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


       #include <sys/mman.h>

       void *mmap(void *start, size_t length, int prot, int flags,
                  int fd, off_t offset);
       int munmap(void *start, size_t length);


       mmap()  creates  a  new  mapping  in  the  virtual address space of the
       calling process.  The starting address for the new mapping is specified
       in start.  The length argument specifies the length of the mapping.

       If  start  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  start  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  the  next  higher 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

       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 caried 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.

                  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:

              Put the mapping into the first 2GB of the process address space.
              Ignored  when  MAP_FIXED  is  set.   This flag is currently only
              supported on x86-64, for 64-bit programs.

              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  only supported on Linux 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 start as a hint: place the  mapping  at  exactly
              that  address.   start  must be a multiple of the page size.  If
              the memory region specified by start 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 downwards in memory.

       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.

              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  file  mapping,  by
              performing read-ahead  on  the  file.   Later  accesses  to  the
              mapping will not be blocked by page faults.

       Of  the  above  flags,  only  MAP_FIXED  is  specified in POSIX.1-2001.
       However, most  systems  also  support  MAP_ANONYMOUS  (or  its  synonym

       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

       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 start 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.

       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


       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  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

       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

       EINVAL We don’t like start, 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

       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 exceeded.

       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

       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


       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 sysconf(3).)


       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.


       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 start.  Since kernel 2.6.12, mmap() fails
       with the error EINVAL for this case.


       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)

       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");

       } /* main */


       getpagesize(2), mincore(2), mlock(2),  mmap2(2),  mremap(2),  msync(2),
       remap_file_pages(2), setrlimit(2), shm_open(3)
       B.O. Gallmeister, POSIX.4, O’Reilly, pp. 128-129 and 389-391.


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