C feature macros that stdlib.h may need

When reading the documentation for a C library function, you may see a note that the function requires a feature test macro to be defined. This is common for functions that are not part of the C standard library but are part of POSIX or another standard. The feature test macro tells the compiler to include the necessary headers and definitions for the function.

Often the “_DEFAULT_SOURCE” macro is sufficient. Try doing in C like:

#if defined(__linux__) && !defined(_DEFAULT_SOURCE)
#define _DEFAULT_SOURCE
#endif