NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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);

       See NOTES for information on feature test macro requirements.

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 supported only 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 supported on Linux only 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 Linux 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: create page tables entries only 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 had effect only 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 supported for private mappings only 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 honored only 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 a file
              mapping 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 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.

       EOVERFLOW
              On 32-bit architecture together with the large file extension
              (i.e., using 64-bit off_t): the number of pages used for
              length plus number of pages used for offset would overflow
              unsigned long (32 bits).

       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

       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).

       Certain flags constants are defined only if either _BSD_SOURCE or
       _SVID_SOURCE is defined.  (Requiring _GNU_SOURCE also suffices, and
       requiring that macro specifically would have been more logical, since
       these flags are all Linux-specific.)  The relevant flags are:
       MAP_32BIT, MAP_ANONYMOUS (and the synonym MAP_ANON), MAP_DENYWRITE,
       MAP_EXECUTABLE, MAP_FILE, MAP_GROWSDOWN, MAP_HUGETLB, MAP_LOCKED,
       MAP_NONBLOCK, MAP_NORESERVE, MAP_POPULATE, and MAP_STACK.

   C library/kernel ABI differences
       This page describes the interface provided by the glibc mmap()
       wrapper function.  Originally, this function invoked a system call of
       the same name.  Since kernel 2.4, that system call has been
       superseded by mmap2(2), and nowadays the glibc mmap() wrapper
       function invokes mmap2(2) with a suitably adjusted value for offset.

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 has effect only 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.

       POSIX specifies that the system shall always zero fill any partial
       page at the end of the object and that system will never write any
       modification of the object beyond its end.  On Linux, when you write
       data to such partial page after the end of the object, the data stays
       in the page cache even after the file is closed and unmapped and even
       though the data is never written to the file itself, subsequent
       mappings may see the modified content.  In some cases, this could be
       fixed by calling msync(2) before the unmap takes place; however, this
       doesn't work on tmpfs (for example, when using POSIX shared memory
       interface documented in shm_overview(7)).

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.

   Program source
       #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)

       The descriptions of the following files in proc(5): /proc/[pid]/maps,
       /proc/[pid]/map_files, and /proc/[pid]/smaps.

       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

COLOPHON         top

       This page is part of release 3.72 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-08-19                          MMAP(2)