map files or devices into
*addr, size_t len,
function causes the contents of fd, starting at
offset, to be mapped in memory at the given
addr. The mapping will extend at least
len bytes, subject to page alignment restrictions.
The addr argument describes the address
where the system should place the mapping. If the
MAP_FIXED flag is specified, the allocation will
happen at the specified address, replacing any previously established
mappings in its range. Otherwise, the mapping will be placed at the
available spot at addr; failing that it will be placed
"close by". If addr is
NULL the system can pick any address. Except for
MAP_FIXED mappings, the system will never replace
The len argument describes the
minimum amount of bytes the mapping will span. Since
pages into memory, len may be rounded up to hit a page
boundary. If len is 0, the mapping will fail with
If an fd and offset are specified, the resulting address may end up not on a page boundary, in order to align the page offset in the addr to the page offset in offset.
The protections (region accessibility) are specified in the
prot argument. It should either be
PROT_NONE (no permissions) or the bitwise OR of one
or more of the following values:
- Pages may be executed.
- Pages may be read.
- Pages may be written.
The flags parameter specifies the type of the mapped object, mapping options, and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Sharing, mapping type, and options are specified in the flags argument by OR'ing the following values. Exactly one of the first two values must be specified:
- Modifications are private.
- Modifications are shared.
Any combination of the following flags may additionally be used:
- Map anonymous memory not associated with any specific file. The file
descriptor used for creating
MAP_ANONmust currently be -1 indicating no name is associated with the region.
- Synonym for
- Demand that the mapping is placed at addr, rather than having the system select a location. addr, len and offset (in the case of fd mappings) must be multiples of the page size. Existing mappings in the address range will be replaced. Use of this option is discouraged.
- Indicate that the mapping is used as a stack. This flag must be used in
- Exclude the mapping from core dumps.
Finally, the following flags are also provided for source compatibility with code written for other operating systems, but are not recommended for use in new OpenBSD code:
- Modifications are private and, unlike
MAP_PRIVATE, modifications made by others are not visible. On OpenBSD this behaves just like
- Mapped from a regular file, character special file, or block special file specified by file descriptor fd. On OpenBSD and all systems conforming to IEEE Std 1003.1-2008 (“POSIX.1”) this is the default mapping type and need not be specified.
- Notify the kernel that the region may contain semaphores and that special handling may be necessary. On OpenBSD this flag is ignored.
- Permit regions to be inherited across execve(2) system calls. On OpenBSD this flag is ignored.
- Attempt to use the hint provided by addr. On OpenBSD this is the default behavior.
The close(2) function does not unmap pages; see munmap(2) for further information.
mmap() function returns a pointer to
the mapped region if successful; otherwise the value
MAP_FAILED is returned and the global variable
errno is set to indicate the error. A successful
mmap() will never return the value
mmap() will fail if:
- The flag
PROT_READwas specified as part of the prot parameter and fd was not open for reading. The flags
PROT_WRITEwere specified as part of the flags and prot parameters and fd was not open for writing.
- fd is not a valid open file descriptor.
MAP_SHAREDwere both specified.
MAP_FIXEDwas specified and the addr parameter was not page aligned.
- addr and len specified a region that would extend beyond the end of the address space.
- fd did not specify a regular, character special, or block special file.
- fd specified a character special or block special file and the underlying device does not support memory mapping.
- The allocation len was 0.
MAP_FIXEDwas specified and the addr parameter wasn't available.
MAP_ANONwas specified and insufficient memory was available.
- The accesses requested in the prot argument are not
allowed. In particular,
PROT_EXECmappings are not permitted unless the filesystem is mounted
wxallowedand the process is link-time tagged with
wxneeded. (See also
kern.wxabortin sysctl(2) for a method to diagnose failure).
madvise(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3), core(5)
mmap() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
A fully functional
mmap() system call
first appeared in SunOS 4.0 and has been available since
IEEE Std 1003.1-2008
(“POSIX.1”) specifies that references to pages beyond
the end of a mapped object shall generate a
signal; however, on some architectures OpenBSD
SIGSEGV signal in this case instead.
Additionally, IEEE Std 1003.1-2008
(“POSIX.1”) specifies that
mmap() shall fail with
EINVAL if neither
is specified by flags; however, for compatibility with
old programs, OpenBSD instead defaults to
MAP_SHARED for mappings of character special files,
MAP_PRIVATE for all other mappings. New
programs should not rely on this behavior.
Due to a limitation of the current vm system (see
uvm_init(9)), mapping descriptors
without also specifying
PROT_READ is useless
(results in a segmentation fault when first accessing the mapping). This
means that such descriptors must be opened with
O_RDWR, which requires both read and write
permissions on the underlying object.