Implementation specifics

Writing shell code

It is necessary to adopt a technique of writing portable Bourne shell code. Often, shell programming tricks you might have learned are actually extensions provided by your favorite shell and are non-portable. When in doubt, check documentation or try the construct on another system's Bourne shell. For a thorough treatment of this topic, the chapter called Writing Portable Bourne Shell.

Using M4 correctly

Writing macros involves interacting with the M4 macro processor, which expands your macros when they are used in configure.in. It is crucial that your macros use M4 correctly-and in particular, that they quote strings correctly. the chapter called M4 for a thorough treatment of this topic.

Caching results

Autoconf provides a caching facility, whereby the results of a test may be stored in a cache file. The cache file is itself a Bourne shell script which is sourced by the configure script to set any `cache variables' to values that are present in the cache file.

The next time configure is run, the cache will be consulted for a prior result. If there is a prior result, the value is re-used and the code that performs that test is skipped. This speeds up subsequent runs of configure and configuration of deep trees, which can share a cache file in the top-level directory (see the chapter called How to run configure and make).

A custom macro is not required to do caching, though it is considered best practice. Sometimes it doesn't make sense for a macro to do caching-tests for system aspects which may frequently change should not be cached. For example, a test for free disk space should not employ caching as it is a dynamic characteristic.

The AC_CACHE_CHECK macro is a convient wrapper for caching the results of tests. You simply provide a description of the test, the name of a cache variable to store the test result to, and the body of the test. If the test has not been run before, the cache will be primed with the result. If the result is already in the cache, then the cache variable will be set and the test will be skipped.

Here is the code for an Autoconf macro that ties together many of the concepts introduced in this chapter:

# AC_PROG_CC_G # ------------ AC_DEFUN(AC_PROG_CC_G, [AC_CACHE_CHECK(whether ${CC-cc} accepts -g, ac_cv_prog_cc_g, [echo 'void f(){}' > conftest.c if test -z "${CC-cc} -g -c conftest.c 2>&1`"; then ac_cv_prog_cc_g=yes else ac_cv_prog_cc_g=no fi rm -f conftest* ])]) # AC_PROG_CC_G