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

       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.

       mmap — map pages of memory

       #include <sys/mman.h>

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

       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

	*  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 rep‐
       resented 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
       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 refer‐
       ences 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 applica‐
       tion 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, modi‐
       fications 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, bar‐
       riers, and conditional variables placed in shared  memory  mapped  with
       MAP_SHARED  becomes  undefined when the last region in any process con‐
       taining the synchronization object is unmapped.

       When fildes represents a typed memory object  opened  with  either  the
       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 rep‐
       resents	 a   typed   memory   object   opened	with	neither	   the
       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 implemen‐
       tation-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

       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 con‐
       straint, the system shall include, in any mapping operation,  any  par‐
       tial  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  mun‐
       map()  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 imple‐

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

       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.

       The mmap() function shall fail if:

       EACCES The fildes argument is not open for read, regardless of the pro‐
	      tection  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 mlock‐
	      all(), 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 mlock‐
	      all(),  because  it  would require more space than the system is
	      able to supply.

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

	      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.

	      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 con‐
	      sidered invalid by the implementation.

       The following sections are informative.


       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 manip‐
       ulations, 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 follow‐

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


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

       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, can‐
       not 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

       If an application requests a mapping that would overlay	existing  map‐
       pings  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 map‐
       pings replace any existing mappings,  following	existing  practice  in
       this regard.

       It  is  not expected that all hardware implementations are able to sup‐
       port all combinations of permissions at all addresses.  Implementations
       are required to disallow write access to mappings without write permis‐
       sion 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  sup‐
       port 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

       The  MAP_FIXED address treatment is likely to fail for non-page-aligned
       values and for certain architecture-dependent address ranges.  Conform‐
       ing 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

       Similarly,  in  order to allow implementations that do not support vir‐
       tual 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, speci‐
       fying NULL as the value for  the	 argument  addr	 and  not  to  specify

       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 map‐
       ping 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 pro‐
       cesses  for  them  to pass in via MAP_FIXED. The availability of a spe‐
       cific 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 applica‐
       tion 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

       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 unsuccess‐
       ful. Since the casting of this value to type void * cannot  be  guaran‐
       teed 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  conform‐
       ing implementation does not return as the result of a successful call.


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

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

       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 Electri‐
       cal 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 .

       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.ker‐ .

IEEE/The Open Group		     2013			      MMAP(3P)

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