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. 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 filebacked
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 default.
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
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.
[EINVAL] MAP_ANON has not been specified and fd did not reference
a regular or character special file.
[EINVAL] The offset argument was not page-aligned. (See BUGS
[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 perprocess
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 2GB. Mmapping slightly more than 2GB does
not work, but it is possible to map a window of size (filesize % 2GB) for
file sizes of slightly less than 2G, 4GB, 6GB and 8GB.
The limit is imposed for a variety of reasons. Most of them have to do
with FreeBSD not wanting to use 64 bit offsets in the VM system due to
the extreme performance penalty. So FreeBSD uses 32bit page indexes and
this gives FreeBSD a maximum of 8TB filesizes. It is actually bugs in
the file system code that causes the limit to be further restricted to
1TB (loss of precision when doing blockno calculations).
Another reason for the 2GB limit is that file system metadata can reside
at negative offsets.
FreeBSD 5.2.1 November 17, 2001 FreeBSD 5.2.1 [ Back ]