MMAP(2)

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

SYNOPSIS
#include <sys/mman.h>

void * mmap(void *start, size_t length, int prot , int flags, int fd, off_t offset);

int munmap(void *start, size_t length);

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 memory region specified by start and len overlaps pages of any existing mapping(s), then the overlapped part of the existing mapping(s) will be discarded. 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 (since Linux 2.5.37)
    Lock the pages of the mapped region into memory in the manner of mlock(). This flag is ignored in older kernels.

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.

MAP_32BIT
    Put the mapping into the first 2GB of the process address space. Ignored when MAP_FIXED is set. This flag is currently only supported on x86-64 for 64bit programs.

MAP_POPULATE (since Linux 2.5.46)
    Populate (prefault) pagetables.

MAP_NONBLOCK (since Linux 2.5.46)
    Do not block on IO.

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, 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).

NOTES
It 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.

ERRORS

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.

EAGAIN
    The file has been locked, or too much memory has been locked (see setrlimit(2)).

EBADF
    fd is not a valid file descriptor (and MAP_ANONYMOUS was not set).

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

ENFILE
    The system limit on the total number of open files has been reached.

ENODEV
    The underlying filesystem of the specified file does not support memory mapping.

ENOMEM
    No memory is available, or the process's maximum number of mappings would have been exceeded.

EPERM
    The prot argument asks for PROT_EXEC but the mapped area belongs to a file on a filesystem that was mounted no-exec.

ETXTBSY
    MAP_DENYWRITE was set but the object specified by fd is open for writing.

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).

AVAILABILITY
On 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 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.

BUGS
On Linux there are no guarantees like those suggested above under MAP_NORESERVE. By default, any process can be killed at any moment when the system runs out of memory.

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