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Asked: 2026-05-27T06:15:34+00:00

In a makefile I work on, gcc is used with the -D XOPEN_SOURCE=500 and

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In a makefile I work on, gcc is used with the -D XOPEN_SOURCE=500 and -D_BSD_SOURCE options. gcc --help does not tell me what these are; a quick google search didn’t help either. I’m quite a newbie with gcc, could someone give me a hint?

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  1. Editorial Team

    According to the GCC documentation (“3.11 Options Controlling the Preprocessor”), the -D switch defines the macros XOPEN_SOURCE and _BSD_SOURCE with the values 500 and 1 respectively. It’s as though you have this code at the beginning of all the source files you pass to GCC:

    #define XOPEN_SOURCE 500
#define _BSD_SOURCE 1

    Build scripts usually take advantage of the compiler’s ability to “insert” macros like these to “communicate” to the source code details about the platform being targeted (e.g. operating system version).

    The “opposite” command-line switch for -D is -U, which #undefs a macro.

    Most (if not all) modern C/C++ compilers include similar switches. For example, Visual C++ compilers accept the /D compiler switch, which essentially serves the same purpose as GCC’s -D switch.

    For future reference, the GCC option index is great if you need to look up compiler switches for the GCC compiler.

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