The free() function shall cause the space pointed to by ptr to be
deallocated; that is, made available for further allocation. If ptr is
a null pointer, no action shall occur. Otherwise, if the argument does not
match a pointer earlier returned by the calloc(), malloc(),
posix_memalign(), realloc(), or strdup() function, or if
the space has been deallocated by a call to free() or realloc(),
the behavior is undefined.
Any use of a pointer that refers to freed space results in undefined behavior.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html