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MMAP(2)                       Linux Programmer's Manual                       MMAP(2)

NAME         top

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

SYNOPSIS         top

       #include <sys/mman.h>

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

DESCRIPTION         top

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

       If addr 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 addr
       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 a nearby 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 initialized 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 carried
       through to the underlying file.  This behavior 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() 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_32BIT (since Linux 2.4.20, 2.6)
              Put the mapping into the first 2 Gigabytes of the process address
              space.  This flag is only supported on x86-64, for 64-bit programs.  It
              was added to allow thread stacks to be allocated somewhere in the first
              2GB of memory, so as to improve context-switch performance on some
              early 64-bit processors.  Modern x86-64 processors no longer have this
              performance problem, so use of this flag is not required on those
              systems.  The MAP_32BIT flag is ignored when MAP_FIXED is set.

       MAP_ANON
              Synonym for MAP_ANONYMOUS.  Deprecated.

       MAP_ANONYMOUS
              The mapping is not backed by any file; its contents are initialized 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_DENYWRITE
              This flag is ignored.  (Long ago, it signaled 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_FILE
              Compatibility flag.  Ignored.

       MAP_FIXED
              Don't interpret addr as a hint: place the mapping at exactly that
              address.  addr must be a multiple of the page size.  If the memory
              region specified by addr 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_GROWSDOWN
              Used for stacks.  Indicates to the kernel virtual memory system that
              the mapping should extend downward in memory.

       MAP_HUGETLB (since Linux 2.6.32)
              Allocate the mapping using "huge pages."  See the kernel source file
              Documentation/vm/hugetlbpage.txt for further information.

       MAP_LOCKED (since Linux 2.5.37)
              Lock the pages of the mapped region into memory in the manner of
              mlock(2).  This flag is ignored in older kernels.

       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.  Since Linux 2.6.23, this flag causes MAP_POPULATE to
              do nothing.  One day the combination of MAP_POPULATE and MAP_NONBLOCK
              may be reimplemented.

       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_POPULATE (since Linux 2.5.46)
              Populate (prefault) page tables for a mapping.  For a file mapping,
              this causes read-ahead on the file.  Later accesses to the mapping will
              not be blocked by page faults.  MAP_POPULATE is only supported for
              private mappings since Linux 2.6.23.

       MAP_STACK (since Linux 2.6.27)
              Allocate the mapping at an address suitable for a process or thread
              stack.  This flag is currently a no-op, but is used in the glibc
              threading implementation so that if some architectures require special
              treatment for stack allocations, support can later be transparently
              implemented for glibc.

       MAP_UNINITIALIZED (since Linux 2.6.33)
              Don't clear anonymous pages.  This flag is intended to improve
              performance on embedded devices.  This flag is only honored if the
              kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED option.
              Because of the security implications, that option is normally enabled
              only on embedded devices (i.e., devices where one has complete control
              of the contents of user memory).

       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.

munmap()

       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 addr 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.

Timestamps changes for file-backed mappings

       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(2) with the MS_SYNC or MS_ASYNC flag, if one occurs.

RETURN VALUE         top

       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).

ERRORS         top

       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 addr, 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 file system 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 file system 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).

CONFORMING TO         top

       SVr4, 4.4BSD, POSIX.1-2001.

AVAILABILITY         top

       On POSIX systems on which mmap(), msync(2) and munmap() are available,
       _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0.  (See
       also sysconf(3).)

NOTES         top

       Since kernel 2.4, this system call has been superseded by mmap2(2).  Nowadays,
       the glibc mmap() wrapper function invokes mmap2(2) with a suitably adjusted
       value for offset.

       On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ.  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.

       The portable way to create a mapping is to specify addr as 0 (NULL), and omit
       MAP_FIXED from flags.  In this case, the system chooses the address for the
       mapping; the address is chosen so as not to conflict with any existing
       mapping, and will not be 0.  If the MAP_FIXED flag is specified, and addr is 0
       (NULL), then the mapped address will be 0 (NULL).

BUGS         top

       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 addr.  Since kernel 2.6.12, mmap() fails with the error EINVAL
       for this case.

EXAMPLE         top

       The following program prints part of the file specified in its first command-
       line argument to standard output.  The range of bytes to be printed is
       specified via offset and length values in the second and third command-line
       arguments.  The program creates a memory mapping of the required pages of the
       file and then uses write(2) to output the desired bytes.

       #include <sys/mman.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

       int
       main(int argc, char *argv[])
       {
           char *addr;
           int fd;
           struct stat sb;
           off_t offset, pa_offset;
           size_t length;
           ssize_t s;

           if (argc < 3 || argc > 4) {
               fprintf(stderr, "%s file offset [length]\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           fd = open(argv[1], O_RDONLY);
           if (fd == -1)
               handle_error("open");

           if (fstat(fd, &sb) == -1)           /* To obtain file size */
               handle_error("fstat");

           offset = atoi(argv[2]);
           pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
               /* offset for mmap() must be page aligned */

           if (offset >= sb.st_size) {
               fprintf(stderr, "offset is past end of file\n");
               exit(EXIT_FAILURE);
           }

           if (argc == 4) {
               length = atoi(argv[3]);
               if (offset + length > sb.st_size)
                   length = sb.st_size - offset;
                       /* Can't display bytes past end of file */

           } else {    /* No length arg ==> display to end of file */
               length = sb.st_size - offset;

           }

           addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
                       MAP_PRIVATE, fd, pa_offset);
           if (addr == MAP_FAILED)
               handle_error("mmap");

           s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
           if (s != length) {
               if (s == -1)
                   handle_error("write");

               fprintf(stderr, "partial write");
               exit(EXIT_FAILURE);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       getpagesize(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2),
       msync(2), remap_file_pages(2), setrlimit(2), shmat(2), shm_open(3),
       shm_overview(7)
       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

COLOPHON         top

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       at http://www.kernel.org/doc/man-pages/.