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msync(2) - phpMan

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



NAME
       msync - synchronize a file with a memory map

SYNOPSIS
       #include <sys/mman.h>

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

DESCRIPTION
       msync()  flushes  changes  made  to the in-core copy of a file that was mapped into memory
       using mmap(2) back to the filesystem.  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 corresponds to the memory area starting at addr  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 specifies that an update be scheduled, but  the  call
       returns immediately.  MS_SYNC asks for an update and waits for it to complete.  MS_INVALI‐
       DATE asks to invalidate other mappings of the same file (so that they can be updated  with
       the fresh values just written).

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

ERRORS
       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.

CONFORMING TO
       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.

AVAILABILITY
       On POSIX systems on which msync() is available, both _POSIX_MAPPED_FILES  and  _POSIX_SYN‐
       CHRONIZED_IO are defined in <unistd.h> to a value greater than 0.  (See also sysconf(3).)

NOTES
       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.   How‐
       ever,  Linux  permits a call to msync() that specifies neither of these flags, with seman‐
       tics 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.

SEE ALSO
       mmap(2)

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

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-04-20                                   MSYNC(2)


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