msync(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

MSYNC(3P)               POSIX Programmer's Manual              MSYNC(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       msync — synchronize memory with physical storage

SYNOPSIS         top

       #include <sys/mman.h>

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

DESCRIPTION         top

       The msync() function shall write all modified data to permanent
       storage locations, if any, in those whole pages containing any
       part of the address space of the process starting at address addr
       and continuing for len bytes. If no such storage exists, msync()
       need not have any effect. If requested, the msync() function
       shall then invalidate cached copies of data.

       The implementation may require that addr be a multiple of the
       page size as returned by sysconf().

       For mappings to files, the msync() function shall ensure that all
       write operations are completed as defined for synchronized I/O
       data integrity completion. It is unspecified whether the
       implementation also writes out other file attributes. When the
       msync() function is called on MAP_PRIVATE mappings, any modified
       data shall not be written to the underlying object and shall not
       cause such data to be made visible to other processes. It is
       unspecified whether data in MAP_PRIVATE mappings has any
       permanent storage locations.  The effect of msync() on a shared
       memory object or a typed memory object is unspecified.  The
       behavior of this function is unspecified if the mapping was not
       established by a call to mmap().

       The flags argument is constructed from the bitwise-inclusive OR
       of one or more of the following flags defined in the <sys/mman.h>
       header:

              ┌──────────────────┬──────────────────────────────┐
              │Symbolic Constant Description          │
              ├──────────────────┼──────────────────────────────┤
              │MS_ASYNC          │ Perform asynchronous writes. │
              │MS_SYNC           │ Perform synchronous writes.  │
              │MS_INVALIDATE     │ Invalidate cached data.      │
              └──────────────────┴──────────────────────────────┘
       When MS_ASYNC is specified, msync() shall return immediately once
       all the write operations are initiated or queued for servicing;
       when MS_SYNC is specified, msync() shall not return until all
       write operations are completed as defined for synchronized I/O
       data integrity completion. Either MS_ASYNC or MS_SYNC shall be
       specified, but not both.

       When MS_INVALIDATE is specified, msync() shall invalidate all
       cached copies of mapped data that are inconsistent with the
       permanent storage locations such that subsequent references shall
       obtain data that was consistent with the permanent storage
       locations sometime between the call to msync() and the first
       subsequent memory reference to the data.

       If msync() causes any write to a file, the file's last data
       modification and last file status change timestamps shall be
       marked for update.

RETURN VALUE         top

       Upon successful completion, msync() shall return 0; otherwise, it
       shall return -1 and set errno to indicate the error.

ERRORS         top

       The msync() function shall fail if:

       EBUSY  Some or all of the addresses in the range starting at addr
              and continuing for len bytes are locked, and MS_INVALIDATE
              is specified.

       EINVAL The value of flags is invalid.

       ENOMEM The addresses in the range starting at addr and continuing
              for len bytes are outside the range allowed for the
              address space of a process or specify one or more pages
              that are not mapped.

       The msync() function may fail if:

       EINVAL The value of addr is not a multiple of the page size as
              returned by sysconf().

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       The msync() function is only supported if the Synchronized Input
       and Output option is supported, and thus need not be available on
       all implementations.

       The msync() function should be used by programs that require a
       memory object to be in a known state; for example, in building
       transaction facilities.

       Normal system activity can cause pages to be written to disk.
       Therefore, there are no guarantees that msync() is the only
       control over when pages are or are not written to disk.

RATIONALE         top

       The msync() function writes out data in a mapped region to the
       permanent storage for the underlying object. The call to msync()
       ensures data integrity of the file.

       After the data is written out, any cached data may be invalidated
       if the MS_INVALIDATE flag was specified. This is useful on
       systems that do not support read/write consistency.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       mmap(3p), sysconf(3p)

       The Base Definitions volume of POSIX.1‐2017, sys_mman.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                         MSYNC(3P)

Pages that refer to this page: sys_mman.h(0p)mmap(3p)posix_typed_mem_open(3p)