C stdio buffers: what is the purpose of fflush? Choose the best description of how fflush operates on streams.

Introduction / Context:
fflush ensures that buffered output is physically written to its destination. The exact scope depends on the argument passed to the function per ISO C and common extensions.

Given Data / Assumptions:

• We are dealing with text or binary output streams from <stdio.h>.
• Some systems support the extension fflush(NULL) to flush all open output streams.

Concept / Approach:

int fflush(FILE *stream) flushes buffered output for the specified stream when stream is non-NULL. Many implementations also define that fflush(NULL) flushes all open output streams. Input streams are not affected by flushing in standard C (except where system-specific behavior exists).

Step-by-Step Solution:

Verification / Alternative check:

Testing with buffered stdout shows that fflush(stdout) forces immediate display; calling fflush(NULL) (where available) flushes all outputs, useful before abnormal termination.

Why Other Options Are Wrong:

flushes only specified stream: Incomplete because many systems also support flushing all streams via NULL.

flushes input/output buffer and flushes file buffer: Vague and do not capture the argument-based behavior.

Common Pitfalls:

• Assuming input buffers are flushed by fflush—the standard does not require that.
• Expecting flushing to fix logical errors such as missing newlines.

Final Answer:

flushes all streams and specified streams.