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Asked: 2026-05-20T22:00:38+00:00

I have an application that uses the mmap system call, I was having an

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I have an application that uses the mmap system call, I was having an issue getting it to compile for hours looking as to why I was getting MAP_ANON and MAP_ANONYMOUS were undeclared, I had a smaller section of code that I used and I saw I could compile it just fine so I tried just a basic compile and that worked, I saw that it fails when you add -std=c99. Is there a specific reason that MAP_ANON and MAP_ANONYMOUS are not valid in the C99 standard? I know that they aren’t defined by POSIX but are defined by BSD SOURCE so I just want to know why that is.

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  1. Editorial Team

    You probably want -std=gnu99 instead of -std=c99. C99 mode explicitly disables (most) GNU extensions.

    I wrote a simple test:

    #include <sys/mman.h>

int a = MAP_ANONYMOUS;

    In C99 mode, it doesn’t find the value:

    $ gcc -std=c99 -c d.c
d.c:3:9: error: ‘MAP_ANONYMOUS’ undeclared here (not in a function)

    Whereas in Gnu99 mode, it does:

    $ gcc -std=gnu99 -c d.c
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