FreeBSD Man Pages

mmap(2)

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 page-
     size, 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 conve-
     nience 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.	Shar-
     ing, 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 can-
		       not be used, mmap() will fail.  If MAP_FIXED is speci-
		       fied, addr must be a multiple of the pagesize.  Use of
		       this option is discouraged.

     MAP_HASSEMAPHORE  Notify the kernel that the region may contain sema-
		       phores 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.

		       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 perfor-
		       mance 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 cre-
		       ating a big hole, and then filling the hole by modify-
		       ing a shared mmap() can lead to severe file fragmenta-
		       tion.  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 ran-
		       dom 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, start-
		       ing 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,

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 negative.

     [EINVAL]		MAP_ANON was specified and the fd argument was not -1.

     [EINVAL]		MAP_ANON has not been specified and fd did not refer-
			ence a regular or character special file.

     [EINVAL]		The offset argument was not page-aligned.  (See BUGS
			below.)

     [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
			sysctl.

SEE ALSO

     madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
     munlock(2), munmap(2), getpagesize(3), make.conf(5)

BUGS

     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.

     Note that an attempt to mmap() zero bytes has no effect and succeeds,
     while an attempt to munmap() zero bytes will return [EINVAL].

FreeBSD 5.4		       November 17, 2001		   FreeBSD 5.4

SPONSORED LINKS