In C, what does the fflush function do when used with a file stream?

Question

BlackTom AI · Accepted Answer

fflush causes any buffered output for a FILE stream to be written immediately to the underlying file or device.

1) Relevant knowledge points and concepts
- C stdio uses buffering: data written with functions like fprintf, fputs, or putc is often placed in a buffer and not sent to the OS/file until the buffer fills or the stream is closed.
- fflush(FILE *stream) forces the transfer of that buffered output to the file/device.
- fflush(NULL) flushes all open output streams.
- fflush on an input-only stream yields undefined behavior (and some implementations may discard unread input), and streams opened for update have additional constraints.

2) Step-by-step reasoning process
- When you call fflush(stream), the runtime checks the stream’s output buffer.
- If there is unwritten data, the library issues the necessary write system call(s) to ensure that buffered bytes are actually written to the underlying file descriptor/device.
- The function then updates internal bookkeeping and returns 0 on success (EOF on error), indicating the flush completed.

3) Intermediate steps (conceptual)
- Data written by fprintf -> placed in stdio buffer.
- fflush(stdout) invoked -> stdio issues write(2) to underlying descriptor and empties buffer.
- After fflush, subsequent reads or program termination will see that data persisted.

4) Why the correct answer is correct
- The provided correct option states “Flushes any buffered data to the associated file,” which exactly matches the standard behavior: moving buffered output to the file/device immediately.

5) Why other options are incorrect
- “Reads a character from the file stream.” — fflush does not perform input; that is fgetc/getchar.
- “Skips the next character in the file stream.” — skipping is done by fseek or reading and discarding, not fflush.
- “Sets the file position indicator to the beginning of the file.” — that is done by rewind or fseek, not fflush.

6) Conclusion
The correct answer is “Flushes any buffered data to the associated file.” This matches the fflush semantics in C and confirms the provided answer.

In C, what does the fflush function do when used with a file stream?单项选择题

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