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

• Anonymous memory objects

• ^[SHM] Shared memory objects

• ^[TYM] 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, or to an anonymous memory object of 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.

^[TYM] 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_ANON	Synonym for MAP_ANONYMOUS.
MAP_ANONYMOUS	Map anonymous memory.
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. ^[XSI]  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.

^[TYM] 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 ^[ML|MLR]  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.

If MAP_ANONYMOUS (or its synonym MAP_ANON) is specified, fildes is -1, and off is 0, then mmap() shall ignore fildes and instead establish a mapping to a new anonymous memory object of size len. The effect of specifying MAP_ANONYMOUS (or MAP_ANON) with other values of fildes or off is unspecified. Anonymous memory objects shall be initialized to all bits zero.

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).

^[XSI] 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:

[EAGAIN]

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

[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).

[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]

^[ML] 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]

^[TYM] 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]

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

[ENXIO]

^[TYM] The fildes argument refers to a typed memory object that is not accessible from the calling process.

In addition, if MAP_ANONYMOUS (or MAP_ANON) is not set in flags, 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.

[EBADF]

The fildes argument is not a valid open file descriptor.

[ENODEV]

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

[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.

[ENXIO]

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

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 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.1-2024 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.1-2024 defines the symbol MAP_FAILED, which a conforming implementation does not return as the result of a successful call.

Some historical implementations only supported MAP_ANON, some only supported MAP_ANONYMOUS, and some supported both spellings. This standard includes both spellings partly for application compatibility and partly because neither spelling was clearly more popular than the other at the time this feature was considered for standardization.

FUTURE DIRECTIONS

None.

SEE ALSO

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

XBD <sys/mman.h>

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Aligned with mmap() in the POSIX Realtime Extension as follows:

• The DESCRIPTION is extensively reworded.
• The [EAGAIN] and [ENOTSUP] mandatory error conditions are added.
• New cases of [ENOMEM] and [ENXIO] are added as mandatory error conditions.
• The value returned on failure is the value of the constant MAP_FAILED; this was previously defined as -1.

Large File Summit extensions are added.

Issue 6

The mmap() function is marked as part of the Memory Mapped Files option.

The Open Group Corrigendum U028/6 is applied, changing (void *)-1 to MAP_FAILED.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The DESCRIPTION is updated to describe the use of MAP_FIXED.
• The DESCRIPTION is updated to describe the addition of an extra reference to the file associated with the file descriptor passed to mmap().
• The DESCRIPTION is updated to state that there may be implementation-defined limits on the number of memory regions that can be mapped.
• The DESCRIPTION is updated to describe constraints on the alignment and size of the off argument.
• The [EINVAL] and [EMFILE] error conditions are added.
• The [EOVERFLOW] error condition is added. This change is to support large files.

The following changes are made for alignment with the ISO POSIX-1:1996 standard:

• The DESCRIPTION is updated to describe the cases when MAP_PRIVATE and MAP_FIXED need not be supported.

The following changes are made for alignment with IEEE Std 1003.1j-2000:

• Semantics for typed memory objects are added to the DESCRIPTION.
• New [ENOMEM] and [ENXIO] errors are added to the ERRORS section.
• The posix_typed_mem_open() function is added to the SEE ALSO section.

The normative text is updated to avoid use of the term "must" for application requirements.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/34 is applied, changing the margin code in the SYNOPSIS from MF|SHM to MC3 (notation for MF|SHM|TYM).

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/60 is applied, updating the DESCRIPTION and ERRORS sections to add the [EINVAL] error when len is zero.

Issue 7

Austin Group Interpretations 1003.1-2001 #078 and #079 are applied, clarifying page alignment requirements and adding a note about the state of synchronization objects becoming undefined when a shared region is unmapped.

Functionality relating to the Memory Protection and Memory Mapped Files options is moved to the Base.

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0229 [852] is applied.

Issue 8

Austin Group Defect 850 is applied, adding anonymous memory objects.