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MMAP !!!MMAP NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS CONFORMING TO SEE ALSO ---- !!NAME mmap, munmap - map or unmap files or devices into memory !!SYNOPSIS __#include __ #include __ __#ifdef _POSIX_MAPPED_FILES__ __void * mmap(void *__''start''__, size_t__ ''length''__, int__ ''prot'' __, int__ ''flags''__, int__ ''fd''__, off_t__ ''offset''__);__ __int munmap(void *__''start''__, size_t__ ''length''__);__ __#endif__ !!DESCRIPTION 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 __PROT_EXEC__ Pages may be executed. __PROT_READ__ Pages may be read. __PROT_WRITE__ Pages may be written. __PROT_NONE__ 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 __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. __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. __MAP_PRIVATE__ 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__ is preserved across fork(2), with the same attributes. 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 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. !!RETURN VALUE 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__ returns 0, on failure -1, and ''errno'' is set (probably to EINVAL). !!ERRORS __EBADF__ ''fd'' is not a valid file descriptor (and MAP_ANONYMOUS was not set). __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) mode. __EINVAL__ We don't like ''start'' or ''length'' or ''offset''. (E.g., they are too large, or not aligned on a PAGESIZE boundary.) __ETXTBSY__ MAP_DENYWRITE was set but the object specified by ''fd'' is open for writing. __EAGAIN__ The file has been locked, or too much memory has been locked. __ENOMEM__ No memory is available. Use of a mapped region can result in these signals: __SIGSEGV__ Attempted write into a region specified to mmap as read-only. __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). !!CONFORMING TO SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD. Svr4 documents additional error codes ENXIO and ENODEV. !!SEE ALSO getpagesize(2), msync(2), __shm_open__(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. ----
6 pages link to
munmap(2)
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Man2m
syscalls(2)
clone(2)
mmap(2)
msync(2)
madvise(2)
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