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MMAP(2)                    Linux Programmer's Manual                   MMAP(2)

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.

Linux 2.3.51                      2000-03-25                           MMAP(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | SEE ALSO

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