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 (which must be greater than 0).

       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

       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:

              Share this mapping.  Updates to the mapping are visible to
              other processes mapping the same region, and (in the case of
              file-backed mappings) are carried through to the underlying
              file.  (To precisely control when updates are carried through
              to the underlying file requires the use of msync(2).)

              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 and POSIX.1-2008.

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

       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 2 GB 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.

              Synonym for MAP_ANONYMOUS.  Deprecated.

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

              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.)

              This flag is ignored.

              Compatibility flag.  Ignored.

              Don't interpret addr as a hint: place the mapping at exactly
              that address.  addr must be suitably aligned: for most
              architectures a multiple of the page size is sufficient;
              however, some architectures may impose additional
              restrictions.  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.  Software
              that aspires to be portable should use this option with care,
              keeping in mind that the exact layout of a process's memory
              mappings is allowed to change significantly between kernel
              versions, C library versions, and operating system releases.

              Furthermore, this option is extremely hazardous (when used on
              its own), because it forcibly removes preexisting mappings,
              making it easy for a multithreaded process to corrupt its own
              address space.

              For example, thread A looks through /proc/<pid>/maps and
              locates an available address range, while thread B
              simultaneously acquires part or all of that same address
              range.  Thread A then calls mmap(MAP_FIXED), effectively
              overwriting the mapping that thread B created.

              Thread B need not create a mapping directly; simply making a
              library call that, internally, uses dlopen(3) to load some
              other shared library, will suffice.  The dlopen(3) call will
              map the library into the process's address space.
              Furthermore, almost any library call may be implemented in a
              way that adds memory mappings to the address space, either
              with this technique, or by simply allocating memory.  Examples
              include brk(2), malloc(3), pthread_create(3), and the PAM
              libraries ⟨⟩.

              This flag is used for stacks.  It indicates to the kernel vir‐
              tual memory system that the mapping should extend downward in
              memory.  The return address is one page lower than the memory
              area that is actually created in the process's virtual address
              space.  Touching an address in the "guard" page below the map‐
              ping will cause the mapping to grow by a page.  This growth
              can be repeated until the mapping grows to within a page of
              the high end of the next lower mapping, at which point touch‐
              ing the "guard" page will result in a SIGSEGV signal.

       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, as well as NOTES, below.

       MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux 3.8)
              Used in conjunction with MAP_HUGETLB to select alternative
              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems
              that support multiple hugetlb page sizes.

              More generally, the desired huge page size can be configured
              by encoding the base-2 logarithm of the desired page size in
              the six bits at the offset MAP_HUGE_SHIFT.  (A value of zero
              in this bit field provides the default huge page size; the
              default huge page size can be discovered vie the Hugepagesize
              field exposed by /proc/meminfo.)  Thus, the above two con‐
              stants are defined as:

                  #define MAP_HUGE_2MB    (21 << MAP_HUGE_SHIFT)
                  #define MAP_HUGE_1GB    (30 << MAP_HUGE_SHIFT)

              The range of huge page sizes that are supported by the system
              can be discovered by listing the subdirectories in /sys/ker‐

       MAP_LOCKED (since Linux 2.5.37)
              Mark the mapped region to be locked in the same way as
              mlock(2).  This implementation will try to populate (prefault)
              the whole range but the mmap() call doesn't fail with ENOMEM
              if this fails.  Therefore major faults might happen later on.
              So the semantic is not as strong as mlock(2).  One should use
              mmap() plus mlock(2) when major faults are not acceptable
              after the initialization of the mapping.  The MAP_LOCKED flag
              is ignored in older kernels.

       MAP_NONBLOCK (since Linux 2.5.46)
              This flag is meaningful only 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 reimple‐

              Do not reserve swap space for this mapping.  When swap space
              is reserved, one has the guarantee that it is possible to mod‐
              ify 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/overcom‐
              mit_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.  This will help
              to reduce blocking on page faults later.  MAP_POPULATE is sup‐
              ported 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 architec‐
              tures 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_UNINI‐
              TIALIZED 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

       Of the above flags, only MAP_FIXED is specified in POSIX.1-2001 and
       POSIX.1-2008.  However, most systems also support MAP_ANONYMOUS (or
       its synonym MAP_ANON).

       Memory mapped by mmap() is preserved across fork(2), with the same

       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 addr must be a multiple of the page size (but length need
       not be).  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.

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 to indicate the cause of the error.

       On success, munmap() returns 0.  On failure, it returns -1, and errno
       is set to indicate the cause of the error (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

       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-wide 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.

       ENOMEM The process's maximum number of mappings would have been
              exceeded.  This error can also occur for munmap(), when
              unmapping a region in the middle of an existing mapping, since
              this results in two smaller mappings on either side of the
              region being unmapped.

       ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described
              in getrlimit(2), would have been exceeded.

              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).

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

       EPERM  The operation was prevented by a file seal; see fcntl(2).

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

       Use of a mapped region can result in these signals:

              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).

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface          Attribute     Value   │
       │mmap(), munmap()   │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.

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

       Certain flags constants are defined only if suitable feature test
       macros are defined (possibly by default): _DEFAULT_SOURCE with glibc
       2.19 or later; or _BSD_SOURCE or _SVID_SOURCE in glibc 2.19 and
       earlier.  (Employing _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,

       An application can determine which pages of a mapping are currently
       resident in the buffer/page cache using mincore(2).

   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.

   Huge page (Huge TLB) mappings
       For mappings that employ huge pages, the requirements for the
       arguments of mmap() and munmap() differ somewhat from the
       requirements for mappings that use the native system page size.

       For mmap(), offset must be a multiple of the underlying huge page
       size.  The system automatically aligns length to be a multiple of the
       underlying huge page size.

       For munmap(), addr and length must both be a multiple of the
       underlying huge page size.

   C library/kernel 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(5) (for example, when using the 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)

       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]);

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

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

           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");

           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)

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

               fprintf(stderr, "partial write");

           munmap(addr, length + offset - pa_offset);


SEE ALSO         top

       ftruncate(2), getpagesize(2), memfd_create(2), mincore(2), mlock(2),
       mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2),
       setrlimit(2), shmat(2), userfaultfd(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 4.15 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

Linux                            2017-12-18                          MMAP(2)

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