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MMAP(2)                             Linux Programmer's Manual                             MMAP(2)

       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 ker‐
       nel  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  pro‐
       cesses  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

       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.

                  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  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  con‐
              text-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 under‐
              lying file should fail with ETXTBUSY.  But this was a source  of  denial-of-service

              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 porta‐
              ble, 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 Documen‐
              tation/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: 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.  Later accesses to the mapping will not be blocked by page
              faults.  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.  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 map‐
       ping on the pages that correspond to added or removed regions of the file is unspecified.

       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.

       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).

       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 val‐

       ENFILE The system 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, or the process's maximum number of mappings would have been

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

              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).

       SVr4, 4.4BSD, POSIX.1-2001.

       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  archi‐
       tecture  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:
       LATE, and MAP_STACK.

   C library/kernel ABI differences
       This  page  describes the interface provided by the glibc mmap() wrapper function.  Origi‐
       nally, this function invoked a system call of the same name.  Since kernel 2.4, that  sys‐
       tem  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  con‐
       tent.   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 map‐
       ping 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)

       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),  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.

       This page is part of release 3.74 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/.

Linux                                       2014-08-19                                    MMAP(2)

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