mmap
- allocate memory, or map files or devices into memory
LIBRARY
Lb libc
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
Fa addr
and continuing for at most
Fa len
bytes to be mapped from the object described by
Fa fd ,
starting at byte offset
Fa offset .
If
Fa 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
Fa 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
Fa addr
is zero, an address will be selected by the system.
The actual starting address of the region is returned.
A successful
Fa mmap
deletes any previous mapping in the allocated address range.
The protections (region accessibility) are specified in the
Fa 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
Fa 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
Fa 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
Fa 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,
Fa 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
Fa addr
to
Fa addr
+
Fa 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 information.
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 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 (Ns 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 (Ns ing);
zero's to the backing file rather than
ftruncate (Ns 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
Fa offset
of 0.
The
Fa fd
argument
must be -1 and
Fa prot
must include at least
PROT_READ
and
PROT_WRITE
This option creates
a memory region that grows to at most
Fa len
bytes in size, starting from the stack top and growing down.
The
stack top is the starting address returned by the call, plus
Fa 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 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:
Bq Er EACCES
The flag
PROT_READ
was specified as part of the
Fa prot
argument and
Fa fd
was not open for reading.
The flags
MAP_SHARED
and
PROT_WRITE
were specified as part of the
Fa flags
and
Fa prot
argument and
Fa fd
was not open for writing.
Bq Er EBADF
The
Fa fd
argument
is not a valid open file descriptor.
Bq Er EINVAL
MAP_FIXED
was specified and the
Fa addr
argument was not page aligned, or part of the desired address space
resides out of the valid address space for a user process.
Bq Er EINVAL
The
Fa len
argument
was negative.
Bq Er EINVAL
MAP_ANON
was specified and the
Fa fd
argument was not -1.
The
Fa offset
argument
was not page-aligned.
(See
Sx BUGS
below.)
Bq Er ENODEV
MAP_ANON
has not been specified and
Fa fd
did not reference a regular or character special file.
Bq Er ENOMEM
MAP_FIXED
was specified and the
Fa 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
sysctl.
The
Fa 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
Fx 2.2 .
Note that an attempt to
mmap ();
zero bytes has no effect and succeeds, while an attempt to
munmap ();
zero bytes will return
Bq Er EINVAL .
