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MMAP(2) OpenBSD Programmer's 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 pages starting at addr and continuing for at
most len bytes to be mapped from the object described by fd, starting at
byte offset offset. If offset or len is not a multiple of the pagesize,
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 conve-
nience to the system, the actual address of the region may differ from
the address supplied.) If addr is zero, an address will be selected by
the system. The actual starting address of the region is returned. A
successful mmap deletes any previous mapping in the allocated address
The protections (region accessibility) are specified in the prot argument
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, mapping op-
tions, and whether modifications made to the mapped copy of the page are
private to the process or are to be shared with other references. Shar-
ing, mapping type, and options are specified in the flags argument by
OR'ing the following values:
MAP_ANON Map anonymous memory not associated with any specific file.
The file descriptor used for creating MAP_ANON must currently
be -1 indicating no name is associated with the region.
MAP_FILE Mapped from a regular file or character-special device memo-
ry. (This is the default mapping type, and need not be spec-
MAP_FIXED Do not permit the system to select a different address than
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 discouraged.
Notify the kernel that the region may contain semaphores and
that special handling may be necessary.
Permit regions to be inherited across exec(3) system calls.
Modifications are private.
MAP_SHARED Modifications are shared.
MAP_COPY Modifications are private and, unlike MAP_PRIVATE, modifica-
tions made by others are not visible. This option is depre-
cated, shouldn't be used and behaves just like MAP_PRIVATE in
the current implementation.
The close(2) function does not unmap pages; see munmap(2) for further in-
The current design does not allow a process to specify the location of
swap space. In the future we may define an additional mapping type,
MAP_SWAP, in which the file descriptor argument specifies a file or de-
vice to which swapping should be done.
Upon successful completion, mmap returns a pointer to the mapped region.
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 <sys/mman.h>.
No successful return from mmap() will return the value MAP_FAILED.
mmap() will fail if:
[EACCES] The flag PROT_READ was specified as part of the prot param-
eter 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] 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 character spe-
[ENOMEM] MAP_FIXED was specified and the addr parameter wasn't
available. MAP_ANON was specified and insufficient memory
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munmap(2),
Due to a limitation of the current vm system (see uvm(9)), mapping de-
scriptors PROT_WRITE without also specifying PROT_READ is useless (re-
sults 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.
OpenBSD 3.8 June 4, 1993 2