Haiku implements a standard library containing the standard functions from C (https://www.iso.org/standard/29237.html) and POSIX (https://pubs.opengroup.org/onlinepubs/9699919799/). These functions are available in libroot, which is linked in by default.

For convenience and POSIX compatibility, empty "m" and "pthread" libraries are also provided, but the math and thread functions are also defined in libroot. However, this is currently not the case for "rt" and "trace" (both also part of the standard libraries required in POSIX).

The C library also contains a number of GNU and BSD extensions to the C and POSIX standards. The BSD extensions are enabled by default unless a specific version of the C standard is requested from the compiler command line (for example by using the –std=c99 switch). In case you want to set the compiler to strict C99 but still use these extensions, you need to define _DEFAULT_SOURCE (with a #define preprocessor directive, or from the command line).

The GNU extensions are not enabled by default, but can be enabled by defining _GNU_SOURCE.

The GNU and BSD extensions are in a separate library, so the -lbsd linker flag may be needed to use them.

On the other hand, network socket functions (socket, connect, bind, accept, ...) are provided in a separate library, libnetwork.so. To use these functions you will need to use the -lnetwork linker command line switch (instead of the POSIX specified -lxnet).