warning: implicit declaration of function

Question

My compiler (GCC) is giving me the warning:

warning: implicit declaration of function

Please help me understand why is it coming

Answer 1

If you have the correct headers defined & are using a non GlibC library (such as Musl C) gcc will also throw error: implicit declaration of function when GNU extensions such as malloc_trim are encountered.

The solution is to wrap the extension & the header:

#if defined (__GLIBC__)
  malloc_trim(0);
#endif