The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


munmap - unmap pages of memory


[MC3] [Option Start] #include <sys/mman.h>

int munmap(void *
addr, size_t len); [Option End]


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 shall require that addr be a multiple of the page size {PAGESIZE}.

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

[ML|MLR] [Option Start] Any memory locks (see mlock() and mlockall() ) associated with this address range shall be removed, as if by an appropriate call to munlock(). [Option End]

[TYM] [Option Start] 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. [Option End]

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


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

The following sections are informative.




The munmap() function is only supported if the Memory Mapped Files option or the Shared Memory Objects option is supported.


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.




mlock(), mlockall(), mmap() , posix_typed_mem_open(), sysconf(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <sys/mman.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Aligned with munmap() in the POSIX Realtime Extension as follows:

Issue 6

The munmap() function is marked as part of the Memory Mapped Files and Shared Memory Objects option.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes are made for alignment with IEEE Std 1003.1j-2000:

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/36 is applied, changing the margin code in the SYNOPSIS from MF|SHM to MC3 (notation for MF|SHM|TYM).

End of informative text.

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