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 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.
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/.
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 44-2. Sample configure output
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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.
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.
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.
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.
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.:
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.
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
Extensions FAQs for more information
on disabling support for building an extension in shared mode.