free - Man Page

free allocated memory


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <stdlib.h>

void free(void *ptr);


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

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 a function in POSIX.1-2008 that allocates memory as if by malloc(), 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.

Return Value

The free() function shall not return a value.


No errors are defined.

The following sections are informative.



Application Usage

There is now no requirement for the implementation to support the inclusion of <malloc.h>.



Future Directions


See Also

calloc(), malloc(), posix_memalign(), realloc()

The Base Definitions volume of POSIX.1-2017, <stdlib.h>

Referenced By

calloc(3p), getdelim(3p), malloc(3p), open_memstream(3p), posix_memalign(3p), putenv(3p), realloc(3p), realpath(3p), stdlib.h(0p), strdup(3p), tempnam(3p), wcsdup(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual