## Talking to the UNIX build system: config.m4

The config.m4 file for an extension tells the UNIX build system what configure options your extension supports, what external libraries and includes you require, and what source files are to be compiled as part of it. A reference to all the commonly used autoconf macros, both PHP-specific and those built into autoconf, is given in the Zend Engine 2 API reference section.

Tip

When developing a PHP extension, it is strongly recommended that autoconf version 2.13 be installed, despite the newer releases which are available. Version 2.13 is recognized as a common denominator of autoconf availability, usability, and user base. Using later versions will sometimes produce cosmetic differences from the expected output of configure.

Example#1 An example config.m4 file

    dnl $Id: configunix.xml,v 1.3 2007/06/28 04:45:10 gwynne Exp $
    dnl config.m4 for extension example

    PHP_ARG_WITH(example, for example support,
    [  --with-example[=FILE]       Include example support. File is the optional path to example-config])
    PHP_ARG_ENABLE(example-debug, whether to enable debugging support in example,
    [  --enable-example-debug        example: Enable debugging support in example], no, no)
    PHP_ARG_WITH(example-extra, for extra libraries for example,
    [  --with-example-extra=DIR      example: Location of extra libraries for example], no, no)

    dnl Check whether the extension is enabled at all
    if test "$PHP_EXAMPLE" != "no"; then

      dnl Check for example-config. First try any path that was given to us, then look in $PATH
      AC_MSG_CHECKING([for example-config])
      EXAMPLE_CONFIG="example-config"
      if test "$PHP_EXAMPLE" != "yes"; then
        EXAMPLE_PATH=$PHP_EXAMPLE
      else
        EXAMPLE_PATH=`$php_shtool path $EXAMPLE_CONFIG`
      fi

      dnl If a usable example-config was found, use it
      if test -f "$EXAMPLE_PATH" && test -x "$EXAMPLE_PATH" && $EXAMPLE_PATH --version > /dev/null 2>&1; then
        AC_MSG_RESULT([$EXAMPLE_PATH])
        EXAMPLE_LIB_NAME=`$EXAMPLE_PATH --libname`
        EXAMPLE_INCDIRS=`$EXAMPLE_PATH --incdirs`
        EXAMPLE_LIBS=`$EXAMPLE_PATH --libs`

        dnl Check that the library works properly
        PHP_CHECK_LIBRARY($EXAMPLE_LIB_NAME, example_critical_function,
        [
          dnl Add the necessary include dirs
          PHP_EVAL_INCLINE($EXAMPLE_INCDIRS)
          dnl Add the necessary libraries and library dirs
          PHP_EVAL_LIBLINE($EXAMPLE_LIBS, EXAMPLE_SHARED_LIBADD)
        ],[
          dnl Bail out
          AC_MSG_ERROR([example library not found. Check config.log for more information.])
        ],[$EXAMPLE_LIBS]
        )
      else
        dnl No usable example-config, bail
        AC_MSG_RESULT([not found])
        AC_MSG_ERROR([Please check your example installation.])
      fi

      dnl Check whether to enable debugging
      if test "$PHP_EXAMPLE_DEBUG" != "no"; then
        dnl Yes, so set the C macro
        AC_DEFINE(USE_EXAMPLE_DEBUG,1,[Include debugging support in example])
      fi

      dnl Check for the extra support
      if test "$PHP_EXAMPLE_EXTRA" != "no"; then
        if test "$PHP_EXAMPLE_EXTRA" == "yes"; then
          AC_MSG_ERROR([You must specify a path when using --with-example-extra])
        fi

        PHP_CHECK_LIBRARY(example-extra, example_critical_extra_function,
        [
          dnl Add the neccessary paths
          PHP_ADD_INCLUDE($PHP_EXAMPLE_EXTRA/include)
          PHP_ADD_LIBRARY_WITH_PATH(example-extra, $PHP_EXAMPLE_EXTRA/lib, EXAMPLE_SHARED_LIBADD)
          AC_DEFINE(HAVE_EXAMPLEEXTRALIB,1,[Whether example-extra support is present and requested])
          EXAMPLE_SOURCES="$EXAMPLE_SOURCES example_extra.c"
        ],[
          AC_MSG_ERROR([example-extra lib not found. See config.log for more information.])
        ],[-L$PHP_EXAMPLE_EXTRA/lib]
        )
      fi

      dnl Finally, tell the build system about the extension and what files are needed
      PHP_NEW_EXTENSION(example, example.c $EXAMPLE_SOURCES, $ext_shared)
      PHP_SUBST(EXAMPLE_SHARED_LIBADD)
    fi

### A short introduction to autoconf syntax

config.m4 files are written using the GNU autoconf syntax. It can be described in a nutshell as shell scripting augmented by a powerful macro language. Comments are delimited by the string dnl, and strings are quoted using left and right brackets (e.g. [ and ]). Quoting of strings can be nested as many times as needed. A full reference to the syntax can be found in the autoconf manual at » http://www.gnu.org/software/autoconf/manual/.

### PHP_ARG_*: Giving users the option

The very first thing seen in the example config.m4 above, aside from a couple of comments, are three lines using PHP_ARG_WITH() and PHP_ARG_ENABLE(). These provide configure with the options and help text seen when running ./configure --help. As the names suggest, the difference between the two is whether they create a --with-* option or an --enable-* option. Every extension should provide at least one or the other with the extension name, so that users can choose whether or not to build the extension into PHP. By convention, PHP_ARG_WITH() is used for an option which takes a parameter, such as the location of a library or program required by an extension, while PHP_ARG_ENABLE() is used for an option which represents a simple flag.

Example#2 Sample configure output

    $ ./configure --help
    ...
      --with-example[=FILE]       Include example support. FILE is the optional path to example-config
      --enable-example-debug        example: Enable debugging support in example
      --with-example-extra=DIR      example: Location of extra libraries for example
    ...

    $ ./configure --with-example=/some/library/path/example-config --disable-example-debug --with-example-extra=/another/library/path
    ...
    checking for example support... yes
    checking whether to enable debugging support in example... no
    checking for extra libraries for example... /another/library/path
    ...

> Note: Regardless of the order in which options are specified on the command line when configure is called, the checks will be run in the order they are specified in config.m4.

### Processing the user's choices

Now that config.m4 can provide the user with some choices of what to do, it's time to act upon those choices. In the example above, the obvious default for all three options, if any of them are unspecified, is "no". As a matter of convention, it is best to use this as the default for the option which enables the extension, as it will be overridden by phpize for extensions built separately, and should not clutter the extension space by default when being built into PHP. The code to process the three options is by far the most complicated.

#### Handling the --with-example[=FILE] option

The first check made of the --with-example[=FILE] option is whether it was set at all. As this option controls the inclusion of the entire extension, if it was unspecified, given in the negative form (--without-example), or given the value "no", nothing else is done at all. In the example above, it is specified with the value /some/library/path/example-config, so the first test succeeds.

Next, the code calls AC_MSG_CHECKING(), an autoconf macro which outputs a standard "checking for something" line, and checks whether the user gave an explicit path to the fictional example-config. In this example, PHP_EXAMPLE got the value /some/library/path/example-config, which is now copied into the EXAMPLE_PATH variable. Had the user specified only --with-example, the code would have executed $php_shtool path $EXAMPLE_CONFIG, which would try to guess the location of example-config using the user's current PATH. Either way, the next step is to check whether the chosen EXAMPLE_PATH is a regular file, is executable, and can be run successfully. If so, AC_MSG_RESULT() is called, which completes the output line started by AC_MSG_CHECKING(). Otherwise, AC_MSG_ERROR() is called, which prints the given message and halts configure immediately.

The code now determines some site-specific configuration information by running example-config several times. The next call is to PHP_CHECK_LIBRARY(), a macro provided by the PHP buildsystem as a wrapper around autoconf's AC_CHECK_LIB(). PHP_CHECK_LIBRARY() attempts to compile, link, and run a program which calls the symbol specified by the second parameter in the library specified by the first, using the string given in the fifth as extra linker options. If the attempt succeeds, the script given in the third parameter is run. This script tells the PHP buildsystem to extract include paths, library paths, and library names from the raw option strings example-config provided. If the attempt fails, the script in the fourth parameter is run instead. In this case, AC_MSG_ERROR() is called to stop processing.

#### Handling the --enable-example-debug option

Processing the --enable-example-debug is much simpler. A simple check for its truth value is performed. If that check succeeds, AC_DEFINE() is called to make the C macro USE_EXAMPLE_DEBUG available to the source of the extension. The third parameter is a comment string for config.h; it is safe to leave this empty, and often is.

#### Handling the --with-example-extra=DIR option

For the sake of this example, the fictional "extra" functionality requested by the --with-example-extra=DIR option does not share the fictional example-config program, nor does it have any default paths to search. Therefore, the user is required to provide the installation prefix of the necessary library. This setup is somewhat unlikely in a real-world extension, but is considered illustrative.

The code begins in a now-familiar way by checking the truth value of PHP_EXAMPLE_EXTRA. If a negative form was provided, no further processing is done; the user did not request extra functionality. If a positive form was provided without a parameter, AC_MSG_ERROR() is called to halt processing. The next step is another invocation of PHP_CHECK_LIBRARY(). This time, since there is no set of predefined compiler options provided, PHP_ADD_INCLUDE() and PHP_ADD_LIBRARY_WITH_PATH() are used to construct the necessary include paths, library paths, and library flags for the extra functionality. AC_DEFINE() is also called to indicate to the code that the extra functionality was both requested and available, and a variable is set to tell later code that there are extra source files to build. If the check fails, the familiar AC_MSG_ERROR() is called. A different way to handle the failure would have been to call AC_MSG_WARNING() instead, e.g.:

          AC_MSG_WARNING([example-extra lib not found. example will be built without extra functionality.])

In this case, configure would print a warning message rather than an error, and continue processing. Which way such failures are handled is a design decision left to the extension developer.

### Telling the buildsystem what was decided

With all the necessary includes and libraries specified, with all the options processed and macros defined, one more thing remains to be done: The build system must be told to build the extension itself, and which files are to be used for that. To do this, the PHP_NEW_EXTENSION() macro is called. The first parameter is the name of the extension, which is the same as the name of the directory containing it. The second parameter is the list of all source files which are part of the extension. See PHP_ADD_BUILD_DIR() for information about adding source files in subdirectories to the build process. The third parameter should always be $ext_shared, a value which was determined by configure when PHP_ARG_WITH() was called for --with-example[=FILE]. The fourth parameter specifies a "SAPI class", and is only useful for extensions which require the CGI or CLI SAPIs specifically. It should be left empty in all other cases. The fifth parameter specifies a list of flags to be added to CFLAGS while building the extension; the sixth is a boolean value which, if "yes", will force the entire extension to be built using $CXX instead of $CC. All parameters after the third are optional. Finally, PHP_SUBST() is called to enable shared builds of the extension. See Extension FAQs for more information on disabling support for building an extension in shared mode.