munmap(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>

       void *mmap(void *start, size_t length, int prot, int flags,
                  int fd, off_t offset);

       int munmap(void *start, size_t length);

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  memory  region  specified by start and len overlaps
                  pages of any existing mapping(s), then the  overlapped  part
                  of the existing mapping(s) will be discarded.  If the speci-
                  fied address cannot be used, mmap() will fail.  If MAP_FIXED
                  is  specified,  start  must  be a multiple of the page size.
                  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.1-2001.  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
              Do  not reserve swap space for this mapping.  When swap space is
              reserved, one has the guarantee that it is  possible  to  modify
              the  mapping.   When  swap  space  is not reserved one might get
              SIGSEGV upon a write if no physical memory  is  available.   See
              also  the  discussion of the file /proc/sys/vm/overcommit_memory
              in proc(5).  In kernels before 2.6, this flag  only  had  effect
              for private writable mappings.

       MAP_LOCKED (since Linux 2.5.37)
              Lock the pages of the mapped region into memory in the manner of
              mlock().  This flag is ignored in older kernels.

       MAP_GROWSDOWN
              Used for stacks. Indicates to the kernel VM system that the map-
              ping should extend downwards in memory.

       MAP_ANONYMOUS
              The  mapping  is not backed by any file; the fd and offset argu-
              ments are ignored.  The use of this  flag  in  conjunction  with
              MAP_SHARED is only supported on Linux since kernel 2.4.

       MAP_ANON
              Alias for MAP_ANONYMOUS.  Deprecated.

       MAP_FILE
              Compatibility flag. Ignored.

       MAP_32BIT
              Put the mapping into the first 2GB of the process address space.
              Ignored when MAP_FIXED is set.  This flag is currently only sup-
              ported on x86-64 for 64bit programs.

       MAP_POPULATE (since Linux 2.5.46)
              Populate  (prefault) page tables for a file mapping, by perform-
              ing read-ahead on the file.  Later accesses to the mapping  will
              not be bocked by page faults.

       MAP_NONBLOCK (since Linux 2.5.46)
              Only meaningful in conjunction with MAP_POPULATE.  Don't perform
              read-ahead: only create page tables entries for pages  that  are
              already present in RAM.

       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.   If
       MAP_ANONYMOUS  is  set,  then  fd  is  ignored on Linux.  However, some
       implementations require fd to be -1 if MAP_ANONYMOUS (or  MAP_ANON)  is
       specified, and portable applications should ensure this.

       offset  should  be  a multiple of the page size as returned by getpage-
       size(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  con-
       taining 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 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, the
       value MAP_FAILED (that is, (void *) -1) is returned, and errno  is  set
       appropriately.   On  success,  munmap()  returns  0, on failure -1, and
       errno is set (probably to EINVAL).

NOTES
       It is architecture dependent whether PROT_READ  includes  PROT_EXEC  or
       not.  Portable  programs  should always set PROT_EXEC if they intend to
       execute code in the new mapping.

ERRORS
       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.

       EAGAIN The file has been locked, or too much  memory  has  been  locked
              (see setrlimit(2)).

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

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

       ENFILE The  system  limit  on  the  total number of open files has been
              reached.

       ENODEV The underlying filesystem of the specified file does not support
              memory mapping.

       ENOMEM No  memory is available, or the process's maximum number of map-
              pings would have been exceeded.

       EPERM  The prot argument asks for PROT_EXEC but the mapped area belongs
              to a file on a filesystem that was mounted no-exec.

       ETXTBSY
              MAP_DENYWRITE was set but the object specified by fd is open for
              writing.

       Use of a mapped region can result in these signals:

       SIGSEGV
              Attempted write into a region mapped as read-only.

       SIGBUS Attempted access to a portion of the buffer that does not corre-
              spond  to  the  file  (for  example, beyond the end of the file,
              including the case  where  another  process  has  truncated  the
              file).

AVAILABILITY
       On  POSIX  systems on which mmap(), msync() and munmap() are available,
       _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0.
       (See also sysconf(3).)

CONFORMING TO
       SVr4, 4.4BSD, POSIX.1-2001.

BUGS
       On  Linux  there  are  no  guarantees  like those suggested above under
       MAP_NORESERVE.  By default, any process can be  killed  at  any  moment
       when the system runs out of memory.

       In  kernels before 2.6.7, the MAP_POPULATE flag only has effect if prot
       is specified as PROT_NONE.

SEE ALSO
       getpagesize(2), mincore(2), mlock(2),  mmap2(2),  mremap(2),  msync(2),
       remap_file_pages(2), setrlimit(2), shm_open(3)
       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

Linux 2.6.9                       2004-12-08                           MMAP(2)

Man(1) output converted with man2html
list of all man pages