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Annotated edit history of munmap(2) version 5, including all changes. View license author blame.
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1 perry 1 !!NAME
2
3 mmap, munmap - map or unmap files or devices into memory
5 PerryLorier 4
1 perry 5 !!SYNOPSIS
5 PerryLorier 6 __#include <unistd.h>__
7 __#include <sys/mman.h>__
1 perry 8
5 PerryLorier 9 __#ifdef _POSIX_MAPPED_FILES__
1 perry 10
5 PerryLorier 11 __void * mmap(void *__''start''__, size_t__ ''length''__, int__ ''prot'' __, int__ ''flags''__, int__ ''fd''__, off_t__ ''offset''__);__
1 perry 12
5 PerryLorier 13 __int munmap(void *__''start''__, size_t__ ''length''__);__
1 perry 14
5 PerryLorier 15 __#endif__
1 perry 16
17 !!DESCRIPTION
5 PerryLorier 18 The __mmap__ 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
1 perry 19
5 PerryLorier 20 ;__PROT_EXEC__: Pages may be executed.
21 ;__PROT_READ__: Pages may be read.
22 ;__PROT_WRITE__: Pages may be written.
23 ;__PROT_NONE__: Pages may not be accessed.
1 perry 24
5 PerryLorier 25 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
1 perry 26
5 PerryLorier 27 ;__MAP_FIXED__: Do not select a different address than the one specified. If the specified address cannot be used, __mmap__ will fail. If MAP_FIXED is specified, ''start'' must be a multiple of the pagesize. Use of this option is discouraged.
1 perry 28
5 PerryLorier 29 ;__MAP_SHARED__: 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.
1 perry 30
5 PerryLorier 31 ;__MAP_PRIVATE__: Create a private copy-on-write mapping. Stores to the region do not affect the original file.
1 perry 32
5 PerryLorier 33 You must specify exactly one of MAP_SHARED and MAP_PRIVATE.
1 perry 34
5 PerryLorier 35 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
1 perry 36 MAP_ANON(YMOUS).
37
5 PerryLorier 38 ''offset'' should ordinarily be a multiple of the page size returned by getpagesize(2).
1 perry 39
5 PerryLorier 40 Memory mapped by __mmap__ is preserved across fork(2), with the same attributes.
1 perry 41
5 PerryLorier 42 The __munmap__ system call deletes the mappings for the specified address range, and causes further references to addresses within the range to generate invalid memory
43 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.
1 perry 44
45 !!RETURN VALUE
5 PerryLorier 46 On success, __mmap__ returns a pointer to the mapped area. On error, MAP_FAILED (-1) is returned, and ''errno'' is set appropriately. On success, __munmap__
47 returns 0, on failure -1, and ''errno'' is set (probably to [EINVAL]).
1 perry 48 !!ERRORS
49
5 PerryLorier 50 ;[EBADF]: ''fd'' is not a valid file descriptor (and MAP_ANONYMOUS was not set).
51 ;[EACCES]: 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)
1 perry 52 mode.
5 PerryLorier 53 ;[EINVAL]: We don't like ''start'' or ''length'' or ''offset''. (E.g., they are too large, or not aligned on a PAGESIZE boundary.)
54 ;[ETXTBSY]: MAP_DENYWRITE was set but the object specified by ''fd'' is open for writing.
55 ;[EAGAIN]: The file has been locked, or too much memory has been locked.
56 ;[ENOMEM]: No memory is available.
1 perry 57
5 PerryLorier 58 Use of a mapped region can result in these signals:
1 perry 59
5 PerryLorier 60 ;[SIGSEGV]: Attempted write into a region specified to mmap as read-only.
61 ;[SIGBUS]: 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).
1 perry 62
63 !!CONFORMING TO
5 PerryLorier 64 SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD. Svr4 documents additional error codes ENXIO and ENODEV.
1 perry 65
66 !!SEE ALSO
5 PerryLorier 67 getpagesize(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
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