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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 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 range.
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 must be 0.
MAP_ANONYMOUS This flag is identical to MAP_ANON and is provided for compatibility.
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 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()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
usually not needed 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_PREFAULT_READ
Immediately update the calling process's lowest-level virtual address translation
structures, such as its page table, so that every memory resident page within the region
is mapped for read access. Ordinarily these structures are updated lazily. The effect
of this option is to eliminate any soft faults that would otherwise occur on the initial
read accesses to the region. Although this option does not preclude prot from including
PROT_WRITE, it does not eliminate soft faults on the initial write accesses to the
region.
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 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.
NOTES
Although this implementation does not impose any alignment restrictions on the offset argument, a portable
program must only use page-aligned values.
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] The len argument was equal to zero.
[EINVAL] MAP_ANON was specified and the fd argument was not -1.
[EINVAL] MAP_ANON was specified and the offset argument was not 0.
[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.
SEE ALSO
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3)
BUGS
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.