MMAP(2)

NAME
mmap - allocate memory, or map files or devices into memory

LIBRARY
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 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 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 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 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 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:

[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 reference 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].