MMAP(2) manual page

NAME

mmap, munmap - map or unmap 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 );
int munmap(caddr_t addr, size_t len);

DESCRIPTION

WARNING: This is a BSD man page. Linux 0.99.11 can't map files, and can't do other things documented here.

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 offset or len is not a multiple of the pagesize, the mapped region may extend past the specified range.

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 regions is used only for naming, and may be specified as -1 if no name is associated with the region.
MAP_FILE: Mapped from a regular file or character-special device memory.
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 exec(2) system calls.
MAP_PRIVATE: Modifications are private.
MAP_SHARED: Modifications are shared.

The munmap system call deletes the mappings for the specified address range, and causes further references to addresses within the range to generate invalid memory references.

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 -1 is returned and errno is set to indicate the error.

ERRORS

For mmap:

EACCES: The flag PROT_READ was specified as part of the prot parameter and fd was not open for reading. The flags PROT_WRITE, MAP_SHARED and MAP_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: One of MAP_ANON or MAP_FILE was not specified as part of the flags parameter. MAP_FIXED was specified and the addr parameter was not page aligned. did not reference a regular or character special file.
ENOMEM: MAP_FIXED was specified and the addr parameter wasn't available. MAP_ANON was specified an insufficient memory was available.

BUGS

This is a BSD man page. Many of the details explained here are not implemented under Linux. Please consult the Linux kernel source code for complete details on the current implementation of the mmap function.