msync - Man Page

synchronize memory with physical storage


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.


#include <sys/mman.h>

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


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 ConstantDescription
MS_ASYNCPerform asynchronous writes.
MS_SYNCPerform synchronous writes.
MS_INVALIDATEInvalidate 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

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


The msync() function shall fail if:


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


The value of flags is invalid.


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:


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

The following sections are informative.



Application Usage

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.


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


See Also

mmap(), sysconf()

The Base Definitions volume of POSIX.1-2017, <sys_mman.h>

Referenced By

mmap(3p), posix_typed_mem_open(3p), sys_mman.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual