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

类似问题

Which is used to open “file.bin” for reading binary data in C?

Here is a typical usage of fopen(): FILE *fp = fopen("numbers.txt", "r"); if (fp == NULL) { fprintf(stderr, "cannot open file\n"); exit(1); } Which of these indicate that a failure has happened?

Which ONE of the following thalamic nuclei is targeted by the trigeminothalamic tract?

Which ONE of the following is disinhibited in the direct pathway of the basal ganglia motor loop

30. What is the main purpose of the lecture?

Section FChoose a, b or c as the best answer.29. Which statement is the speaker most likely to agree with?

28. 15-minute neighbourhoods provide residents with inexpensive ____ .

27. ____ % of global car ownership is in the U.S.A.

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单