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


       #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.


       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:

              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

              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.

              Synonym for MAP_ANONYMOUS.  Deprecated.

              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.

              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  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.

              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.

              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

       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

       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.


       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).


       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

       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).

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

              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:

              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).


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

       │InterfaceAttributeValue   │
       │mmap(), munmap()   │ Thread safety │ MT-Safe │


       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


       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 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:

   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.


       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

       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)).


       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

   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");



       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.


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