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Annotated edit history of madvise(2) version 4, including all changes. View license author blame.
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1 perry 1 !!NAME
2 PerryLorier 2 madvise - give advice about use of memory
1 perry 3
4 !!SYNOPSIS
2 PerryLorier 5 __#include <sys/mman.h>__
1 perry 6
2 PerryLorier 7 __int madvise(void *__''start''__, size_t__ ''length''__, int__ ''advice'' __);__
1 perry 8
9 !!DESCRIPTION
2 PerryLorier 10 The madvise(2) 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 application 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, but may influence its performance. The kernel is free to ignore the advice.
1 perry 11
2 PerryLorier 12 The advice is indicated in the ''advice'' parameter which can be
1 perry 13
3 PerryLorier 14 ;[MADV_NORMAL]: No special treatment. This is the default.
15 ;[MADV_RANDOM]: Expect page references in random order. (Hence, read ahead may be less useful than normally.)
2 PerryLorier 16 ;[MADV_SEQUENTIAL]: 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.)
17 ;[MADV_WILLNEED]: Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.)
18 ;[MADV_DONTNEED]: 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.)
1 perry 19
20 !!RETURN VALUE
2 PerryLorier 21 On success madvise(2) returns zero. On error, it returns -1 and ''errno'' is set appropiately.
1 perry 22
23 !!ERRORS
2 PerryLorier 24 ;[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_DONTNEED).
25 ;[ENOMEM]: addresses in the specified range are not currently mapped, or are outside the address space of the process.
26 ;[ENOMEM]: (for MADV_WILLNEED) Not enough memory - paging in failed.
27 ;[EIO]: (for MADV_WILLNEED) Paging in this area would exceed the process's maximum resident set size.
28 ;[EBADF]: the map exists, but the area maps something that isn't a file.
29 ;[EAGAIN]: a kernel resource was temporarily unavailable.
1 perry 30
31 !!LINUX NOTES
2 PerryLorier 32 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 behaviour.
1 perry 33
2 PerryLorier 34 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
35 that are not mapped, the Linux version of madvise(2) ignores them and applies the call to the rest (but returns [ENOMEM] from the system call, as it
1 perry 36 should).
37 !!HISTORY
2 PerryLorier 38 The __madvise__ function first appeared in 4.4BSD.
1 perry 39
40 !!CONFORMING TO
4 JohnMcPherson 41 [POSIX].1b (POSIX.4). The Austin draft describes __posix_madvise__ with constants POSIX_MADV_NORMAL, etc., with a behaviour close to that described here. There is a
2 PerryLorier 42 similar ''posix_fadvise'' for file access.
1 perry 43
44 !!SEE ALSO
2 PerryLorier 45 getrlimit(2), mmap(2), mincore(2), mprotect(2), msync(2), munmap(2)
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