mmap - map or unmap files or devices into memory


  1. include <unistd.h>
  2. include <sys/mman.h>

void *mmap(void *start, size_t length, in prot, int flags, int fd, off_t offset);

void munmap(void *start, size_t length);

  1. endif


The mmap(2) function asks to map length bytes starting at offset offset from the file (or other object) specified by the file descriptor fd into memory, preferably at address start. This latter address is a hint only, and is usually specified as 0. The actual place where the object is mapped is returned by mmap. The prot argument describes the desired memory protection (and must not conflict with the open mode of the file). It has bits

Pages may be executed.
Pages may be read.
Pages may be written.
Pages may not be accessed.

The flags parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. It has bits

Do not select a different address than the one specified. If the specified address cannot be used, mmap(2) will fail. If MAP_FIXED is specified, start must be a multiple of the pagesize. Use of this option is discouraged.
Share this mapping with all other processes that map this object. Storing to the region is equivalent to writing to the file. The file may not actually be updated until msync(2) or munmap(2) are called.
Create a private copy-on-write mapping. Stores to the region do not affect the original file.

You must specify exactly one of MAP_SHARED and MAP_PRIVATE.

The above three flags are described in POSIX.1b (formerly POSIX.4). Linux also knows about MAP_DENYWRITE, MAP_EXECUTABLE, MAP_NORESERVE, MAP_LOCKED, MAP_GROWSDOWN and MAP_ANON(YMOUS).

offset should ordinarily be a multiple of the page size returned by getpagesize(2).

Memory mapped by mmap(2) is preserved across fork(2), with the same attributes.

The munmap(2) system call deletes the mappings for the specified address range, and causes further references to addresses within the range to generate invalid memory references. The region is also automatically unmapped when the process is terminated. On the other hand, closing the file descriptor does not unmap the region.


On success, mmap returns a pointer to the mapped area. On error, MAP_FAILED (-1) is returned, and errno is set appropriately.


fd is not a valid file descriptor (and MAP_ANONYMOUS was not set).
MAP_PRIVATE was requested, but fd is not open for reading. Or MAP_SHARED was requested and PROT_WRITE is set, but fd is not open in read/write (O_RDWR) mode.
We don't like start or length or offset. (E.g., they are too large, or not aligned on a PAGESIZE boundary.)
MAP_DENYWRITE was set but the object specified by fd is open for writing.
The file has been locked, or too much memory has been locked.
No memory is available.

Use of a mapped region can result in these signals:

Attempted write into a region specified to mmap as read-only.
Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file).


On POSIX systems on which mmap, msync and munmap are available, _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)


SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD, SUSv2. Svr4 documents additional error codes ENXIO and ENODEV. SUSv2 documents additional error codes EMFILE and EOVERFLOW.


getpagesize(2), msync(2), shm_open(2)?, B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

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