mmap - map files or devices into memory
mmap(void *addr, size_t len, int prot, int flags, int fd,
The mmap function causes the pages starting at addr and continuing for at
most len bytes to be mapped from the object described by fd,
byte offset offset. If offset or len is not a multiple of
the mapped region may extend past the specified range.
If addr is non-zero, it is used as a hint to the system.
(As a convenience
to the system, the actual address of the region may
the address supplied.) If addr is zero, an address will be
the system. The actual starting address of the region is
successful mmap deletes any previous mapping in the allocated address
The protections (region accessibility) are specified in the
by OR'ing the following values:
PROT_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_NONE No permissions.
The flags parameter specifies the type of the mapped object,
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
OR'ing the following values:
MAP_ANON Map anonymous memory not associated with any
The file descriptor used for creating MAP_ANON
be -1 indicating no name is associated with the
MAP_FILE Mapped from a regular file or character-special
(This is the default mapping type, and need
not be specified.)
MAP_FIXED Do not permit the system to select a different
the one specified. If the specified address
cannot be used,
mmap will fail. If MAP_FIXED is specified, addr
must be a
multiple of the pagesize. Use of this option is
Notify the kernel that the region may contain
that special handling may be necessary.
Permit regions to be inherited across exec(3)
Modifications are private.
MAP_SHARED Modifications are shared.
MAP_COPY Modifications are private and, unlike MAP_PRIVATE, modifications
made by others are not visible. This option is deprecated,
shouldn't be used and behaves just like
the current implementation.
The close(2) function does not unmap pages; see munmap(2)
for further information.
The current design does not allow a process to specify the
swap space. In the future we may define an additional mapping type,
MAP_SWAP, in which the file descriptor argument specifies a
file or device
to which swapping should be done.
Upon successful completion, mmap returns a pointer to the
Otherwise, a value of MAP_FAILED is returned and errno is
set to indicate
the error. The symbol MAP_FAILED is defined in the header
No successful return from mmap() will return the value
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
and PROT_WRITE were specified as part of the
flags and prot
parameters and fd was not open for writing.
[EBADF] fd is not a valid open file descriptor.
[EINVAL] MAP_FIXED was specified and the addr parameter
was not page
aligned. fd did not reference a regular or
[ENOMEM] MAP_FIXED was specified and the addr parameter
available. MAP_ANON was specified and insufficient memory
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2),
Due to a limitation of the current vm system (see uvm(9)),
PROT_WRITE without also specifying PROT_READ is
in a segmentation fault when first accessing the mapping). This
means that such descriptors must be opened with O_RDWR,
both read and write permissions on the underlying object.
OpenBSD 3.6 June 4, 1993
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