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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 that map this file, and
              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 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, 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(2) 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)
              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.   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).

       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 (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(2), 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.

       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.

       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).

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 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.

   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  (for  example,  when  using  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);
           }

           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), 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 4.04 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/.