munmap - Man Page

unmap pages of 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 <sys/mman.h>

int munmap(void *addr, size_t len);


The munmap() function shall remove any mappings for those entire pages containing any part of the address space of the process starting at addr and continuing for len bytes. Further references to these pages shall result in the generation of a SIGSEGV signal to the process. If there are no mappings in the specified address range, then munmap() has no effect.

The implementation may require that addr be a multiple of the page size as returned by sysconf().

If a mapping to be removed was private, any modifications made in this address range shall be discarded.

Any memory locks (see mlock() and mlockall()) associated with this address range shall be removed, as if by an appropriate call to munlock().

If a mapping removed from a typed memory object causes the corresponding address range of the memory pool to be inaccessible by any process in the system except through allocatable mappings (that is, mappings of typed memory objects opened with the POSIX_TYPED_MEM_MAP_ALLOCATABLE flag), then that range of the memory pool shall become deallocated and may become available to satisfy future typed memory allocation requests.

A mapping removed from a typed memory object opened with the POSIX_TYPED_MEM_MAP_ALLOCATABLE flag shall not affect in any way the availability of that typed memory for allocation.

The behavior of this function is unspecified if the mapping was not established by a call to mmap().

Return Value

Upon successful completion, munmap() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.


The munmap() function shall fail if:


Addresses in the range [addr,addr+len) are outside the valid range for the address space of a process.


The len argument is 0.

The munmap() function may fail if:


The addr argument is not a multiple of the page size as returned by sysconf().

The following sections are informative.



Application Usage



The munmap() function corresponds to SVR4, just as the mmap() function does.

It is possible that an application has applied process memory locking to a region that contains shared memory. If this has occurred, the munmap() call ignores those locks and, if necessary, causes those locks to be removed.

Most implementations require that addr is a multiple of the page size as returned by sysconf().

Future Directions


See Also

mlock(), mlockall(), mmap(), posix_typed_mem_open(), sysconf()

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

Referenced By

_Exit(3p), mlock(3p), mlockall(3p), mmap(3p), shm_unlink(3p), sys_mman.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual