Home Man
Search
Today's Posts
Register

Linux & Unix Commands - Search Man Pages

RedHat 9 (Linux i386) - man page for munmap (redhat section 2)

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 under-
	      lying 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 argu-
       ment 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  map-
       ping 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)


All times are GMT -4. The time now is 04:05 PM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
UNIX.COM Login
Username:
Password:  
Show Password