msync(2) - Linux manual page (original) (raw)

msync(2) System Calls Manual msync(2)

NAME top

   msync - synchronize a file with a memory map

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <sys/mman.h>**

   **int msync(void** _addr_**[.**_length_**], size_t** _length_**, int** _flags_**);**

DESCRIPTION top

   **msync**() flushes changes made to the in-core copy of a file that
   was mapped into memory using [mmap(2)](../man2/mmap.2.html) back to the filesystem.
   Without use of this call, there is no guarantee that changes are
   written back before [munmap(2)](../man2/munmap.2.html) is called.  To be more precise, the
   part of the file that corresponds to the memory area starting at
   _addr_ and having length _length_ is updated.

   The _flags_ argument should specify exactly one of **MS_ASYNC** and
   **MS_SYNC**, and may additionally include the **MS_INVALIDATE** bit.
   These bits have the following meanings:

   **MS_ASYNC**
          Specifies that an update be scheduled, but the call returns
          immediately.

   **MS_SYNC**
          Requests an update and waits for it to complete.

   **MS_INVALIDATE**
          Asks to invalidate other mappings of the same file (so that
          they can be updated with the fresh values just written).

RETURN VALUE top

   On success, zero is returned.  On error, -1 is returned, and _[errno](../man3/errno.3.html)_
   is set to indicate the error.

ERRORS top

   **EBUSY  MS_INVALIDATE** was specified in _flags_, and a memory lock
          exists for the specified address range.

   **EINVAL** _addr_ is not a multiple of PAGESIZE; or any bit other than
          **MS_ASYNC** | **MS_INVALIDATE** | **MS_SYNC** is set in _flags_; or both
          **MS_SYNC** and **MS_ASYNC** are set in _flags_.

   **ENOMEM** The indicated memory (or part of it) was not mapped.

VERSIONS top

   According to POSIX, either **MS_SYNC** or **MS_ASYNC** must be specified
   in _flags_, and indeed failure to include one of these flags will
   cause **msync**() to fail on some systems.  However, Linux permits a
   call to **msync**() that specifies neither of these flags, with
   semantics that are (currently) equivalent to specifying **MS_ASYNC**.
   (Since Linux 2.6.19, **MS_ASYNC** is in fact a no-op, since the kernel
   properly tracks dirty pages and flushes them to storage as
   necessary.)  Notwithstanding the Linux behavior, portable, future-
   proof applications should ensure that they specify either **MS_SYNC**
   or **MS_ASYNC** in _flags_.

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001.

   This call was introduced in Linux 1.3.21, and then used **EFAULT**
   instead of **ENOMEM**.  In Linux 2.4.19, this was changed to the POSIX
   value **ENOMEM**.

   On POSIX systems on which **msync**() is available, both
   **_POSIX_MAPPED_FILES** and **_POSIX_SYNCHRONIZED_IO** are defined in
   _<unistd.h>_ to a value greater than 0.  (See also [sysconf(3)](../man3/sysconf.3.html).)

SEE ALSO top

   [mmap(2)](../man2/mmap.2.html)

   B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.

COLOPHON top

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Linux man-pages 6.10 2024-07-23 msync(2)

Pages that refer to this page:madvise(2), mmap2(2), mmap(2), remap_file_pages(2), sync_file_range(2), syscalls(2), nfs(5), systemd.exec(5), fanotify(7), inotify(7), xfs_io(8)