OpenBSD manual page server

NAME

mmap — map files or devices into memory

SYNOPSIS

#include <sys/types.h>
#include <sys/mman.h>

void *
mmap(void *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 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:

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 currently be -1 indicating no name is associated with the region.

MAP_FILE

Mapped from a regular file or character-special device memory. (This is the default mapping type, and need not be specified.)

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(3) system calls.

MAP_PRIVATE

Modifications are private.

MAP_SHARED

Modifications are shared.

MAP_TRYFIXED

Attempt to use the hint provided by addr even if the kernel would normally prefer a different address, but do not fail if the address is not available. This option is provided for compatibility with other operating systems and its use in OpenBSD is discouraged.

MAP_COPY

Modifications are private and, unlike MAP_PRIVATE, modifications made by others are not visible. This option is deprecated, shouldn't be used and behaves just like MAP_PRIVATE in the current implementation.

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. The symbol MAP_FAILED is defined in the header ⟨sys/mman.h⟩. No successful return from mmap() will return the value MAP_FAILED.

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 addr and size specify a region that would extend beyond the end of the address space. fd 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 and insufficient memory was available.

SEE ALSO

madvise(2), mincore(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3)

BUGS

Due to a limitation of the current vm system (see uvm(9)), mapping descriptors PROT_WRITE without also specifying PROT_READ is useless (results in a segmentation fault when first accessing the mapping). This means that such descriptors must be opened with O_RDWR, which requires both read and write permissions on the underlying object.