Using Autoconf Macros to Check for Headers

In the context of Autotools/Autoconf, you can use the AC_CHECK_HEADER or AC_CHECK_HEADERS macros to test if specific headers are present in the system. These macros check for the existence of header files in the standard include paths or in paths specified by the user.

AC_CHECK_HEADER Macro

The AC_CHECK_HEADER macro checks for the presence of a single header file.

Syntax

AC_CHECK_HEADER(header-file, [action-if-found], [action-if-not-found], [includes])

• header-file: The name of the header file to check (e.g., stdio.h).

• action-if-found: Optional shell commands to execute if the header is found.

• action-if-not-found: Optional shell commands to execute if the header is not found.

• includes: Optional additional includes needed for the check.

Example

AC_CHECK_HEADER([stdio.h],
               [AC_DEFINE([HAVE_STDIO_H], [1], [Define to 1 if you have <stdio.h>.])],
               [AC_MSG_ERROR([stdio.h is required])])

AC_CHECK_HEADERS Macro

The AC_CHECK_HEADERS macro checks for the presence of multiple header files at once.

Syntax

AC_CHECK_HEADERS(header-file1 header-file2 ..., [action-if-found], [action-if-not-found], [includes])

• header-file1 header-file2 ...: A list of header files to check.

• action-if-found: Optional shell commands to execute if all headers are found.

• action-if-not-found: Optional shell commands to execute if any header is not found.

• includes: Optional additional includes needed for the check.

Example

AC_CHECK_HEADERS([stdio.h stdlib.h string.h],
                [AC_DEFINE([HAVE_STDIO_H], [1], [Define to 1 if you have <stdio.h>.])],
                [AC_MSG_ERROR([Required headers not found])])

Example Usage in configure.ac

Here’s how you might use AC_CHECK_HEADER in your configure.ac file:

AC_CHECK_HEADER([myheader.h],
               [AC_DEFINE([HAVE_MYHEADER_H], [1], [Define to 1 if you have <myheader.h>.])],
               [AC_MSG_ERROR([myheader.h is required])])

This will check for the presence of myheader.h and define a preprocessor macro HAVE_MYHEADER_H if it is found. If the header is not found, it will stop the configuration process with an error message.

Summary

• Use AC_CHECK_HEADER to check for a single header.

• Use AC_CHECK_HEADERS to check for multiple headers.

• These macros are essential for ensuring that the necessary headers are available on the system before proceeding with the build.