mmap.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
       mmap() creates a new mapping in the virtual address space of the calling
       process.  The starting address for the new mapping is specified in start.
       The length argument specifies the length of the mapping.

       If start is NULL, then the kernel chooses the address at which to create
       the mapping; this is the most portable method of creating a new mapping.
       If start is not NULL, then the kernel takes it as a hint about where to
       place the mapping; on Linux, the mapping will be created at the next
       higher page boundary.  The address of the new mapping is returned as the
       result of the call.

       The contents of a file mapping (as opposed to an anonymous mapping; see
       MAP_ANONYMOUS below), are initialised using length bytes starting at
       offset offset in the file (or other object) referred to by the file
       descriptor fd.  offset must be a multiple of the page size as returned by
       sysconf(_SC_PAGE_SIZE).

       The prot argument describes the desired memory protection of the mapping
       (and must not conflict with the open mode of the file).  It is either
       PROT_NONE or the bitwise OR of one or more of the following 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 argument determines whether updates to the mapping are visible
       to other processes mapping the same region, and whether updates are
       caried through to the underlying file.  This behaviour is determined by
       including exactly one of the following values in flags:

       MAP_SHARED Share this mapping.  Updates to the mapping are visible to
                  other processes that map this file, and are carried through to
                  the underlying file.  The file may not actually be updated
                  until msync(2) or munmap(2) is called.

       MAP_PRIVATE
                  Create a private copy-on-write mapping.  Updates to the
                  mapping are not visible to other processes mapping the same
                  file, and are not carried through to the underlying file.  It
                  is unspecified whether changes made to the file after the
                  mmap() call are visible in the mapped region.

       Both of these flags are described in POSIX.1-2001.

       In addition, zero or more of the following values can be ORed in flags:

       MAP_FIXED
              Don't interpret start as a hint: place the mapping at exactly that
              address.  start must be a multiple of the page size.  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 specified address cannot be
              used, mmap() will fail.  Because requiring a fixed address for a
              mapping is less portable, the use of this option is discouraged.

       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 virtual memory system
              that the mapping should extend downwards in memory.

       MAP_ANON
              Synonym for MAP_ANONYMOUS.  Deprecated.

       MAP_ANONYMOUS
              The mapping is not backed by any file; its contents are
              initialised to zero.  The fd and offset arguments are ignored;
              however, some implementations require fd to be -1 if MAP_ANONYMOUS
              (or MAP_ANON) is specified, and portable applications should
              ensure this.  The use of MAP_ANONYMOUS in conjunction with
              MAP_SHARED is only supported on Linux since kernel 2.4.

       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
              supported on x86-64 for 64bit programs.

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

       Of the above flags, only MAP_FIXED is specified in POSIX.1-2001.
       However, most systems also support MAP_ANONYMOUS (or its synonym
       MAP_ANON).

       Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
       MAP_COPY, and MAP_LOCAL.

       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
       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, 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 implies 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, length, or offset (e.g., they are too large,
              or not aligned on a page boundary).

       EINVAL (since Linux 2.6.12), length was 0.

       EINVAL flags contained neither MAP_PRIVATE or MAP_SHARED, or contained
              both of these values.

       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
              mappings 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
              correspond 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.

       SUSv3 specifies that mmap() should fail if length is 0.  However, in
       kernels before 2.6.12, mmap() succeeded in this case: no mapping was
       created and the call returned start.  Since kernel 2.6.12, mmap() fails
       with the error EINVAL for this case.

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 2.6.19                       2006-12-04                            MMAP(2)