mmap - map files or devices into memory
Standard C Library (libc, -lc)
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 len is not a multiple of the pagesize, the mapped
region may extend past the specified range. Any such extension beyond
the end of the mapped object will be zero-filled.
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 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 Pages may not be accessed.
Note that, due to hardware limitations, on some platforms PROT_WRITE may
imply PROT_READ, and PROT_READ may imply PROT_EXEC. Portable programs
should not rely on these flags being separately enforcable.
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. Note
that either MAP_SHARED, MAP_PRIVATE or MAP_COPY must be specified. Sharing,
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 regions is
used only for naming, and may be specified as -1 if no name
is associated with the region.
MAP_FILE Mapped from a regular file or character-special device memory.
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 execve(2) system calls.
Attempt to use the address addr even if it falls within the
normally protected process data or text segment memory
regions. If the requested region of memory is actually present
in the memory map, a different address will be selected
as if MAP_TRYFIXED had not been specified. If addr is NULL,
this flag is ignored and the system will select a mapping
Modifications made by this process are private, however modifications
made by other processes using MAP_SHARED will be
MAP_SHARED Modifications are shared.
MAP_COPY Modifications are private, including other processes.
The close(2) function does not unmap pages, see munmap(2) for further
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
device 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
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_FIXED was specified and the addr parameter was not
page aligned or was outside of the valid address range
for a process. MAP_ANON was specified and fd was not
-1. len was less than zero.
[ENODEV] fd did not reference a regular or character special
[ENOMEM] MAP_FIXED was specified and the addr parameter wasn't
available. MAP_ANON was specified and insufficient
memory was available.
[EOVERFLOW] fd references a regular file and the value of offset
plus len would exceed the offset maximum established
in its open file description.
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munmap(2),
The MAP_COPY flag is not implemented. The current MAP_COPY semantics are
the same as those of the MAP_PRIVATE flag.
BSD May 11, 1995 BSD
[ Back ]