MADVISE(2)

MADVISE(2) | System Calls Manual | MADVISE(2)

# NAME

`madvise`, `posix_madvise` —
give advice about use of memory

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/mman.h>`

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

int
`posix_madvise`(void *addr, size_t len, int advice);

# 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 is identical and is provided for standards conformance.

The known behaviors are:

`MADV_NORMAL`
  Tells the system to revert to the default paging behavior.
`MADV_RANDOM`
  Is a hint that pages will be accessed randomly, and prefetching is likely not advantageous.
`MADV_SEQUENTIAL`
  Is a hint that pages will be accessed sequentially, from the lower address to higher address. It might cause the VM system to depress the priority of pages immediately preceding a given page when it is faulted in.
`MADV_WILLNEED`
  Is a hint that pages will be accessed in the near future. It might cause the VM system to make pages that are in a given virtual address range to temporarily have higher priority, and if they are in memory, decrease the likelihood of them being freed. It might immediately map the pages that are already in memory into the process, thereby eliminating unnecessary overhead of going through the entire process of faulting the pages in. It might or might not fault pages in from backing store.
`MADV_DONTNEED`
  Is a hint that pages will not be accessed in the near future. It might allow the VM system to decrease the in-memory priority of pages in the specified range.
`MADV_FREE`
  Gives the VM system the freedom to free pages, and tells the system that information in the specified page range is no longer important.

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

# RETURN VALUES

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

# ERRORS

`madvise`() will fail if:

[`EINVAL`]
  Invalid parameters were provided.

# SEE ALSO

mincore(2), mprotect(2), msync(2), munmap(2), posix_fadvise(2)

# STANDARDS

The `posix_madvise`() system call is expected to conform to the IEEE Std 1003.1-2001 (“POSIX.1”) standard.

# HISTORY

The `madvise` system call first appeared in 4.4BSD, but until NetBSD 1.5 it did not perform any of the requests on, or change any behavior of the address range given. The `posix_madvise`() call was added in NetBSD 5.0.

March 29, 2011 | NetBSD 9.5