bionic (2) msync.2.gz

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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 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

       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, POSIX.1-2008.

       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_SYNCHRONIZED_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.  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.

SEE ALSO

       mmap(2)

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

COLOPHON

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