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Linux / Unix Command: mmap
Command Library

NAME

mmap, munmap - map or unmap files or devices into memory  

SYNOPSIS

#include <sys/mman.h>

#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, 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.

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. It is unspecified whether changes made to the file after the mmap call are visible in the mapped region.

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:

MAP_DENYWRITE
This flag is ignored. (Long ago, it signalled that attempts to write to the underlying file should fail with ETXTBUSY. But this was a source of denial-of-service attacks.)
MAP_EXECUTABLE
This flag is ignored.
MAP_NORESERVE
(Used together with MAP_PRIVATE.) Do not reserve swap space pages for this mapping. When swap space is reserved, one has the guarantee that it is possible to modify this private copy-on-write region. When it is not reserved one might get SIGSEGV upon a write when no memory is available.
MAP_LOCKED
This flag is ignored.
MAP_GROWSDOWN
Used for stacks. Indicates to the kernel VM system that the mapping should extend downwards in memory.
MAP_ANONYMOUS
The mapping is not backed by any file; the fd and offset arguments are ignored. This flag in conjunction with MAP_SHARED is implemented since Linux 2.4.
MAP_ANON
Alias for MAP_ANONYMOUS. Deprecated.
MAP_FILE
Compatibility flag. Ignored.

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 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
A file descriptor refers to a non-regular file. Or 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. Or PROT_WRITE is set, but the file is append-only.
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, or the process's maximum number of mappings would have been exceeded.
ENODEV
The underlying filesystem of the specified file does not support memory mapping.

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, SUSv2. SVr4 documents additional error codes ENXIO and ENODEV. SUSv2 documents additional error codes EMFILE and EOVERFLOW.  

SEE ALSO

getpagesize(2), mmap2(2), mremap(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.


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