c/program/system

Calls the host environment's command processor with the parameter. Returns an implementation-defined value (usually the value that the invoked program returns).

If command is a null pointer, checks if the host environment has a command processor and returns a nonzero value if and only if the command processor exists.

Return value
Implementation-defined value. If is a null pointer, returns a nonzero value if and only if the command processor exists.

Example
In this example there is a system call of the unix command date +%A and a system call to (possibly installed) gcc compiler with command-line argument (--version):