*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->Linux man pages -> mmap (2)              



NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

       #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);


DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]
		  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,

       offset should ordinarily be a multiple of the  page  size  returned  by

       Memory  mapped  by  mmap  is  preserved	across	fork(2), with the same

       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    [Toc]    [Back]

       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

ERRORS    [Toc]    [Back]

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

       EACCES 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.

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

       ETXTBSY    [Toc]    [Back]
	      MAP_DENYWRITE was set but the object specified by fd is open for

       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    [Toc]    [Back]
	      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

CONFORMING TO    [Toc]    [Back]

       SVr4,  POSIX.1b	(formerly POSIX.4), 4.4BSD.  Svr4 documents additional
       error codes ENXIO and ENODEV.

SEE ALSO    [Toc]    [Back]

       getpagesize(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)
[ Back ]
 Similar pages
Name OS Title
mmap FreeBSD allocate memory, or map files or devices into memory
mmap NetBSD map files or devices into memory
mmap OpenBSD map files or devices into memory
munmap IRIX unmap pages of memory
shmat NetBSD map/unmap shared memory
shmdt NetBSD map/unmap shared memory
munmap HP-UX unmap pages of memory
shmat OpenBSD map/unmap shared memory
shmdt OpenBSD map/unmap shared memory
readl OpenBSD Alpha devices I/O ports and memory access functions
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service