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NAME

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

SYNOPSIS

       #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

       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 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_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 and POSIX.1-2008.

       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.  Deprecated.

       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,  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
              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/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 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  mmaped  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, 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 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 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 (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

       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

       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-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 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

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

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

CONFORMING TO

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

AVAILABILITY

       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

       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).

   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

       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

       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

       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

       This  page  is  part of release 4.13 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/.