIX=yes],
             [AC_MSG_RESULT([no]); XENIX=])
Macro: AC_YYTEXT_POINTER
AC_DECL_YYTEXT

Upgrading From Version 1

Autoconf version 2 is mostly backward compatible with version 1. However, it introduces better ways to do some things, and doesn't support some of the ugly things in version 1. So, depending on how sophisticated your `configure.ac' files are, you might have to do some manual work in order to upgrade to version 2. This chapter points out some problems to watch for when upgrading. Also, perhaps your configure scripts could benefit from some of the new features in version 2; the changes are summarized in the file `NEWS' in the Autoconf distribution.

Changed File Names

If you have an `aclocal.m4' installed with Autoconf (as opposed to in a particular package's source directory), you must rename it to `acsite.m4'. See section Using autoconf to Create configure.

If you distribute `install.sh' with your package, rename it to `install-sh' so make builtin rules won't inadvertently create a file called `install' from it. AC_PROG_INSTALL looks for the script under both names, but it is best to use the new name.

If you were using `config.h.top', `config.h.bot', or `acconfig.h', you still can, but you will have less clutter if you use the AH_ macros. See section Autoheader Macros.

Changed Makefiles

Add `@CFLAGS@', `@CPPFLAGS@', and `@LDFLAGS@' in your `Makefile.in' files, so they can take advantage of the values of those variables in the environment when configure is run. Doing this isn't necessary, but it's a convenience for users.

Also add `@configure_input@' in a comment to each input file for AC_OUTPUT, so that the output files will contain a comment saying they were produced by configure. Automatically selecting the right comment syntax for all the kinds of files that people call AC_OUTPUT on became too much work.

Add `config.log' and `config.cache' to the list of files you remove in distclean targets.

If you have the following in `Makefile.in':

prefix = /usr/local
exec_prefix = $(prefix)

you must change it to:

prefix = @prefix@
exec_prefix = @exec_prefix@

The old behavior of replacing those variables without `@' characters around them has been removed.

Changed Macros

Many of the macros were renamed in Autoconf version 2. You can still use the old names, but the new ones are clearer, and it's easier to find the documentation for them. See section Obsolete Macros, for a table showing the new names for the old macros. Use the autoupdate program to convert your `configure.ac' to using the new macro names. See section Using autoupdate to Modernize `configure.ac'.

Some macros have been superseded by similar ones that do the job better, but are not call-compatible. If you get warnings about calling obsolete macros while running autoconf, you may safely ignore them, but your configure script will generally work better if you follow the advice it prints about what to replace the obsolete macros with. In particular, the mechanism for reporting the results of tests has changed. If you were using echo or AC_VERBOSE (perhaps via AC_COMPILE_CHECK), your configure script's output will look better if you switch to AC_MSG_CHECKING and AC_MSG_RESULT. See section Printing Messages. Those macros work best in conjunction with cache variables. See section Caching Results.

Changed Results

If you were checking the results of previous tests by examining the shell variable DEFS, you need to switch to checking the values of the cache variables for those tests. DEFS no longer exists while configure is running; it is only created when generating output files. This difference from version 1 is because properly quoting the contents of that variable turned out to be too cumbersome and inefficient to do every time AC_DEFINE is called. See section Cache Variable Names.

For example, here is a `configure.ac' fragment written for Autoconf version 1:

AC_HAVE_FUNCS(syslog)
case "$DEFS" in
*-DHAVE_SYSLOG*) ;;
*) # syslog is not in the default libraries.  See if it's in some other.
  saved_LIBS="$LIBS"
  for lib in bsd socket inet; do
    AC_CHECKING(for syslog in -l$lib)
    LIBS="$saved_LIBS -l$lib"
    AC_HAVE_FUNCS(syslog)
    case "$DEFS" in
    *-DHAVE_SYSLOG*) break ;;
    *) ;;
    esac
    LIBS="$saved_LIBS"
  done ;;
esac

Here is a way to write it for version 2:

AC_CHECK_FUNCS(syslog)
if test $ac_cv_func_syslog = no; then
  # syslog is not in the default libraries.  See if it's in some other.
  for lib in bsd socket inet; do
    AC_CHECK_LIB($lib, syslog, [AC_DEFINE(HAVE_SYSLOG)
      LIBS="$LIBS -l$lib"; break])
  done
fi

If you were working around bugs in AC_DEFINE_UNQUOTED by adding backslashes before quotes, you need to remove them. It now works predictably, and does not treat quotes (except back quotes) specially. See section Setting Output Variables.

All of the boolean shell variables set by Autoconf macros now use `yes' for the true value. Most of them use `no' for false, though for backward compatibility some use the empty string instead. If you were relying on a shell variable being set to something like 1 or `t' for true, you need to change your tests.

Changed Macro Writing

When defining your own macros, you should now use AC_DEFUN instead of define. AC_DEFUN automatically calls AC_PROVIDE and ensures that macros called via AC_REQUIRE do not interrupt other macros, to prevent nested `checking...' messages on the screen. There's no actual harm in continuing to use the older way, but it's less convenient and attractive. See section Macro Definitions.

You probably looked at the macros that came with Autoconf as a guide for how to do things. It would be a good idea to take a look at the new versions of them, as the style is somewhat improved and they take advantage of some new features.

If you were doing tricky things with undocumented Autoconf internals (macros, variables, diversions), check whether you need to change anything to account for changes that have been made. Perhaps you can even use an officially supported technique in version 2 instead of kludging. Or perhaps not.

To speed up your locally written feature tests, add caching to them. See whether any of your tests are of general enough usefulness to encapsulate into macros that you can share.

Upgrading From Version 2.13

The introduction of the previous section (see section Upgrading From Version 1) perfectly suits this section...

Autoconf version 2.50 is mostly backward compatible with version 2.13. However, it introduces better ways to do some things, and doesn't support some of the ugly things in version 2.13. So, depending on how sophisticated your `configure.ac' files are, you might have to do some manual work in order to upgrade to version 2.50. This chapter points out some problems to watch for when upgrading. Also, perhaps your configure scripts could benefit from some of the new features in version 2.50; the changes are summarized in the file `NEWS' in the Autoconf distribution.

Changed Quotation

The most important changes are invisible to you: the implementation of most macros have completely changed. This allowed more factorization of the code, better error messages, a higher uniformity of the user's interface etc. Unfortunately, as a side effect, some construct which used to (miraculously) work might break starting with Autoconf 2.50. The most common culprit is bad quotation.

For instance, in the following example, the message is not properly quoted:

AC_INIT
AC_CHECK_HEADERS(foo.h,,
AC_MSG_ERROR(cannot find foo.h, bailing out))
AC_OUTPUT

Autoconf 2.13 simply ignores it:

$ autoconf-2.13; ./configure --silent
creating cache ./config.cache
configure: error: cannot find foo.h
$

while Autoconf 2.50 will produce a broken `configure':

$ autoconf-2.50; ./configure --silent
configure: error: cannot find foo.h
./configure: exit: bad non-numeric arg `bailing'
./configure: exit: bad non-numeric arg `bailing'
$

The message needs to be quoted, and the AC_MSG_ERROR invocation too!

AC_INIT
AC_CHECK_HEADERS(foo.h,,
                 [AC_MSG_ERROR([cannot find foo.h, bailing out])])
AC_OUTPUT

Many many (and many more) Autoconf macros were lacking proper quotation, including no less than... AC_DEFUN itself!

$ cat configure.in
AC_DEFUN([AC_PROG_INSTALL],
[# My own much better version
])
AC_INIT
AC_PROG_INSTALL
AC_OUTPUT
$ autoconf-2.13
autoconf: Undefined macros:
***BUG in Autoconf--please report*** AC_FD_MSG
***BUG in Autoconf--please report*** AC_EPI
configure.in:1:AC_DEFUN([AC_PROG_INSTALL],
configure.in:5:AC_PROG_INSTALL
$ autoconf-2.50
$

New Macros

Because Autoconf has been dormant for years, Automake provided Autoconf-like macros for a while. Autoconf 2.50 now provides better versions of these macros, integrated in the AC_ namespace, instead of AM_. But in order to ease the upgrading via @command{autoupdate}, bindings to such AM_ macros are provided.

Unfortunately Automake did not quote the name of these macros! Therefore, when @command{m4} find in `aclocal.m4' something like `AC_DEFUN(AM_TYPE_PTRDIFF_T, ...)', AM_TYPE_PTRDIFF_T is expanded, replaced with its Autoconf definition.

Fortunately Autoconf catches pre-AC_INIT expansions, and will complain, in its own words:

$ cat configure.in
AC_INIT
AM_TYPE_PTRDIFF_T
$ aclocal-1.4
$ autoconf
./aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
actypes.m4:289: AM_TYPE_PTRDIFF_T is expanded from...
./aclocal.m4:17: the top level
$

Future versions of Automake will simply no longer define most of these macros, and will properly quote the names of the remaining macros. But you don't have to wait for it to happen to do the right thing right now: do not depend upon macros from Automake as it is simply not its job to provide macros (but the one it requires by itself):

$ cat configure.in
AC_INIT
AM_TYPE_PTRDIFF_T
$ rm aclocal.m4
$ autoupdate
autoupdate: `configure.in' is updated
$ cat configure.in
AC_INIT
AC_CHECK_TYPES([ptrdiff_t])
$ aclocal-1.4
$ autoconf
$


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