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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, 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.
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 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.
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.1‐2008, sys_mman.h(0p)
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(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)