MMAP(2)

NAME
mmap - map files or devices into memory

SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
caddr_t
mmap(caddr_t addr, size_t len, int prot, int flags, int fd, off_t offset);

DESCRIPTION
The mmap() function 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_EXEC
    Pages may be executed.

PROT_READ
    Pages may be read.

PROT_WRITE
    Pages may be written.

The flags parameter 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 parameter 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
    Permit regions to be inherited across execve(2) system calls.

MAP_PRIVATE
    Modifications are private.

MAP_SHARED
    Modifications are shared.

The close(2) function 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
Mmap() will fail if:

[EACCES]
    The flag PROT_READ was specified as part of the prot parameter and fd was not open for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and prot parameters and fd was not open for writing.

[EBADF]
    Fd is not a valid open file descriptor.

[EINVAL]
    MAP_FIXED was specified and the addr parameter was not page aligned, or part of the desired address space resides out of the valid address space for a user process.

[EINVAL]
    Len was negative.

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

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

[EINVAL]
    Offset was not page-aligned. (See BUGS below.)

[ENOMEM]
    MAP_FIXED was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.

SEE ALSO
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3)

BUGS
len is limited to 2GB. Mmapping slightly more than 2GB doesn't 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's actually bugs in the filesystem 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 filesystem metadata can reside at negative offsets.

We currently can only deal with page aligned file offsets.