MMAP(2) BSD System Calls Manual MMAP(2)
mmap -- map files or devices into memory
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
The mmap() 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 sys-
tem will never replace existing mappings.
The len argument describes the minimum amount of bytes the mapping will
span. Since mmap() maps pages into memory, len may be rounded up to hit
a page boundary. If len is 0, the mapping will fail with EINVAL.
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 argu-
ment. It should either be PROT_NONE (no permissions) or the bitwise OR
of one or more of the following values:
PROT_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE 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
MAP_PRIVATE Modifications are private.
MAP_SHARED Modifications are shared.
Any combination of the following flags may additionally be used:
MAP_ANON Map anonymous memory not associated with any spe-
cific file. The file descriptor used for creating
MAP_ANON must currently be -1 indicating no name is
associated with the region.
MAP_ANONYMOUS Synonym for MAP_ANON.
MAP_FIXED 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 mul-
tiples of the page size. Existing mappings in the
address range will be replaced. Use of this option
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:
MAP_COPY Modifications are private and, unlike MAP_PRIVATE,
modifications made by others are not visible. On
OpenBSD this behaves just like MAP_PRIVATE.
MAP_FILE 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 sema-
phores and that special handling may be necessary.
On OpenBSD this flag is ignored.
MAP_INHERIT Permit regions to be inherited across exec(3) system
calls. On OpenBSD this flag is ignored.
MAP_TRYFIXED 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
The 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 return from mmap() will never
return the value MAP_FAILED.
mmap() will fail if:
[EACCES] The flag PROT_READ was specified as part of the prot
parameter and fd was not open for reading. The flags
MAP_SHARED and PROT_WRITE were specified as part of
the flags and prot parameters and fd was not open for
[EBADF] fd is not a valid open file descriptor.
[EINVAL] MAP_PRIVATE and MAP_SHARED were both specified.
[EINVAL] MAP_FIXED was specified and the addr parameter was not
[EINVAL] addr and len specified a region that would extend
beyond the end of the address space.
[EINVAL] fd did not specify a regular, character special, or
block special file.
[EINVAL] The allocation len was 0.
[ENOMEM] MAP_FIXED was specified and the addr parameter wasn't
available. MAP_ANON was specified and insufficient
memory was available.
madvise(2), mincore(2), mlock(2), mprotect(2), mquery(2), msync(2),
The mmap() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').
The mmap() system call first appeared in 4.1cBSD.
IEEE Std 1003.1-2008 (``POSIX.1'') specifies that references to pages
beyond the end of a mapped object shall generate a SIGBUS signal; how-
ever, OpenBSD generates a SIGSEGV signal in this case instead.
Additionally, IEEE Std 1003.1-2008 (``POSIX.1'') specifies that mmap()
shall fail with EINVAL if neither MAP_PRIVATE nor MAP_SHARED is specified
by flags; however, for compatibility with old programs, OpenBSD instead
defaults to MAP_SHARED for mappings of character special files, and to
MAP_PRIVATE for all other mappings. New programs should not rely on this
Due to a limitation of the current vm system (see uvm(9)), mapping
descriptors PROT_WRITE 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.
BSD March 28, 2017 BSD