In Darwin, much is built into the system library, /usr/lib/libSystem.dylib. In particular, the following libraries are included in libSystem.
The standard C library. This library contains the functions used by C programmers on all platforms.
The NetInfo library.
The kernel virtual memory library.
The math library, which contains arithmetic functions.
The POSIX threads library, which allows multiple tasks to run concurrently within a single program.
Symbolic links are provided as placeholders for these libraries. For example, libm.dylib is a symbolic link in /usr/lib that points to libSystem.dylib. Thus, -lm or -lpthread do no harm, but are unnecessary. The -lm option links to the math library, and -lpthread links to the POSIX threads library. Since libSystem provides these functions, you don’t need to use these options. However, you should use them to make sure your application is portable to other systems. (Since libm.dylib and libpthread.dylib are symbolic links to libSystem.dylib, the extra -l options will refer to the same library.)
In Mac OS X 10.1 and earlier versions, the curses screen library (a set of functions for controlling a terminal display) was part of libSystem.dylib. In Mac OS X 10.2 (Jaguar), the ncurses library (/usr/lib/libncurses.5.dylib) took the place of curses. You may still encounter source code releases that look for curses in libSystem.dylib, which will result in linking errors. You ...