mmap

MMAP(3POSIX)               POSIX Programmer's Manual              MMAP(3POSIX)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the corresponding
       Linux manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.


NAME
       mmap — map pages of memory

SYNOPSIS
       #include <sys/mman.h>

       void *mmap(void *addr, size_t len, int prot, int flags,
           int fildes, off_t off);

DESCRIPTION
       The mmap() function shall establish a mapping between an address space
       of a process and a memory object.

       The mmap() function shall be supported for the following memory
       objects:

        *  Regular files

        *  Shared memory objects

        *  Typed memory objects

       Support for any other type of file is unspecified.

       The format of the call is as follows:

           pa=mmap(addr, len, prot, flags, fildes, off);

       The mmap() function shall establish a mapping between the address space
       of the process at an address pa for len bytes to the memory object
       represented by the file descriptor fildes at offset off for len bytes.
       The value of pa is an implementation-defined function of the parameter
       addr and the values of flags, further described below. A successful
       mmap() call shall return pa as its result. The address range starting
       at pa and continuing for len bytes shall be legitimate for the possible
       (not necessarily current) address space of the process. The range of
       bytes starting at off and continuing for len bytes shall be legitimate
       for the possible (not necessarily current) offsets in the memory object
       represented by fildes.

       If fildes represents a typed memory object opened with either the
       POSIX_TYPED_MEM_ALLOCATE flag or the POSIX_TYPED_MEM_ALLOCATE_CONTIG
       flag, the memory object to be mapped shall be that portion of the typed
       memory object allocated by the implementation as specified below. In
       this case, if off is non-zero, the behavior of mmap() is undefined. If
       fildes refers to a valid typed memory object that is not accessible
       from the calling process, mmap() shall fail.

       The mapping established by mmap() shall replace any previous mappings
       for those whole pages containing any part of the address space of the
       process starting at pa and continuing for len bytes.

       If the size of the mapped file changes after the call to mmap() as a
       result of some other operation on the mapped file, the effect of
       references to portions of the mapped region that correspond to added or
       removed portions of the file is unspecified.

       If len is zero, mmap() shall fail and no mapping shall be established.

       The parameter prot determines whether read, write, execute, or some
       combination of accesses are permitted to the data being mapped. The
       prot shall be either PROT_NONE or the bitwise-inclusive OR of one or
       more of the other flags in the following table, defined in the
       <sys/mman.h> header.

                   ┌──────────────────┬──────────────────────────┐
                   │Symbolic Constant Description        │
                   ├──────────────────┼──────────────────────────┤
                   │PROT_READ         │ Data can be read.        │
                   │PROT_WRITE        │ Data can be written.     │
                   │PROT_EXEC         │ Data can be executed.    │
                   │PROT_NONE         │ Data cannot be accessed. │
                   └──────────────────┴──────────────────────────┘
       If an implementation cannot support the combination of access types
       specified by prot, the call to mmap() shall fail.

       An implementation may permit accesses other than those specified by
       prot; however, the implementation shall not permit a write to succeed
       where PROT_WRITE has not been set and shall not permit any access where
       PROT_NONE alone has been set. The implementation shall support at least
       the following values of prot: PROT_NONE, PROT_READ, PROT_WRITE, and the
       bitwise-inclusive OR of PROT_READ and PROT_WRITE. The file descriptor
       fildes shall have been opened with read permission, regardless of the
       protection options specified. If PROT_WRITE is specified, the
       application shall ensure that it has opened the file descriptor fildes
       with write permission unless MAP_PRIVATE is specified in the flags
       parameter as described below.

       The parameter flags provides other information about the handling of
       the mapped data.  The value of flags is the bitwise-inclusive OR of
       these options, defined in <sys/mman.h>:

                    ┌──────────────────┬─────────────────────────┐
                    │Symbolic Constant Description       │
                    ├──────────────────┼─────────────────────────┤
                    │MAP_SHARED        │ Changes are shared.     │
                    │MAP_PRIVATE       │ Changes are private.    │
                    │MAP_FIXED         │ Interpret addr exactly. │
                    └──────────────────┴─────────────────────────┘
       It is implementation-defined whether MAP_FIXED shall be supported.
       MAP_FIXED shall be supported on XSI-conformant systems.

       MAP_SHARED and MAP_PRIVATE describe the disposition of write references
       to the memory object. If MAP_SHARED is specified, write references
       shall change the underlying object. If MAP_PRIVATE is specified,
       modifications to the mapped data by the calling process shall be
       visible only to the calling process and shall not change the underlying
       object.  It is unspecified whether modifications to the underlying
       object done after the MAP_PRIVATE mapping is established are visible
       through the MAP_PRIVATE mapping. Either MAP_SHARED or MAP_PRIVATE can
       be specified, but not both. The mapping type is retained across fork().

       The state of synchronization objects such as mutexes, semaphores,
       barriers, and conditional variables placed in shared memory mapped with
       MAP_SHARED becomes undefined when the last region in any process
       containing the synchronization object is unmapped.

       When fildes represents a typed memory object opened with either the
       POSIX_TYPED_MEM_ALLOCATE flag or the POSIX_TYPED_MEM_ALLOCATE_CONTIG
       flag, mmap() shall, if there are enough resources available, map len
       bytes allocated from the corresponding typed memory object which were
       not previously allocated to any process in any processor that may
       access that typed memory object. If there are not enough resources
       available, the function shall fail. If fildes represents a typed memory
       object opened with the POSIX_TYPED_MEM_ALLOCATE_CONTIG flag, these
       allocated bytes shall be contiguous within the typed memory object. If
       fildes represents a typed memory object opened with the
       POSIX_TYPED_MEM_ALLOCATE flag, these allocated bytes may be composed of
       non-contiguous fragments within the typed memory object. If fildes
       represents a typed memory object opened with neither the
       POSIX_TYPED_MEM_ALLOCATE_CONTIG flag nor the POSIX_TYPED_MEM_ALLOCATE
       flag, len bytes starting at offset off within the typed memory object
       are mapped, exactly as when mapping a file or shared memory object. In
       this case, if two processes map an area of typed memory using the same
       off and len values and using file descriptors that refer to the same
       memory pool (either from the same port or from a different port), both
       processes shall map the same region of storage.

       When MAP_FIXED is set in the flags argument, the implementation is
       informed that the value of pa shall be addr, exactly. If MAP_FIXED is
       set, mmap() may return MAP_FAILED and set errno to [EINVAL].  If a
       MAP_FIXED request is successful, the mapping established by mmap()
       replaces any previous mappings for the pages in the range [pa,pa+len)
       of the process.

       When MAP_FIXED is not set, the implementation uses addr in an
       implementation-defined manner to arrive at pa.  The pa so chosen shall
       be an area of the address space that the implementation deems suitable
       for a mapping of len bytes to the file. All implementations interpret
       an addr value of 0 as granting the implementation complete freedom in
       selecting pa, subject to constraints described below. A non-zero value
       of addr is taken to be a suggestion of a process address near which the
       mapping should be placed. When the implementation selects a value for
       pa, it never places a mapping at address 0, nor does it replace any
       extant mapping.

       If MAP_FIXED is specified and addr is non-zero, it shall have the same
       remainder as the off parameter, modulo the page size as returned by
       sysconf() when passed _SC_PAGESIZE or _SC_PAGE_SIZE. The implementation
       may require that off is a multiple of the page size. If MAP_FIXED is
       specified, the implementation may require that addr is a multiple of
       the page size. The system performs mapping operations over whole pages.
       Thus, while the parameter len need not meet a size or alignment
       constraint, the system shall include, in any mapping operation, any
       partial page specified by the address range starting at pa and
       continuing for len bytes.

       The system shall always zero-fill any partial page at the end of an
       object. Further, the system shall never write out any modified portions
       of the last page of an object which are beyond its end.  References
       within the address range starting at pa and continuing for len bytes to
       whole pages following the end of an object shall result in delivery of
       a SIGBUS signal.

       An implementation may generate SIGBUS signals when a reference would
       cause an error in the mapped object, such as out-of-space condition.

       The mmap() function shall add an extra reference to the file associated
       with the file descriptor fildes which is not removed by a subsequent
       close() on that file descriptor. This reference shall be removed when
       there are no more mappings to the file.

       The last data access timestamp of the mapped file may be marked for
       update at any time between the mmap() call and the corresponding
       munmap() call. The initial read or write reference to a mapped region
       shall cause the file's last data access timestamp to be marked for
       update if it has not already been marked for update.

       The last data modification and last file status change timestamps of a
       file that is mapped with MAP_SHARED and PROT_WRITE shall be marked for
       update at some point in the interval between a write reference to the
       mapped region and the next call to msync() with MS_ASYNC or MS_SYNC for
       that portion of the file by any process.  If there is no such call and
       if the underlying file is modified as a result of a write reference,
       then these timestamps shall be marked for update at some time after the
       write reference.

       There may be implementation-defined limits on the number of memory
       regions that can be mapped (per process or per system).

       If such a limit is imposed, whether the number of memory regions that
       can be mapped by a process is decreased by the use of shmat() is
       implementation-defined.

       If mmap() fails for reasons other than [EBADF], [EINVAL], or [ENOTSUP],
       some of the mappings in the address range starting at addr and
       continuing for len bytes may have been unmapped.

RETURN VALUE
       Upon successful completion, the mmap() function shall return the
       address at which the mapping was placed (pa); otherwise, it shall
       return a value of MAP_FAILED and set errno to indicate the error. The
       symbol MAP_FAILED is defined in the <sys/mman.h> header. No successful
       return from mmap() shall return the value MAP_FAILED.

ERRORS
       The mmap() function shall fail if:

       EACCES The fildes argument is not open for read, regardless of the
              protection specified, or fildes is not open for write and
              PROT_WRITE was specified for a MAP_SHARED type mapping.

       EAGAIN The mapping could not be locked in memory, if required by
              mlockall(), due to a lack of resources.

       EBADF  The fildes argument is not a valid open file descriptor.

       EINVAL The value of len is zero.

       EINVAL The value of flags is invalid (neither MAP_PRIVATE nor
              MAP_SHARED is set).

       EMFILE The number of mapped regions would exceed an implementation-
              defined limit (per process or per system).

       ENODEV The fildes argument refers to a file whose type is not supported
              by mmap().

       ENOMEM MAP_FIXED was specified, and the range [addr,addr+len) exceeds
              that allowed for the address space of a process; or, if
              MAP_FIXED was not specified and there is insufficient room in
              the address space to effect the mapping.

       ENOMEM The mapping could not be locked in memory, if required by
              mlockall(), because it would require more space than the system
              is able to supply.

       ENOMEM Not enough unallocated memory resources remain in the typed
              memory object designated by fildes to allocate len bytes.

       ENOTSUP
              MAP_FIXED or MAP_PRIVATE was specified in the flags argument and
              the implementation does not support this functionality.

                   The implementation does not support the combination of
                   accesses requested in the prot argument.

       ENXIO  Addresses in the range [off,off+len) are invalid for the object
              specified by fildes.

       ENXIO  MAP_FIXED was specified in flags and the combination of addr,
              len, and off is invalid for the object specified by fildes.

       ENXIO  The fildes argument refers to a typed memory object that is not
              accessible from the calling process.

       EOVERFLOW
              The file is a regular file and the value of off plus len exceeds
              the offset maximum established in the open file description
              associated with fildes.

       The mmap() function may fail if:

       EINVAL The addr argument (if MAP_FIXED was specified) or off is not a
              multiple of the page size as returned by sysconf(), or is
              considered invalid by the implementation.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       Use of mmap() may reduce the amount of memory available to other memory
       allocation functions.

       Use of MAP_FIXED may result in unspecified behavior in further use of
       malloc() and shmat().  The use of MAP_FIXED is discouraged, as it may
       prevent an implementation from making the most effective use of
       resources. Most implementations require that off and addr are multiples
       of the page size as returned by sysconf().

       The application must ensure correct synchronization when using mmap()
       in conjunction with any other file access method, such as read() and
       write(), standard input/output, and shmat().

       The mmap() function allows access to resources via address space
       manipulations, instead of read()/write().  Once a file is mapped, all a
       process has to do to access it is use the data at the address to which
       the file was mapped. So, using pseudo-code to illustrate the way in
       which an existing program might be changed to use mmap(), the
       following:

           fildes = open(...)
           lseek(fildes, some_offset)
           read(fildes, buf, len)
           /* Use data in buf. */

       becomes:

           fildes = open(...)
           address = mmap(0, len, PROT_READ, MAP_PRIVATE, fildes, some_offset)
           /* Use data at address. */

RATIONALE
       After considering several other alternatives, it was decided to adopt
       the mmap() definition found in SVR4 for mapping memory objects into
       process address spaces. The SVR4 definition is minimal, in that it
       describes only what has been built, and what appears to be necessary
       for a general and portable mapping facility.

       Note that while mmap() was first designed for mapping files, it is
       actually a general-purpose mapping facility. It can be used to map any
       appropriate object, such as memory, files, devices, and so on, into the
       address space of a process.

       When a mapping is established, it is possible that the implementation
       may need to map more than is requested into the address space of the
       process because of hardware requirements. An application, however,
       cannot count on this behavior. Implementations that do not use a paged
       architecture may simply allocate a common memory region and return the
       address of it; such implementations probably do not allocate any more
       than is necessary. References past the end of the requested area are
       unspecified.

       If an application requests a mapping that would overlay existing
       mappings in the process, it might be desirable that an implementation
       detect this and inform the application. However, the default, portable
       (not MAP_FIXED) operation does not overlay existing mappings. On the
       other hand, if the program specifies a fixed address mapping (which
       requires some implementation knowledge to determine a suitable address,
       if the function is supported at all), then the program is presumed to
       be successfully managing its own address space and should be trusted
       when it asks to map over existing data structures. Furthermore, it is
       also desirable to make as few system calls as possible, and it might be
       considered onerous to require an munmap() before an mmap() to the same
       address range. This volume of POSIX.1‐2008 specifies that the new
       mappings replace any existing mappings, following existing practice in
       this regard.

       It is not expected that all hardware implementations are able to
       support all combinations of permissions at all addresses.
       Implementations are required to disallow write access to mappings
       without write permission and to disallow access to mappings without any
       access permission. Other than these restrictions, implementations may
       allow access types other than those requested by the application. For
       example, if the application requests only PROT_WRITE, the
       implementation may also allow read access. A call to mmap() fails if
       the implementation cannot support allowing all the access requested by
       the application. For example, some implementations cannot support a
       request for both write access and execute access simultaneously. All
       implementations must support requests for no access, read access, write
       access, and both read and write access. Strictly conforming code must
       only rely on the required checks. These restrictions allow for
       portability across a wide range of hardware.

       The MAP_FIXED address treatment is likely to fail for non-page-aligned
       values and for certain architecture-dependent address ranges.
       Conforming implementations cannot count on being able to choose address
       values for MAP_FIXED without utilizing non-portable, implementation-
       defined knowledge. Nonetheless, MAP_FIXED is provided as a standard
       interface conforming to existing practice for utilizing such knowledge
       when it is available.

       Similarly, in order to allow implementations that do not support
       virtual addresses, support for directly specifying any mapping
       addresses via MAP_FIXED is not required and thus a conforming
       application may not count on it.

       The MAP_PRIVATE function can be implemented efficiently when memory
       protection hardware is available. When such hardware is not available,
       implementations can implement such ``mappings'' by simply making a real
       copy of the relevant data into process private memory, though this
       tends to behave similarly to read().

       The function has been defined to allow for many different models of
       using shared memory. However, all uses are not equally portable across
       all machine architectures. In particular, the mmap() function allows
       the system as well as the application to specify the address at which
       to map a specific region of a memory object. The most portable way to
       use the function is always to let the system choose the address,
       specifying NULL as the value for the argument addr and not to specify
       MAP_FIXED.

       If it is intended that a particular region of a memory object be mapped
       at the same address in a group of processes (on machines where this is
       even possible), then MAP_FIXED can be used to pass in the desired
       mapping address. The system can still be used to choose the desired
       address if the first such mapping is made without specifying MAP_FIXED,
       and then the resulting mapping address can be passed to subsequent
       processes for them to pass in via MAP_FIXED. The availability of a
       specific address range cannot be guaranteed, in general.

       The mmap() function can be used to map a region of memory that is
       larger than the current size of the object. Memory access within the
       mapping but beyond the current end of the underlying objects may result
       in SIGBUS signals being sent to the process. The reason for this is
       that the size of the object can be manipulated by other processes and
       can change at any moment. The implementation should tell the
       application that a memory reference is outside the object where this
       can be detected; otherwise, written data may be lost and read data may
       not reflect actual data in the object.

       Note that references beyond the end of the object do not extend the
       object as the new end cannot be determined precisely by most virtual
       memory hardware. Instead, the size can be directly manipulated by
       ftruncate().

       Process memory locking does apply to shared memory regions, and the
       MEMLOCK_FUTURE argument to mlockall() can be relied upon to cause new
       shared memory regions to be automatically locked.

       Existing implementations of mmap() return the value −1 when
       unsuccessful. Since the casting of this value to type void * cannot be
       guaranteed by the ISO C standard to be distinct from a successful
       value, this volume of POSIX.1‐2008 defines the symbol MAP_FAILED, which
       a conforming implementation does not return as the result of a
       successful call.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exec, fcntl(), fork(), lockf(), msync(), munmap(), mprotect(),
       posix_typed_mem_open(), shmat(), sysconf()

       The Base Definitions volume of POSIX.1‐2008, <sys_mman.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be obtained online
       at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2013                         MMAP(3POSIX)