Output not printing without fflush(stdout)

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?

Overmatter

stdout is just a pointer, it doesn't "do" anything by itself. The real question would be, "doesn't the OS flush all open files?" – Haleyhalf

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.

Raki

Answer 2

No, why should it. The program gets killed by the operating system. If a segfault occurs, the program is no longer in a meaningful state, so nothing can safely happen at that point other than immediate termination.

(And don't nobody try to register a signal handler for SIGSEGV.)

Haleyhalf

In some cases, registering a signal handler for SIGSEGV is valid - for instance if you're making a read-only mapping with mmap, you can catch and longjmp out of write attempts on it with a signal handler, and this is 100% (as far as POSIX is concerned) portable and valid as long as the write was not being performed by an async-signal-unsafe function. – Aspectual

@R..: as usual with "pseudo-advice": once you understand when it's safe to ignore, you can safely ignore it ;-) In any event, you must terminate the program eventually; you cannot recover from the signal. – Haleyhalf

Answer 3

"I cannot figure out why fflush (stdout) is called here in this code I try to comment this line and behavior was exactly the same."

Because you're not guaranteed to see previous printf() output if that output doesn't end in a newline.

Basically, you only need it if you're displaying say a prompt without a newline, and you want to make sure the user can see it.

See this site.

Rosalindrosalinda