...
The C99 exit() function is used for normal program termination (see ERR04-C. Choose an appropriate termination strategy). Nested calls to exit() result in undefined behavior (see also undefined behavior #172 in Annex J). This can only occur when exit() is invoked from a function registered with atexit(), or when exit() is called from within a signal handler (see SIG30-C. Call only asynchronous-safe functions within signal handlers).
...