Question

I don’t understand why sometimes I need to use fflush() and sometimes not.

My program is segfaulting at the moment and I am debugging it with print statements. When a program segfaults, does stdout not flush its buffer automatically?

Answer 1

I don’t understand why sometimes I need to use fflush() and sometimes
not.

Sometimes the stdio buffers are flushed sometimes they aren’t. For example simply including a “\n” in the printed stuff will typically flush it (because stdout is by default line-buffered when attached to a terminal).

When a program segfaults, does stdout not flush its buffer
automatically ?

Stdio buffers are flushed by exit. When a signal (such as SIGSEGV) kills a process, exit is not called. Another way to exit a process without flushing the stdio buffers is to use the Unix-specific call _exit.