mmap#

Synopsis#

#include <sys/mman.h>

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

Status#

Partially implemented

Conformance#

IEEE Std 1003.1-2017

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
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 writ 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 provide 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.
`MAP_ANONYMOUS`	Map is not backed by a file.
`MAP_ANON`	Alias to `MAP_ANONYMOUS`.
`MAP_NONE`	Noop flag.
`MAP_FIXED`	Interpret addr exactly.
`MAP_DEVICE`	Map hardware device.
`MAP_NEEDSCOPY`	Mark that pages need copy on first access.
`MAP_CONTIGUOUS`	Map using contiguous frames.
`MAP_UNCACHED`	Pages will not be cached.
`MAP_NOINHERIT`	Child processes will not inherit this mapping.
`MAP_PHYSMEM`	Mapping of physical memory at given address is requested.

Flags MAP_PHYSMEM and MAP_CONTIGUOUS require flag MAP_ANONYMOUS to be used as well.

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 [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.

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 writ 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 writ 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 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:

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.

Tests#

Untested

Known bugs#

None