Why gcc gives warning: implicit declaration of function qsort_r?

Question

I do include<stdlib.h> where qsort_r is given. And I use gcc -std=c99 -O3 myfun.c -o myfun to compile.

It compiles, links and runs well. I don't know why I got this warning and what is potential risk of this warning ?

BTW, my compiler is gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)

Answer 1

It does so because you use -std=c99 , there's no qsort_r function in stdlib.h in c99.

Use -std=gnu99 to make the extensions available, or add a #define _GNU_SOURCE to your source files before including the header files.

Answer 2

qsort_r is not supported by C99. Specification says nothing about it.

Language Standards Supported by GCC :

By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard ₁ . See Extensions to the C Language Family. Use of the -std options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with -std=gnu89 (for C89 with GNU extensions) or -std=gnu99 (for C99 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu89; this will change to -std=gnu99 in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.

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_{1. Emphasis is mine}