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NAMEmmap, munmap - map or unmap files or devices into memory
void * mmap(void *start, size_t length, int prot , int flags, int fd, off_t offset);
int munmap(void *start, size_t length);
DESCRIPTIONThe 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, and is never 0.
The prot argument describes the desired memory protection (and must not conflict with the open mode of the file). It is either PROT_NONE or is the bitwise OR of one or more of the other PROT_* flags.
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
You must specify exactly one of MAP_SHARED and MAP_PRIVATE.
The above three flags are described in POSIX.1b (formerly POSIX.4) and SUSv2. Linux also knows about the following non-standard flags:
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV, MAP_COPY, and MAP_LOCAL.
fd should be a valid file descriptor, unless MAP_ANONYMOUS is set, in which case the argument is ignored.
offset should be a multiple of the page size as returned by getpagesize(2).
Memory mapped by mmap is preserved across fork(2), with the same attributes.
A file is mapped in multiples of the page size. For a file that is not a multiple of the page size, the remaining memory is zeroed when mapped, and writes to that region are not written out to the file. The effect of changing the size of the underlying file of a mapping on the pages that correspond to added or removed regions of the file is unspecified.
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.
The address start must be a multiple of the page size. All pages containing a part of the indicated range are unmapped, and subsequent references to these pages will generate SIGSEGV. It is not an error if the indicated range does not contain any mapped pages.
For file-backed mappings, the st_atime field for the mapped file may be updated at any time between the mmap() and the corresponding unmapping; the first reference to a mapped page will update the field if it has not been already.
The st_ctime and st_mtime field for a file mapped with PROT_WRITE and MAP_SHARED will be updated after a write to the mapped region, and before a subsequent msync() with the MS_SYNC or MS_ASYNC flag, if one occurs.
RETURN VALUEOn success, mmap returns a pointer to the mapped area. On error, the value MAP_FAILED (that is, (void *) -1) is returned, and errno is set appropriately. On success, munmap returns 0, on failure -1, and errno is set (probably to EINVAL).
NOTESIt is architecture dependent whether PROT_READ includes PROT_EXEC or not. Portable programs should always set PROT_EXEC if they intend to execute code in the new mapping.
Use of a mapped region can result in these signals:
AVAILABILITYOn 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).)
CONFORMING TOSVr4, POSIX.1b (formerly POSIX.4), 4.4BSD, SUSv2. SVr4 documents additional error codes ENXIO and ENODEV. SUSv2 documents additional error codes EMFILE and EOVERFLOW.
SEE ALSOgetpagesize(2), mlock(2), mmap2(2), mremap(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.