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MMAP(2) BSD System Calls Manual MMAP(2)
NAME
mmap -- allocate memory, or map files or devices into memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
DESCRIPTION
The mmap() system call 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 mul-tiple multiple
tiple of the pagesize, the mapped region may extend past the specified range. Any extension beyond the
end of the mapped object will be zero-filled.
The addr argument is used by the system to determine the starting address of the mapping, and its
interpretation is dependent on the setting of the MAP_FIXED flag. If MAP_FIXED is specified in flags,
the system will try to place the mapping at the specified address, possibly removing a mapping that
already exists at that location. If MAP_FIXED is not specified, then the system will attempt to use
the range of addresses starting at addr if they do not overlap any existing mappings, including memory
allocated by malloc(3) and other such allocators. Otherwise, the system will choose an alternate
address for the mapping (using an implementation dependent algorithm) that does not overlap any exist-ing existing
ing mappings. In other words, without MAP_FIXED the system will attempt to find an empty location in
the address space if the specified address range has already been mapped by something else. If addr is
zero and MAP_FIXED is not specified, then an address will be selected by the system so as not to over-lap overlap
lap any existing mappings in the address space. In all cases, the actual starting address of the
region is returned. If MAP_FIXED is specified, a successful mmap deletes any previous mapping in the
allocated address range. Previous mappings are never deleted if MAP_FIXED is not specified.
The protections (region accessibility) are specified in the prot argument by or'ing the following val-ues: values:
ues:
PROT_NONE Pages may not be accessed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_EXEC Pages may be executed.
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 argument 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 (copy-on-write) 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:
MAP_ANON Map anonymous memory not associated with any specific file. The offset argument is
ignored. Mac OS X specific: the file descriptor used for creating MAP_ANON regions
can be used to pass some Mach VM flags, and can be specified as -1 if no such flags
are associated with the region. Mach VM flags are defined in <mach/vm_statistics.h>
and the ones that currently apply to mmap are:
VM_FLAGS_PURGABLE to create Mach purgable (i.e. volatile) memory
VM_MAKE_TAG(tag) to associate an 8-bit tag with the region
<mach/vm_statistics.h> defines some preset tags (with a VM_MEMORY_ prefix). Users
are encouraged to use tags between 240 and 255. Tags are used by tools such as
vmmap(1) to help identify specific memory regions.
MAP_FILE Mapped from a regular file. (This is the default mapping type, and need not be spec-ified.) specified.)
ified.)
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. If a MAP_FIXED request is successful, the
mapping established by mmap() replaces any previous mappings for the process' pages
in the range from addr to addr + len. Use of this option is discouraged.
MAP_HASSEMAPHORE Notify the kernel that the region may contain semaphores and that special handling
may be necessary.
MAP_PRIVATE Modifications are private (copy-on-write).
MAP_SHARED Modifications are shared.
MAP_NOCACHE Pages in this mapping are not retained in the kernel's memory cache. If the system
runs low on memory, pages in MAP_NOCACHE mappings will be among the first to be
reclaimed. This flag is intended for mappings that have little locality and provides
a hint to the kernel that pages in this mapping are unlikely to be needed again in
the near future.
Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
The close(2) system call does not unmap pages, see munmap(2) for further information.
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.
RETURN VALUES
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.
ERRORS
The mmap() system call will fail if:
[EACCES] The flag PROT_READ was specified as part of the prot argument and fd was not open
for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the
flags and prot argument and fd was not open for writing.
[EBADF] The fd argument is not a valid open file descriptor.
[EINVAL] MAP_FIXED was specified and the addr argument was not page aligned, or part of the
desired address space resides out of the valid address space for a user process.
[EINVAL] flags does not include either MAP_PRIVATE or MAP_SHARED.
[EINVAL] The len argument was negative.
[EINVAL] The offset argument was not page-aligned based on the page size as returned by get-pagesize(3). getpagesize(3).
pagesize(3).
[ENODEV] MAP_ANON has not been specified and the file fd refers to does not support mapping.
[ENOMEM] MAP_FIXED was specified and the addr argument was not available. MAP_FIXED was
specified and the address range specified exceeds the address space limit for the
process. MAP_ANON was specified and insufficient memory was available.
[ENXIO] Addresses in the specified range are invalid for fd.
[EOVERFLOW] Addresses in the specified range exceed the maximum offset set for fd.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
The include file <sys/types.h> is necessary.
COMPATIBILITY
mmap() now returns with errno set to EINVAL in places that historically succeeded. The rules have
changed as follows:
• The flags parameter must specify either MAP_PRIVATE or MAP_SHARED.
• The size parameter must not be 0.
• The off parameter must be a multiple of pagesize, as returned by sysconf().
SEE ALSO
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), shmat(2),
getpagesize(3)
BSD April 21, 2006 BSD
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