NOTE: click here if you get an empty page.

MSYNC(2)		   Linux Programmer's Manual		      MSYNC(2)


msync - synchronize a file with a memory map


#include <sys/mman.h> int msync(void *start, size_t length, int flags);


msync() flushes changes made to the in-core copy of a file that was mapped into memory using mmap(2) back to disk. Without use of this call there is no guarantee that changes are written back before munmap(2) is called. To be more precise, the part of the file that corre- sponds to the memory area starting at start and having length length is updated. The flags argument may have the bits MS_ASYNC, MS_SYNC and MS_INVALIDATE set, but not both MS_ASYNC and MS_SYNC. MS_ASYNC speci- fies that an update be scheduled, but the call returns immediately. MS_SYNC asks for 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).


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


EINVAL start 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.


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).)


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.


mmap(2) B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. Linux 2.4 2003-08-21 MSYNC(2)

1994 Man-cgi 1.15, Panagiotis Christias <>