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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 <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 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.  The prot argument describes the
       desired memory protection (and must not conflict with the open mode of
       the file). 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.  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.

       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, MAP_NORESERVE,
       MAP_LOCKED, MAP_GROWSDOWN and MAP_ANON(YMOUS).

       offset should ordinarily be a multiple of the page size returned by
       getpagesize(2).

       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.

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 in
              read/write (O_RDWR) mode.

       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.

       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.  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.

Linux 2.3.51                     25 March 2000                         MMAP(2)

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

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