Manual browser: msync(2)

MSYNC(2) System Calls Manual MSYNC(2)


msyncsynchronize a mapped region


Standard C Library (libc, -lc)


#include <sys/mman.h>

msync(void *addr, size_t len, int flags);


The msync() system call writes all pages with shared modifications in the specified region of the process's address space back to permanent storage, and, if requested, invalidates cached data mapped in the region. If len is 0, all modified pages within the region containing addr will be flushed; if len is non-zero, only modified pages containing addr and len succeeding locations will be flushed. Any required synchronization of memory caches will also take place at this time. Filesystem operations on a file that is mapped for shared modifications are unpredictable except after an msync().

The flags argument is formed by or'ing the following values

MS_ASYNC Perform asynchronous writes.
MS_SYNC Perform synchronous writes.
MS_INVALIDATE Invalidate cached data after writing.


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


The following errors may be reported:
The MS_INVALIDATE flag was specified and a portion of the specified region was locked with mlock(2).
The specified flags argument was invalid.
The addr parameter was not page aligned.
The addr parameter did not specify an address part of a mapped region.
An I/O error occurred while writing to the file system.
Addresses in the specified region are outside the range allowed for the address space of the process, or specify one or more pages which are unmapped.


The msync() function conforms to IEEE Std 1003.1b-1993 (“POSIX.1b”)


The msync() function first appeared in 4.4BSD. It was modified to conform to IEEE Std 1003.1b-1993 (“POSIX.1b”) in NetBSD 1.3.
May 9, 2014 NetBSD 7.0