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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 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_NONE Pages may not be accessed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_EXEC Pages may be executed.
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 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 file descriptor used for creating MAP_ANON
must be -1. The offset argument is ignored.
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_INHERIT This flag never operated as advertised and is no longer
supported. Please refer to minherit(2) for further
MAP_NOCORE Region is not included in a core file.
MAP_NOSYNC Causes data dirtied via this VM map to be flushed to
physical media only when necessary (usually by the
pager) rather than gratuitously. Typically this
prevents the update daemons from flushing pages dirtied
through such maps and thus allows efficient sharing of
memory across unassociated processes using a file-
backed shared memory map. Without this option any VM
pages you dirty may be flushed to disk every so often
(every 30-60 seconds usually) which can create
performance problems if you do not need that to occur
(such as when you are using shared file-backed mmap
regions for IPC purposes). Note that VM/file system
coherency is maintained whether you use MAP_NOSYNC or
not. This option is not portable across UNIX platforms
(yet), though some may implement the same behavior by
WARNING! Extending a file with ftruncate(2), thus
creating a big hole, and then filling the hole by
modifying a shared mmap() can lead to severe file
fragmentation. In order to avoid such fragmentation
you should always pre-allocate the file’s backing store
by write()ing zero’s into the newly extended area prior
to modifying the area via your mmap(). The
fragmentation problem is especially sensitive to
MAP_NOSYNC pages, because pages may be flushed to disk
in a totally random order.
The same applies when using MAP_NOSYNC to implement a
file-based shared memory store. It is recommended that
you create the backing store by write()ing zero’s to
the backing file rather than ftruncate()ing it. You
can test file fragmentation by observing the KB/t
(kilobytes per transfer) results from an “iostat 1”
while reading a large file sequentially, e.g. using “dd
if=filename of=/dev/null bs=32k”.
The fsync(2) system call will flush all dirty data and
metadata associated with a file, including dirty NOSYNC
VM data, to physical media. The sync(8) command and
sync(2) system call generally do not flush dirty NOSYNC
VM data. The msync(2) system call is obsolete since
BSD implements a coherent file system buffer cache.
However, it may be used to associate dirty VM pages
with file system buffers and thus cause them to be
flushed to physical media sooner rather than later.
MAP_PRIVATE Modifications are private.
MAP_SHARED Modifications are shared.
MAP_STACK MAP_STACK implies MAP_ANON, and offset of 0. The fd
argument must be -1 and prot must include at least
PROT_READ and PROT_WRITE. This option creates a memory
region that grows to at most len bytes in size,
starting from the stack top and growing down. The
stack top is the starting address returned by the call,
plus len bytes. The bottom of the stack at maximum
growth is the starting address returned by the call.
The close(2) system call 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 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
[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
[EINVAL] The len argument was negative.
[EINVAL] MAP_ANON was specified and the fd argument was not -1.
The offset argument was not page-aligned. (See BUGS
[ENODEV] MAP_ANON has not been specified and fd did not
reference a regular or character special file.
[ENOMEM] MAP_FIXED was specified and the addr argument was not
available. MAP_ANON was specified and insufficient
memory was available. The system has reached the per-
process mmap limit specified in the vm.max_proc_mmap
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
munlock(2), munmap(2), getpagesize(3), make.conf(5)
The len argument is limited to the maximum file size or available
userland address space. Files may not be able to be made more than 1TB
large on 32 bit systems due to file systems restrictions and bugs, but
address space is far more restrictive. Larger files may be possible on
64 bit systems.
The previous documented limit of 2GB was a documentation bug. That limit
has not existed since FreeBSD 2.2.
Note that an attempt to mmap() zero bytes has no effect and succeeds,
while an attempt to munmap() zero bytes will return [EINVAL].