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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);

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.   The  file  may
                  not actually be updated until msync(2) or munmap() is called.

       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.

       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  only  supported  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  only
              supported on Linux 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  kernel  source  file
              Documentation/vm/hugetlbpage.txt for further information.

       MAP_LOCKED (since Linux 2.5.37)
              Lock the pages of the mapped region into memory in the manner  of  mlock(2).   This
              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: only
              create page tables entries 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 only had effect 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.  Later accesses to the mapping will not be blocked by page
              faults.  MAP_POPULATE is only supported for private mappings 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 only honored 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.  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.  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.

   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.

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 appropriately.  On success, munmap()
       returns 0, on failure -1, and errno is set (probably to EINVAL).

ERRORS

       EACCES A file descriptor refers to a non-regular file.  Or MAP_PRIVATE 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 limit on the total number of open files has been reached.

       ENODEV The underlying file system of the specified file does not support memory mapping.

       ENOMEM No memory is available, or the process's maximum number of mappings would have been
              exceeded.

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

       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).

CONFORMING TO

       SVr4, 4.4BSD, POSIX.1-2001.

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

       Since kernel 2.4, this system call has been superseded by mmap2(2).  Nowadays,  the  glibc
       mmap() wrapper function invokes mmap2(2) with a suitably adjusted value for offset.

       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).

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 only has effect 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.

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.

       #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),   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)
       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.