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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →To flush a buffer is to send pending buffered data onward to its destination. For a C output stream, fflush(stdout) asks the C runtime to write output it is holding rather than waiting for an automatic flush condition. It changes when output is sent; it does not simply erase the buffer or guarantee that data is physically saved to disk.
What a buffer flush does
Software may collect output in a buffer before sending it onward, reducing the need for many small writes. Flushing tells the relevant stream or API to pass pending data along. In C, fflush() sends buffered output through the stream’s underlying write function. The exact meaning depends on the API and the kind of buffer, so C’s rules should not be assumed to apply to every language or library. Linux man-pages: fflush(3)
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Why flush output before waiting for input?
A common use is an interactive prompt that has no newline. Without an explicit flush, a program might print the prompt into a buffer and then wait for input before the prompt appears. Calling fflush(stdout) asks the C runtime to send the prompt first:
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fflush(stdout);POSIX describes this prompt-and-input pattern because standard output is usually buffered. The Open Group: POSIX fflush(3p), POSIX.1-2017
The same timing issue matters in interactive programming problems: the judge or other program needs to receive your response before it can provide the next input. A contest problem, for example, may explicitly instruct participants to flush output in an interactive task. CALICO Spring ’23 contest problem
When output may flush automatically
C streams can be unbuffered, block-buffered, or line-buffered. On Linux, terminal-connected stdout is normally line-buffered, so a newline can trigger output. Other buffering modes and destinations behave differently; an explicit flush requests output before those automatic conditions occur. Linux man-pages: setbuf(3)
What flushing does not guarantee
A C stream flush is not the same as guaranteeing that data has reached physical storage. The Linux manual explains that fflush() flushes the C library’s user-space buffer through the underlying write function; data may still be held in kernel buffers. When physical-storage synchronization matters, the manual points to operations such as fsync() or sync(). Linux man-pages: fflush(3)
Output and input flushing are not interchangeable
For output, flushing sends pending data onward. Input-buffer behavior is more specialized and depends on the API and stream. Linux documents a particular fflush() behavior for seekable input files; that rule does not extend to pipes or terminals. Consult the documentation for the specific language, library, and stream rather than treating every buffer flush as an output operation. Linux man-pages: fflush(3)
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Check for output errors when they matter
On Linux, fflush() returns 0 on success and EOF if an error occurs, with errno set. Code that must know whether output succeeded should check the return value. Closing a C stream also flushes buffered output, but applications that depend on successful output should still handle errors. Linux man-pages: fflush(3) Linux man-pages: fclose(3)
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