mmap(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLES | 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 (which must be greater than 0).

       If addr is NULL, then the kernel chooses the (page-aligned)
       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 kernel will pick a nearby page boundary (but always
       above or equal to the value specified by
       /proc/sys/vm/mmap_min_addr) and attempt to create the mapping
       there.  If another mapping already exists there, the kernel picks
       a new address that may or may not depend on the hint.  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).

       After the mmap() call has returned, the file descriptor, fd, can
       be closed immediately without invalidating the mapping.

       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
       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 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).)

       MAP_SHARED_VALIDATE (since Linux 4.15)
              This flag provides the same behavior as MAP_SHARED except
              that MAP_SHARED mappings ignore unknown flags in flags.
              By contrast, when creating a mapping using
              MAP_SHARED_VALIDATE, the kernel verifies all passed flags
              are known and fails the mapping with the error EOPNOTSUPP
              for unknown flags.  This mapping type is also required to
              be able to use some mapping flags (e.g., MAP_SYNC).

       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 MAP_SHARED and MAP_PRIVATE are described in POSIX.1-2001 and
       POSIX.1-2008.  MAP_SHARED_VALIDATE is a Linux extension.

       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 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.

       MAP_ANON
              Synonym for MAP_ANONYMOUS; provided for compatibility with
              other implementations.

       MAP_ANONYMOUS
              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.

       MAP_DENYWRITE
              This flag is ignored.  (Long ago—Linux 2.0 and earlier—it
              signaled that attempts to write to the underlying file
              should fail with ETXTBSY.  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 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 the
              MAP_FIXED flag 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.  Carefully read the
              discussion of this flag in NOTES!

       MAP_FIXED_NOREPLACE (since Linux 4.17)
              This flag provides behavior that is similar to MAP_FIXED
              with respect to the addr enforcement, but differs in that
              MAP_FIXED_NOREPLACE never clobbers a preexisting mapped
              range.  If the requested range would collide with an
              existing mapping, then this call fails with the error
              EEXIST.  This flag can therefore be used as a way to
              atomically (with respect to other threads) attempt to map
              an address range: one thread will succeed; all others will
              report failure.

              Note that older kernels which do not recognize the
              MAP_FIXED_NOREPLACE flag will typically (upon detecting a
              collision with a preexisting mapping) fall back to a "non-
              MAP_FIXED" type of behavior: they will return an address
              that is different from the requested address.  Therefore,
              backward-compatible software should check the returned
              address against the requested address.

       MAP_GROWSDOWN
              This flag is used for stacks.  It indicates to the kernel
              virtual 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 mapping 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 touching 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/admin-
              guide/mm/hugetlbpage.rst 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 via the Hugepagesize field exposed by
              /proc/meminfo.)  Thus, the above two constants 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/kernel/mm/hugepages.

       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 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.  This will
              help to reduce blocking on page faults later.
              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 on Linux.  However, by
              employing this flag, applications can ensure that they
              transparently obtain support if the flag is implemented in
              the future.  Thus, it is used in the glibc threading
              implementation to allow for the fact that some
              architectures may (later) require special treatment for
              stack allocations.  A further reason to employ this flag
              is portability: MAP_STACK exists (and has an effect) on
              some other systems (e.g., some of the BSDs).

       MAP_SYNC (since Linux 4.15)
              This flag is available only with the MAP_SHARED_VALIDATE
              mapping type; mappings of type MAP_SHARED will silently
              ignore this flag.  This flag is supported only for files
              supporting DAX (direct mapping of persistent memory).  For
              other files, creating a mapping with this flag results in
              an EOPNOTSUPP error.

              Shared file mappings with this flag provide the guarantee
              that while some memory is mapped writable in the address
              space of the process, it will be visible in the same file
              at the same offset even after the system crashes or is
              rebooted.  In conjunction with the use of appropriate CPU
              instructions, this provides users of such mappings with a
              more efficient way of making data modifications
              persistent.

       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
       and POSIX.1-2008.  However, most systems also support
       MAP_ANONYMOUS (or its synonym MAP_ANON).

   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 (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 set).

       EEXIST MAP_FIXED_NOREPLACE was specified in flags, and the range
              covered by addr and length clashes with an existing
              mapping.

       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 none of MAP_PRIVATE, MAP_SHARED or
              MAP_SHARED_VALIDATE.

       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.

       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).

       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).

       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 page of the buffer that lies beyond
              the end of the mapped file.  For an explanation of the
              treatment of the bytes in the page that corresponds to the
              end of a mapped file that is not a multiple of the page
              size, see NOTES.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────────┬───────────────┬─────────┐
       │Interface          Attribute     Value   │
       ├───────────────────┼───────────────┼─────────┤
       │mmap(), munmap()   │ Thread safety │ MT-Safe │
       └───────────────────┴───────────────┴─────────┘

CONFORMING TO         top

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

       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

       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 bytes in the
       partial page at the end of the mapping are zeroed when mapped,
       and modifications 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.

       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 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, MAP_EXECUTABLE, MAP_FILE, MAP_GROWSDOWN,
       MAP_HUGETLB, MAP_LOCKED, MAP_NONBLOCK, MAP_NORESERVE,
       MAP_POPULATE, and MAP_STACK.

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

   Using MAP_FIXED safely
       The only safe use for MAP_FIXED is where the address range
       specified by addr and length was previously reserved using
       another mapping; otherwise, the use of MAP_FIXED is hazardous
       because it forcibly removes preexisting mappings, making it easy
       for a multithreaded process to corrupt its own address space.

       For example, suppose that thread A looks through /proc/<pid>/maps
       in order to locate an unused address range that it can map using
       MAP_FIXED, while thread B simultaneously acquires part or all of
       that same address range.  When thread A subsequently employs
       mmap(MAP_FIXED), it will effectively clobber the mapping that
       thread B created.  In this scenario, 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 ⟨http://www.linux-pam.org⟩.

       Since Linux 4.17, a multithreaded program can use the
       MAP_FIXED_NOREPLACE flag to avoid the hazard described above when
       attempting to create a mapping at a fixed address that has not
       been reserved by a preexisting mapping.

   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)).

EXAMPLES         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);
           }

           munmap(addr, length + offset - pa_offset);
           close(fd);

           exit(EXIT_SUCCESS);
       }

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 5.10 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-12-21                        MMAP(2)

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