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To flush a buffer means to send pending buffered data onward to its destination. For program output, that usually means asking the runtime to write data it has temporarily collected instead of waiting for the buffer to fill or an automatic flush condition.
What does “flush your buffer” mean?
A buffer is temporary storage that lets software collect data before passing it along, often reducing the overhead of many small writes. Flushing changes when that data moves; it does not simply erase the buffer.
In C, for example, fflush(stdout) asks the C runtime to pass pending standard-output data through the stream’s underlying write function. POSIX describes the operation for an output stream this way: “fflush() shall cause any unwritten data for that stream to be written to the file.” See the POSIX fflush manual.
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Why flush output explicitly?
Make a prompt appear before waiting for input
A program may print a prompt without a newline and then wait for the user. If the output is buffered, the prompt might not appear before the program waits. Explicitly flushing standard output sends the prompt onward first:
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printf("Name: ");
fflush(stdout);The POSIX manual uses this kind of prompt as an example of why a flush is useful. The same timing issue can occur in interactive programming problems: the judge needs to receive a response before it can provide the next input.
Send output before an automatic condition occurs
C streams can be unbuffered, block-buffered, or line-buffered. With line buffering, a newline can trigger output; other buffering modes may wait for different conditions. On Linux, terminal-connected stdout is normally line-buffered. An explicit fflush() requests output sooner than those automatic conditions. The Linux setbuf manual describes these buffering modes.
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What a stream flush does not guarantee
A C stream flush is not a guarantee that data has been physically committed to disk. The Linux fflush manual says fflush() flushes the C library’s user-space buffers; when physical-storage synchronization matters, it distinguishes that operation from synchronizing kernel buffers, for which fsync() is an example.
Keep the layers separate: flushing a stream passes its pending data through the stream’s write mechanism, while ensuring storage durability is a different requirement.
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Does “flush” mean the same thing for input?
Not necessarily. Output flushing generally sends pending output onward, but input-buffer behavior depends on the language, API, and stream. The Linux manual describes a particular input-stream behavior for seekable files and excludes pipes and terminals from that rule. Do not assume C’s input behavior—or its meaning of fflush()—applies to every kind of buffer or library.
How to handle flush errors in C
In the Linux manual, fflush() returns 0 on success and EOF on failure, with errno set. If successful output matters, check the return value rather than assuming the requested write succeeded.
Closing a C stream also flushes its buffered output as part of closing it, according to the Linux fclose manual. Checking for errors remains important when the output must be confirmed.
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