A tricky bug

Today I debugged a piece of code that looked like this:

printf("foobar\n");
if(fork() == 0)
  exit(0);

When the program is run from the terminal, the output is as expected: a single line containing "foobar". However, when the output is sent to a pipe or redirected to a file, two lines appear in the output. (Actually, the problem was a bit more complicated: there were n fork() calls and there were exactly n additional copies of the output). After a bit of digging around and not finding any obvious fault, I asked about it on IRC.

It turned out that stdout is fully buffered when it is attached to a pipe or a file. What happened is that fork() duplicated also the internal I/O buffers, which got flushed after the child process exited, thus producing extra output. I solved the problem by inserting a fflush(NULL); statement before forking, which flushes buffers of all output streams.