Go to main content

man pages section 3: Basic Library Functions

Exit Print View

Updated: Wednesday, July 27, 2022



aio_fsync - asynchronous file synchronization


#include <aio.h>

int aio_fsync(int op, struct aiocb *aiocbp);


The aio_fsync() function asynchronously forces all I/O operations associated with the file indicated by the file descriptor aio_fildes member of the aiocb structure referenced by the aiocbp argument and queued at the time of the call to aio_fsync() to the synchronized I/O completion state. The function call returns when the synchronization request has been initiated or queued to the file or device (even when the data cannot be synchronized immediately).

If op is O_DSYNC, all currently queued I/O operations are completed as if by a call to fdatasync(3C); that is, as defined for synchronized I/O data integrity completion. If op is O_SYNC, all currently queued I/O operations are completed as if by a call to fsync(3C); that is, as defined for synchronized I/O file integrity completion. If the aio_fsync() function fails, or if the operation queued by aio_fsync() fails, then, as for fsync(3C) and fdatasync(3C), outstanding I/O operations are not guaranteed to have been completed.

If aio_fsync() succeeds, then it is only the I/O that was queued at the time of the call to aio_fsync() that is guaranteed to be forced to the relevant completion state. The completion of subsequent I/O on the file descriptor is not guaranteed to be completed in a synchronized fashion.

The aiocbp argument refers to an asynchronous I/O control block. The aiocbp value may be used as an argument to aio_error(3C) and aio_return(3C) in order to determine the error status and return status, respectively, of the asynchronous operation while it is proceeding. When the request is queued, the error status for the operation is EINPROGRESS. When all data has been successfully transferred, the error status will be reset to reflect the success or failure of the operation. If the operation does not complete successfully, the error status for the operation will be set to indicate the error. The aio_sigevent member determines the asynchronous notification to occur when all operations have achieved synchronized I/O completion (see signal.h(3HEAD)). All other members of the structure referenced by aiocbp are ignored. If the control block referenced by aiocbp becomes an illegal address prior to asynchronous I/O completion, then the behavior is undefined.

If the aio_fsync() function fails or the aiocbp indicates an error condition, data is not guaranteed to have been successfully transferred.

If aiocbp is NULL, then no status is returned in aiocbp, and no signal is generated upon completion of the operation.

Return Values

The aio_fsync() function returns 0 to the calling process if the I/O operation is successfully queued; otherwise, the function returns −1 and sets errno to indicate the error.


The aio_fsync() function will fail if:


The requested asynchronous operation was not queued due to temporary resource limitations.


The aio_fildes member of the aiocb structure referenced by the aiocbp argument is not a valid file descriptor open for writing.


The system does not support synchronized I/O for this file.


A value of op other than O_DSYNC or O_SYNC was specified.

In the event that any of the queued I/O operations fail, aio_fsync() returns the error condition defined for read(2) and write(2). The error will be returned in the error status for the asynchronous fsync(3C) operation, which can be retrieved using aio_error(3C).


The aio_fsync() function has a transitional interface for 64-bit file offsets. See lf64(7).


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

fcntl(2), open(2), read(2), write(2), aio_error(3C), aio_return(3C), fdatasync(3C), fsync(3C), aio.h(3HEAD), fcntl.h(3HEAD), signal.h(3HEAD), attributes(7), lf64(7), standards(7)