2.7. Feature Test Macros

The headers define numerous POSIX.1 and XSI symbols, as we've described. But most implementations can add their own definitions to these headers, in addition to the POSIX.1 and XSI definitions. If we want to compile a program so that it depends only on the POSIX definitions and doesn't use any implementation-defined limits, we need to define the constant _POSIX_C_SOURCE. All the POSIX.1 headers use this constant to exclude any implementation-defined definitions when _POSIX_C_SOURCE is defined.

Previous versions of the POSIX.1 standard defined the _POSIX_SOURCE constant. This has been superseded by the _POSIX_C_SOURCE constant in the 2001 version of POSIX.1.

The constants _POSIX_C_SOURCE and _XOPEN_SOURCE are called feature test macros. All feature test macros begin with an underscore. When used, they are typically defined in the cc command, as in

   cc -D_POSIX_C_SOURCE=200112 file.c

This causes the feature test macro to be defined before any header files are included by the C program. If we want to use only the POSIX.1 definitions, we can also set the first line of a source file to

   #define _POSIX_C_SOURCE 200112

To make the functionality of Version 3 of the Single UNIX Specification available to applications, we need to define the constant _XOPEN_SOURCE to be 600. This has the same effect as defining _POSIX_C_SOURCE to be 200112L as far as POSIX.1 functionality is concerned.

The Single UNIX Specification defines the c99 utility as the interface to the C compilation environment. With it we can compile a file as follows:

   c99 -D_XOPEN_SOURCE=600 file.c -o file

To enable the 1999 ISO C extensions in the gcc C compiler, we use the -std=c99 option, as in

   gcc -D_XOPEN_SOURCE=600 -std=c99 file.c -o file

Another feature test macro is _ _STDC_ _, which is automatically defined by the C compiler if the compiler conforms to the ISO C standard. This allows us to write C programs that compile under both ISO C compilers and non-ISO C compilers. For example, to take advantage of the ISO C prototype feature, if supported, a header could contain

   #ifdef _ _STDC_ _
   void  *myfunc(const char *, int);
   #else
   void  *myfunc();
   #endif

Although most C compilers these days support the ISO C standard, this use of the _ _STDC_ _ feature test macro can still be found in many header files.