MMAP(2) BSD System Calls Manual MMAP(2)
mmap -- allocate memory, or 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() 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 multiple 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 existing mappings. In other words, without MAP_FIXED the system will attempt to find an empty location in the address space if the spec-
ified 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 overlap 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 map-
pings are never deleted if MAP_FIXED is not specified.
The protections (region accessibility) are specified in the prot argument by or'ing the following values:
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 pro-
grams 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.
VM_FLAGS_SUPERPAGE_SIZE_* to use superpages for the allocation. See <mach/vm_statistics.h> for supported architectures
and sizes (or use VM_FLAGS_SUPERPAGE_SIZE_ANY to have the kernel choose a size). The specified size must be divisible by
the superpage size (except for VM_FLAGS_SUPERPAGE_SIZE_ANY), and if you use MAP_FIXED, the specified address must be prop-
erly aligned. If the system cannot satisfy the request with superpages, the call will fail. Note that currently, superpages
are always wired and not inherited by children of the process.
MAP_FILE Mapped from a regular file. (This is the default mapping type, and need not be specified.)
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 success-
ful, 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.
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 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 getpagesize(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.
The include file <sys/types.h> is necessary.
mmap() now returns with errno set to EINVAL in places that historically succeeded. The rules have changed as follows:
o The flags parameter must specify either MAP_PRIVATE or MAP_SHARED.
o The size parameter must not be 0.
o The off parameter must be a multiple of pagesize, as returned by sysconf().
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), shmat(2), getpagesize(3)
April 21, 2006 BSD