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.