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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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
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, then any previous mappings
or memory locks for those whole pages containing any part of the
address range [pa,pa+len) shall be removed, as if by an
appropriate call to munmap(), before the new mapping is
established.
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.
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
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.
None.
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. */
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 overlaps existing
mappings in the process, it might be desirable that an
implementation detect this and inform the application. However, 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.1‐2017
specifies that the new mapping replaces any existing mappings
(implying an automatic munmap() on the address range), following
existing practice in this regard. The standard developers also
considered whether there should be a way for new mappings to
overlay existing mappings, but found no existing practice for
this.
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 MCL_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.1‐2017 defines the symbol
MAP_FAILED, which a conforming implementation does not return as
the result of a successful call.
None.
exec(1p), fcntl(3p), fork(3p), lockf(3p), msync(3p), munmap(3p),
mprotect(3p), posix_typed_mem_open(3p), shmat(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, sys_mman.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 MMAP(3P)
Pages that refer to this page: sys_mman.h(0p), exec(3p), lockf(3p), mprotect(3p), msync(3p), munmap(3p), posix_madvise(3p), posix_mem_offset(3p), posix_typed_mem_get_info(3p), posix_typed_mem_open(3p), shm_open(3p), shm_unlink(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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