MMAP(2) | System Calls Manual | MMAP(2) |
mmap
— map files
or devices into memory
#include
<sys/mman.h>
void *
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 system 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 argument. It should either be
PROT_NONE
(no permissions) or the bitwise OR of one
or more of the following values:
PROT_EXEC
PROT_READ
PROT_WRITE
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:
MAP_PRIVATE
MAP_SHARED
Any combination of the following flags may additionally be used:
MAP_ANON
MAP_ANON
must
currently be -1 indicating no name is associated with the region.MAP_ANONYMOUS
MAP_ANON
.MAP_FIXED
MAP_STACK
MAP_ANON
and
MAP_PRIVATE
.MAP_CONCEAL
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
MAP_PRIVATE
,
modifications made by others are not visible. On
OpenBSD this behaves just like
MAP_PRIVATE
.MAP_FILE
MAP_HASSEMAPHORE
MAP_INHERIT
MAP_TRYFIXED
The close(2) function does not unmap pages; see munmap(2) for further information.
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
]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 writing.EBADF
]EINVAL
]MAP_PRIVATE
and MAP_SHARED
were both specified.EINVAL
]MAP_FIXED
was specified and the addr parameter was not page
aligned.EINVAL
]EINVAL
]EINVAL
]EINVAL
]ENOMEM
]MAP_FIXED
was specified and the addr parameter wasn't
available. MAP_ANON
was specified and insufficient
memory was available.ENOTSUP
]PROT_WRITE
|
PROT_EXEC
mappings are not permitted unless the filesystem is mounted
wxallowed
and the process is link-time tagged with
wxneeded
. (See also
kern.wxabort
in
sysctl(2) for a method to diagnose
failure).madvise(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3), core(5)
The 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
4.3BSD-Net/2.
IEEE Std 1003.1-2008
(“POSIX.1”) specifies that references to pages beyond
the end of a mapped object shall generate a SIGBUS
signal; however, on some architectures 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 behavior.
Due to a limitation of the current vm system (see
uvm_init(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.
March 31, 2022 | OpenBSD-current |