mmap
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 a process'
address space and a file, shared memory object, or typed memory
object. 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 file, shared
memory object, or typed 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 refer-
ences to portions of the mapped region that correspond to added or
removed portions of the file is unspecified.
The mmap() function shall be supported for regular files, shared memory
objects, and typed memory objects. Support for any other type of
file is unspecified.
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.
prot; however, if the Memory Protection option is supported, the
implementation shall not permit a write to succeed where PROT_WRITE has
not been set or shall not permit any access where PROT_NONE alone has
been set. The implementation shall support at least the following val-
ues of prot: PROT_NONE, PROT_READ, PROT_WRITE, and the bitwise-inclu-
sive OR of PROT_READ and PROT_WRITE. If the Memory Protection option
is not supported, the result of any access that conflicts with the
specified protection is undefined. 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 permis-
sion 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.
Implementations that do not support the Memory Mapped Files option are
not required to support MAP_PRIVATE.
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().
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 rep-
resents 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
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
mapping.
The off argument is constrained to be aligned and sized according to
the value returned by sysconf() when passed _SC_PAGESIZE or
_SC_PAGE_SIZE. When MAP_FIXED is specified, the application shall
ensure that the argument addr also meets these constraints. The imple-
mentation performs mapping operations over whole pages. Thus, while the
argument len need not meet a size or alignment constraint, the imple-
mentation shall include, in any mapping operation, any partial page
specified by the range [pa,pa+len).
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 st_atime field 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 st_atime field to be marked for update if it has not already
been marked for update.
The st_ctime and st_mtime fields 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 fields
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
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 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 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.
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.
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.
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.
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-
ing:
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, 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
unspecified.
new mappings replace any existing mappings, following existing practice
in this regard.
It is not expected, when the Memory Protection option is supported,
that all hardware implementations are able to support all combinations
of permissions at all addresses. When this option is supported, imple-
mentations 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 exam-
ple, if the application requests only PROT_WRITE, the implementation
may also allow read access. A call to mmap() fails if the implementa-
tion cannot support allowing all the access requested by the applica-
tion. For example, some implementations cannot support a request for
both write access and execute access simultaneously. All implementa-
tions supporting the Memory Protection option 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. 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
available.
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
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 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-
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 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, <sys/mman.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 MMAP(P)
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