fflush_unlocked - Linux


fflush_unlocked is a powerful system call that provides fine-grained control over the flushing of buffered output. It ensures that all unwritten data in a stream is immediately written to the disk, bypassing the typical buffering mechanisms. This can be useful in applications that require instant persistence of data, such as databases or logging frameworks.


int fflush_unlocked ( FILE *stream );


  • stream: Pointer to the FILE object representing the stream to be flushed.




1. Flushing an Output File:

FILE *fp = fopen("output.txt", "w");
fprintf(fp, "This is an example of fflush_unlocked.\n");

2. Ensuring Data Integrity in Logging:

FILE *logfile = fopen("log.txt", "a");
fprintf(logfile, "Critical error occurred: %s\n", error_message);
fflush_unlocked(logfile);  // Immediately write the log entry to disk

Common Issues

  • Incorrect Usage: Calling fflush_unlocked with an invalid FILE pointer can lead to runtime errors. Ensure that the stream has been properly opened before attempting to flush it.
  • Blocking: fflush_unlocked is a blocking operation. If the device associated with the stream is slow, the call may pause the execution of the program until the flush operation completes.


fflush_unlocked can be integrated with other system calls and tools to enhance its functionality:

  • fsync: After flushing the stream with fflush_unlocked, use fsync to ensure that the data is not only written to the system’s internal buffers but also committed to the permanent storage device.
  • flock: Use flock to lock the file associated with the stream to prevent other processes from modifying the data while it is being flushed.

Related Commands

  • fflush: Simulates the behavior of fflush_unlocked but uses locks to ensure that the flush operation is serialized.
  • fsync: Ensures that data is physically written to a permanent storage device.
  • flock: Provides file locking mechanisms for inter-process synchronization.