Msync

msync is a POSIX-compliant system call used to synchronize a memory-mapped region with its underlying file on disk. It serves to ensure that changes made to a mapped region are persisted and, in some cases, to refresh the region from the file after external updates.

Prototype and flags

The function has the form: int msync(void *addr, size_t length, int flags). Flags commonly include MS_SYNC, MS_ASYNC, and MS_INVALIDATE. MS_SYNC requests that all modified pages within the specified range be flushed to storage and that the call blocks until the I/O completes. MS_ASYNC enqueues the write operations and may return before the I/O finishes. MS_INVALIDATE, when specified, invalidates any cached copies of the mapped region in the calling process, so subsequent accesses will reread data from the file.

Return values

On success, msync returns 0. On failure, -1 is returned and errno is set to indicate the error (for example, EINVAL for an invalid address or length, EIO for an I/O error, or ENOMEM for insufficient memory).

Usage considerations

msync is relevant when working with mmap’d files. Using MS_SYNC provides strong persistence guarantees but may incur latency due to synchronous I/O. MS_ASYNC can improve responsiveness at the risk of delayed persistence in the event of a crash. MS_INVALIDATE is useful in scenarios where the underlying file may have been changed by another process, allowing the calling process to refresh its view of the data.

Portability and related APIs

msync is available on most Unix-like systems implementing POSIX mmap, including Linux, macOS, and BSD variants. Windows uses different memory-mapped I/O mechanisms (such as FlushViewOfFile) rather than msync. See also mmap and munmap.