GeezerVison

NAME

mmap — map files or devices into memory

SYNOPSIS

#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 contents of fd, starting at offset, to be mapped in memory at the given addr. The mapping will extend at least len bytes, subject to page alignment restrictions.

The addr argument describes the address where the system should place the mapping. If the MAP_FIXED flag is specified, the allocation will happen at the specified address, replacing any previously established mappings in its range. Otherwise, the mapping will be placed at the available spot at addr; failing that it will be placed "close by". If addr is NULL, the system can pick any address. Except for MAP_FIXED mappings, the system will never replace existing mappings.

The len argument describes the minimum amount of bytes the mapping will span. Since mmap() maps pages into memory, len may be rounded up to hit a page boundary. If len is 0, the mapping will fail with EINVAL.

If an fd and offset are specified, the resulting address may end up not on a page boundary, in order to align the page offset in the addr to the page offset in offset.

The protections (region accessibility) are specified in the prot argument. It should either be PROT_NONE (no permissions) or the bitwise OR of one or more of 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. Exactly one of the first two values must be specified:

Any combination of the following flags may additionally be used:

Finally, the following flags are also provided for source compatibility with code written for other operating systems, but are not recommended for use in new OpenBSD code:

The close(2) function does not unmap pages; see munmap(2) for further information.

RETURN VALUES

The mmap() function returns a pointer to the mapped region if successful; otherwise the value MAP_FAILED is returned and the global variable errno is set to indicate the error. A successful return from mmap() will never 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_PRIVATE and MAP_SHARED were both specified.

[EINVAL]

MAP_FIXED was specified and the addr parameter was not page aligned.

[EINVAL]

addr and len specified a region that would extend beyond the end of the address space.

[EINVAL]

fd did not specify a regular, character special, or block special file.

[EINVAL]

fd specified a character special or block special file and the underlying device does not support memory mapping.

[EINVAL]

The allocation len was 0.

[ENOMEM]

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

[ENOTSUP]

The accesses requested in the prot argument are not allowed. In particular, PROT_WRITE | PROT_EXEC mappings are not permitted unless the filesystem is mounted wxallowed and the process is link-time tagged with wxneeded. (See also kern.wxabort in sysctl(2) for a method to diagnose failure).

[EPERM]

The addr and len parameters specify a region which contains at least one page marked immutable.

SEE ALSO

madvise(2), mimmutable(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3), core(5)

STANDARDS

The mmap() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

A fully functional mmap() system call first appeared in SunOS 4.0 and has been available since 4.3BSD-Net/2.

CAVEATS

IEEE Std 1003.1-2008 (“POSIX.1”) specifies that references to pages beyond the end of a mapped object shall generate a SIGBUS signal; however, on some architectures OpenBSD generates a SIGSEGV signal in this case instead.

Additionally, IEEE Std 1003.1-2008 (“POSIX.1”) specifies that mmap() shall fail with EINVAL if neither MAP_PRIVATE nor MAP_SHARED is specified by flags; however, for compatibility with old programs, OpenBSD instead defaults to MAP_SHARED for mappings of character special files, and to MAP_PRIVATE for all other mappings. New programs should not rely on this behavior.

BUGS

Due to a limitation of the current vm system (see uvm_init(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.