MMAP(2)                   (2020-12-21)                    MMAP(2)

     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 fd, off_t offset);
          int munmap(void *addr, size_t length);

          See NOTES for information on feature test macro require-
          ments.

     DESCRIPTION
          mmap() creates a new mapping in the virtual address space of
          the calling process.  The starting address for the new map-
          ping is specified in addr. The length argument specifies the
          length of the mapping (which must be greater than 0).

          If addr is NULL, then the kernel chooses the (page-aligned)
          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 kernel will pick a nearby
          page boundary (but always above or equal to the value speci-
          fied by /proc/sys/vm/mmap_min_addr) and attempt to create
          the mapping there.  If another mapping already exists there,
          the kernel picks a new address that may or may not depend on
          the hint.  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).

          After the mmap() call has returned, the file descriptor, fd,
          can be closed immediately without invalidating the mapping.

          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.

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          PROT_WRITE Pages may be written.

          PROT_NONE  Pages may not be accessed.

        The flags argument
          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 mapping the same region, and (in the
               case of file-backed mappings) 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_SHARED_VALIDATE (since Linux 4.15)
               This flag provides the same behavior as MAP_SHARED
               except that MAP_SHARED mappings ignore unknown flags in
               flags. By contrast, when creating a mapping using
               MAP_SHARED_VALIDATE, the kernel verifies all passed
               flags are known and fails the mapping with the error
               EOPNOTSUPP for unknown flags.  This mapping type is
               also required to be able to use some mapping flags
               (e.g., MAP_SYNC).

          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 underly-
               ing file.  It is unspecified whether changes made to
               the file after the mmap() call are visible in the
               mapped region.

          Both MAP_SHARED and MAP_PRIVATE are described in POSIX.1-
          2001 and POSIX.1-2008.  MAP_SHARED_VALIDATE is a Linux
          extension.

          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 pro-
               cess 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
               2 GB of memory, so as to improve context-switch perfor-
               mance on some early 64-bit processors.  Modern x86-64
               processors no longer have this performance problem, so

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               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; provided for compatibility
               with other implementations.

          MAP_ANONYMOUS
               The mapping is not backed by any file; its contents are
               initialized to zero.  The fd argument is ignored; how-
               ever, some implementations require fd to be -1 if
               MAP_ANONYMOUS (or MAP_ANON) is specified, and portable
               applications should ensure this.  The offset argument
               should be zero.  The use of MAP_ANONYMOUS in conjunc-
               tion with MAP_SHARED is supported on Linux only since
               kernel 2.4.

          MAP_DENYWRITE
               This flag is ignored.  (Long ago-Linux 2.0 and
               earlier-it signaled that attempts to write to the
               underlying file should fail with ETXTBSY.  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 suitably aligned:
               for most architectures a multiple of the page size is
               sufficient; however, some architectures may impose
               additional restrictions.  If the memory region speci-
               fied 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.

               Software that aspires to be portable should use the
               MAP_FIXED flag with care, keeping in mind that the
               exact layout of a process's memory mappings is allowed
               to change significantly between kernel versions, C
               library versions, and operating system releases.
               Carefully read the discussion of this flag in NOTES!
               .}f

          MAP_FIXED_NOREPLACE (since Linux 4.17)
               This flag provides behavior that is similar to
               MAP_FIXED with respect to the addr enforcement, but
               differs in that MAP_FIXED_NOREPLACE never clobbers a

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               preexisting mapped range.  If the requested range would
               collide with an existing mapping, then this call fails
               with the error EEXIST. This flag can therefore be used
               as a way to atomically (with respect to other threads)
               attempt to map an address range: one thread will suc-
               ceed; all others will report failure.

               Note that older kernels which do not recognize the
               MAP_FIXED_NOREPLACE flag will typically (upon detecting
               a collision with a preexisting mapping) fall back to a
               "non-MAP_FIXED" type of behavior: they will return an
               address that is different from the requested address.
               Therefore, backward-compatible software should check
               the returned address against the requested address.

          MAP_GROWSDOWN
               This flag is used for stacks.  It indicates to the ker-
               nel virtual memory system that the mapping should
               extend downward in memory.  The return address is one
               page lower than the memory area that is actually cre-
               ated in the process's virtual address space.  Touching
               an address in the "guard" page below the mapping will
               cause the mapping to grow by a page.  This growth can
               be repeated until the mapping grows to within a page of
               the high end of the next lower mapping, at which point
               touching the "guard" page will result in a SIGSEGV sig-
               nal.

          MAP_HUGETLB (since Linux 2.6.32)
               Allocate the mapping using "huge pages."  See the Linux
               kernel source file Documentation/admin-
               guide/mm/hugetlbpage.rst 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 alterna-
               tive 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 con-
               figured 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 dis-
               covered via 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

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               system can be discovered by listing the subdirectories
               in /sys/kernel/mm/hugepages.

          MAP_LOCKED (since Linux 2.5.37)
               Mark the mapped 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() 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)
               This flag is meaningful only 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 phys-
               ical 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 pri-
               vate 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 pro-
               cess or thread stack.

               This flag is currently a no-op on Linux.  However, by
               employing this flag, applications can ensure that they
               transparently obtain support if the flag is implemented
               in the future.  Thus, it is used in the glibc threading
               implementation to allow for the fact that some archi-
               tectures may (later) require special treatment for
               stack allocations.  A further reason to employ this
               flag is portability: MAP_STACK exists (and has an
               effect) on some other systems (e.g., some of the BSDs).

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          MAP_SYNC (since Linux 4.15)
               This flag is available only with the
               MAP_SHARED_VALIDATE mapping type; mappings of type
               MAP_SHARED will silently ignore this flag.  This flag
               is supported only for files supporting DAX (direct map-
               ping of persistent memory).  For other files, creating
               a mapping with this flag results in an EOPNOTSUPP
               error.

               Shared file mappings with this flag provide the guaran-
               tee that while some memory is mapped writable in the
               address space of the process, it will be visible in the
               same file at the same offset even after the system
               crashes or is rebooted.  In conjunction with the use of
               appropriate CPU instructions, this provides users of
               such mappings with a more efficient way of making data
               modifications persistent.

          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).

        munmap()
          The munmap() system call deletes the mappings for the speci-
          fied address range, and causes further references to
          addresses within the range to generate invalid memory refer-
          ences.  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,

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          and errno is set to indicate the cause of the error (proba-
          bly 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).

          EEXIST
               MAP_FIXED_NOREPLACE was specified in flags, and the
               range covered by addr and length clashes with an exist-
               ing mapping.

          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 none of MAP_PRIVATE, MAP_SHARED or
               MAP_SHARED_VALIDATE.

          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(),
               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.

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          ENOMEM
               (since Linux 4.7) The process's RLIMIT_DATA limit,
               described in getrlimit(2), would have been exceeded.

          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).

          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.

          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 page of the buffer that lies
               beyond the end of the mapped file.  For an explanation
               of the treatment of the bytes in the page that corre-
               sponds to the end of a mapped file that is not a multi-
               ple of the page size, see NOTES.

     ATTRIBUTES
          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lbw18 lb lb l l l.
          Interface Attribute Value T{ mmap(), munmap() T}   Thread
          safety  MT-Safe

     CONFORMING TO
          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 sysconf(3).)

     NOTES
          Memory mapped by mmap() is preserved across fork(2), with
          the same attributes.

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          A file is mapped in multiples of the page size.  For a file
          that is not a multiple of the page size, the remaining bytes
          in the partial page at the end of the mapping are zeroed
          when mapped, and modifications to that region are not writ-
          ten 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.

          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 suitable feature
          test macros are defined (possibly by default):
          _DEFAULT_SOURCE with glibc 2.19 or later; or _BSD_SOURCE or
          _SVID_SOURCE in glibc 2.19 and earlier.  (Employing
          _GNU_SOURCE also suffices, and requiring that macro specifi-
          cally 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.

          An application can determine which pages of a mapping are
          currently resident in the buffer/page cache using
          mincore(2).

        Using MAP_FIXED safely
          The only safe use for MAP_FIXED is where the address range
          specified by addr and length was previously reserved using
          another mapping; otherwise, the use of MAP_FIXED is haz-
          ardous because it forcibly removes preexisting mappings,
          making it easy for a multithreaded process to corrupt its
          own address space.

          For example, suppose that thread A looks through
          /proc/<pid>/maps in order to locate an unused address range
          that it can map using MAP_FIXED, while thread B simultane-
          ously acquires part or all of that same address range.  When
          thread A subsequently employs mmap(MAP_FIXED), it will
          effectively clobber the mapping that thread B created.  In
          this scenario, thread B need not create a mapping directly;

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          simply making a library call that, internally, uses
          dlopen(3) to load some other shared library, will suffice.
          The dlopen(3) call will map the library into the process's
          address space.  Furthermore, almost any library call may be
          implemented in a way that adds memory mappings to the
          address space, either with this technique, or by simply
          allocating memory.  Examples include brk(2), malloc(3),
          pthread_create(3), and the PAM libraries

          Since Linux 4.17, a multithreaded program can use the
          MAP_FIXED_NOREPLACE flag to avoid the hazard described above
          when attempting to create a mapping at a fixed address that
          has not been reserved by a preexisting mapping.

        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 sys-
          tem call has been superseded by mmap2(2), and nowadays the
          glibc mmap() wrapper function invokes mmap2(2) with a suit-
          ably 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

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          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 map-
          pings 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(5) (for example,
          when using the POSIX shared memory interface documented in
          shm_overview(7)).

     EXAMPLES
          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 argu-
          ments.  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) {

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                  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 & ti(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;
                          /* Canaqt 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);
              }

              munmap(addr, length + offset - pa_offset);
              close(fd);

              exit(EXIT_SUCCESS);
          }

     SEE ALSO
          ftruncate(2), getpagesize(2), memfd_create(2), mincore(2),

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          mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2),
          remap_file_pages(2), setrlimit(2), shmat(2), userfaultfd(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. 128en129 and
          389en391.

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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