madvise - give advice about use of memory


#include <sys/mman.h>

int madvise(void *addr, size_t length, int advice);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

madvise(): _BSD_SOURCE


The madvise() system call advises the kernel about how to handle paging input/output in the address range beginning at address addr 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 (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 argument which can be

No special treatment. This is the default.
Expect page references in random order. (Hence, read ahead may be less useful than normally.)
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 pages ahead.)
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 reloading of the memory contents from the underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file.
MADV_REMOVE (Since Linux 2.6.16)
Free up a given range of pages and its associated backing store. Currently, only shmfs/tmpfs supports this; other file systems return with the error ENOSYS.
MADV_DONTFORK (Since Linux 2.6.16)
Do not make the pages in this range available to the child after a fork(2). This is useful to prevent copy-on-write semantics from changing the physical location of a page(s) if the parent writes to it after a fork(2). (Such page relocations cause problems for hardware that DMAs into the page(s).)
MADV_DOFORK (Since Linux 2.6.16)
Undo the effect of MADV_DONTFORK, restoring the default behavior, whereby a mapping is inherited across fork(2).
MADV_HWPOISON (Since Linux 2.6.32)
Poison a page and handle it like a hardware memory corruption. This operation is only available for privileged (CAP_SYS_ADMIN) processes. This operation may result in the calling process receiving a SIGBUS and the page being unmapped. This feature is intended for testing of memory error-handling code; it is only available if the kernel was configured with CONFIG_MEMORY_FAILURE.
MADV_SOFT_OFFLINE (Since Linux 2.6.33)
Soft offline the pages in the range specified by addr and length. The memory of each page in the specified range is copied to a new page, and the original page is offlined (i.e., no longer used, and taken out of normal memory management). The effect of the MADV_SOFT_OFFLINE operation is normally invisible to (i.e., does not change the semantics of) the calling process. This feature is intended for testing of memory error-handling code; it is only available if the kernel was configured with CONFIG_MEMORY_FAILURE.
MADV_MERGEABLE (since Linux 2.6.32)
Enable Kernel Samepage Merging (KSM) for the pages in the range specified by addr and length. The kernel regularly scans those areas of user memory that have been marked as mergeable, looking for pages with identical content. These are replaced by a single write-protected page (which is automatically copied if a process later wants to update the content of the page). KSM only merges private anonymous pages (see mmap(2)). The KSM feature is intended for applications that generate many instances of the same data (e.g., virtualization systems such as KVM). It can consume a lot of processing power; use with care. See the kernel source file Documentation/vm/ksm.txt for more details. The MADV_MERGEABLE and MADV_UNMERGEABLE operations are only available if the kernel was configured with CONFIG_KSM.
MADV_UNMERGEABLE (since Linux 2.6.32)
Undo the effect of an earlier MADV_MERGEABLE operation on the specified address range; KSM unmerges whatever pages it had merged in the address range specified by addr and length.


On success madvise() returns zero. On error, it returns -1 and errno is set appropriately.


A kernel resource was temporarily unavailable.
The map exists, but the area maps something that isn't a file.
This error can occur for the following reasons:
The value len is negative.
addr is not page-aligned.
advice is not a valid value
The application is attempting to release locked or shared pages (with MADV_DONTNEED).
MADV_MERGEABLE or MADV_UNMERGEABLE was specified in advice, but the kernel was not configured with CONFIG_KSM.
(for MADV_WILLNEED) Paging in this area would exceed the process's maximum resident set size.
(for MADV_WILLNEED) Not enough memory: paging in failed.
Addresses in the specified range are not currently mapped, or are outside the address space of the process.


POSIX.1b. POSIX.1-2001 describes posix_madvise(3) with constants POSIX_MADV_NORMAL, etc., with a behavior close to that described here. There is a similar posix_fadvise(2) for file access.



Linux Notes

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 behavior.

The Linux implementation requires that the address addr 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).


getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)


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