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MMAP(2) System Calls Manual MMAP(2)

mmap — map files or devices into memory

#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);

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:

Pages may be executed.
Pages may be read.
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 anonymous memory not associated with any specific file. The file descriptor used for creating MAP_ANON must be -1. The offset parameter is ignored.
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.
Notify the kernel that the region may contain semaphores and that special handling may be necessary.
Permit regions to be inherited across exec(2) system calls.
Modifications are private.
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.

Upon successful completion, mmap returns a pointer to the mapped region. Otherwise, a value of -1 is returned and errno is set to indicate the error.

Mmap() will fail if:

[]
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.
[]
Fd is not a valid open file descriptor.
[]
was specified and the addr parameter was not page aligned or was outside of the valid address range for a process. MAP_ANON was specified and fd was not -1. Fd did not reference a regular or character special file. Len was less than zero.
[]
was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.

getpagesize(2), msync(2), munmap(2), mprotect(2), madvise(2), mincore(2)

BSD 4 May 11, 1995 MMAP(2)