Statement before fork() printing twice [duplicate]

Answer 1

It seems that the ERROR! WHY DONT U UNDERSTAND is still buffered after forking and gets written by both processes.

If you add

fflush(stdout);

right after your first printf() the internal buffer is flushed and it only appears once in your file.

Answer 2

flushing the buffer will solve the problem. use fflush just after the print statement.

Answer 3

Since after reopen the stream is non-interactive, it's fully buffered and doesn't flush on '\n'. Before fork is called the buffer still contains the message, and after fork this buffered message was duplicated (because both processes got their own copies of stdout) and then flushed by both the parent and the child. See part 7.19.3 of C standard.

You can avoid such behavior by calling fflush just before fork.

Answer 4

It's because of buffering. Do a fflush right after printf.

Both processes end up with the same copy of stdio's internal stuff and both proceed to flush it at exit. You might also prevent it from happening if you call _exit in the child.