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 guaran-
	      tee 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 imple-
	      mented 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 ref-
       erences 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 unmap-
       ping; 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)