MMAP

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
SEE ALSO

NAME

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

SYNOPSIS

#include <unistd.h>
#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 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. 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

MAP_PRIVATE

Create a private copy-on-write mapping.

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 and MAP_ANON(YMOUS).

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.

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 asked, but fd is not open for reading. Or MAP_SHARED was asked and PROT_WRITE is set, fd is not open for writing.

EINVAL

We don’t like start or length or offset. (E.g., they are too large, or not aligned on a PAGESIZE boundary.)

ETXTBUSY

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.

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.