NAME

madvise, posix_madvise — give advice about use of memory

SYNOPSIS

#include <sys/mman.h>

int
madvise(void *addr, size_t len, int behav);

int
posix_madvise(void *addr, size_t len, int behav);

DESCRIPTION

The madvise() system call allows a process that has knowledge of its memory behavior to describe it to the system. The posix_madvise() interface has the same effect, but returns the error value instead of only setting errno.

The possible behaviors are:

MADV_NORMAL

No further special treatment needed.

MADV_RANDOM

Expect random page access patterns.

MADV_SEQUENTIAL

Expect sequential page references.

MADV_WILLNEED

The pages will be referenced soon.

MADV_DONTNEED

The pages will not be referenced soon.

MADV_SPACEAVAIL

Ensure that resources are reserved.

MADV_FREE

The pages don't contain any useful data and can be recycled.

Portable programs that call the posix_madvise() interface should use the aliases POSIX_MADV_NORMAL, POSIX_MADV_RANDOM, POSIX_MADV_SEQUENTIAL, POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the flags described above.

RETURN VALUES

The madvise() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

If successful, the posix_madvise() function will return zero. Otherwise an error number will be returned to indicate the error.

SEE ALSO

minherit(2), mprotect(2), msync(2), munmap(2)

STANDARDS

The posix_madvise() system call conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

The madvise() function first appeared in SunOS 4.0 and has been available since OpenBSD 2.7. The posix_madvise() function first appeared in OpenBSD 4.8.

BUGS

The MADV_WILLNEED behavior is ignored. The MADV_SPACEAVAIL behavior is not implemented and will always fail.