printf statements not printing until Interrupt

Question

I've added some printf statements to some code that is run at the start of a program, but the statements aren't printing to stdout until I terminate the program with a ctrl c in my terminal, and they are showing up on the same line as my terminal prompt. For example the printf in the demo code below would show like this

[notice] Interrupt: exiting.
yabbadabbadoo/usr/local/bin 13:24:21 :

Why is that and how to change it? i.e. would there be somewhere in the application code that the developer has somehow set printf statements to print at exit?

some_function_name(){
  printf("yabbadabbadoo");

}

@WeatherVane add it to where? to the program code, or to my terminal? — Leahcim, Mar 07 '17 at 18:29
Add it to the program code, after the `printf` statement whose output you want to see immediately. Including a final `\n` newline often flushes the output too, but I am unsure if that is a standard feature, and there may be occasions when you do not *want* a newline. — Weather Vane, Mar 07 '17 at 18:31
error: linker command failed with exit code 1: warning: implicit declaration of function 'flush' is invalid in C99 — Leahcim, Mar 07 '17 at 18:32
@AlexLop the answer below seems more complete because it also provides the \n option — Leahcim, Mar 07 '17 at 18:34
@Leahcim The answer in the link I posted also provides it in the accepted answer (see the first line). — Alex Lop., Mar 07 '17 at 18:35

Sourav Ghosh · Accepted Answer · 2017-03-07T18:35:28.837

would there be somewhere in the application code that the developer has somehow set printf statements to print at exit?

No, in general, the standard output stdout is line buffered. So unless the buffer is either full, or received a newline, the content is not flushed.

While the program is terminating, all open buffers are flushed automatically, that is why you get to see the output after terminating the program.

In your case, you need to force the buffer to be flushed to make the output appear on the terminal. You can either

add a newline '\n' at the end of format string.
use fflush(stdout) after the printf() call

to ensure the flushing.

printf statements not printing until Interrupt

1 Answers1