MADVISE(2) Linux Programmer's Manual MADVISE(2)
madvise - give advice about use of memory
int madvise(void *start, size_t length, int advice);
The madvise system call advises the kernel about how to handle paging input/output in the
address range beginning at address start and with size length bytes. It allows an applica-
tion to tell the kernel how it expects to use some mapped or shared memory areas, so that
the kernel can choose appropriate read-ahead and caching techniques. This call does not
influence the semantics of the application (except in the case of MADV_DONTNEED), but may
influence its performance. The kernel is free to ignore the advice.
The advice is indicated in the advice parameter which can be
No special treatment. This is the default.
Expect page references in random order. (Hence, read ahead may be less useful than
Expect page references in sequential order. (Hence, pages in the given range can
be aggressively read ahead, and may be freed soon after they are accessed.)
Expect access in the near future. (Hence, it might be a good idea to read some
Do not expect access in the near future. (For the time being, the application is
finished with the given range, so the kernel can free resources associated with
it.) Subsequent accesses of pages in this range will succeed, but will result
either in re-loading of the memory contents from the underlying mapped file (see
mmap) or zero-fill-on-demand pages for mappings without an underlying file.
On success madvise returns zero. On error, it returns -1 and errno is set appropiately.
EINVAL the value len is negative, start is not page-aligned, advice is not a valid value,
or the application is attempting to release locked or shared pages (with MADV_DONT-
ENOMEM addresses in the specified range are not currently mapped, or are outside the
address space of the process.
ENOMEM (for MADV_WILLNEED) Not enough memory - paging in failed.
EIO (for MADV_WILLNEED) Paging in this area would exceed the process's maximum resident
EBADF the map exists, but the area maps something that isn't a file.
EAGAIN a kernel resource was temporarily unavailable.
The current Linux implementation (2.4.0) views this system call more as a command than as
advice and hence may return an error when it cannot do what it usually would do in
response to this advice. (See the ERRORS description above.) This is nonstandard behav-
The Linux implementation requires that the address start be page-aligned, and allows
length to be zero. If there are some parts of the specified address range that are not
mapped, the Linux version of madvise ignores them and applies the call to the rest (but
returns ENOMEM from the system call, as it should).
The madvise function first appeared in 4.4BSD.
POSIX.1b (POSIX.4). POSIX 1003.1-2001 describes posix_madvise with constants
POSIX_MADV_NORMAL, etc., with a behaviour close to that described here. There is a similar
posix_fadvise for file access.
getrlimit(2), mmap(2), mincore(2), mprotect(2), msync(2), munmap(2)
Linux 2.4.5 2001-06-10 MADVISE(2)