Question

In my application I am creating a bunch of child processes. After fork() I open a per process file, set the stdout/stderr of the created process to point to that file and then exec the intended program.

Is there an option for the parent process to setup things such a way that when the child process does a printf it gets flushed immediately to the output file without having to call flush() ? Or is there an API that can be called from the child process itself (before exec) to disable buffered I/O ?

Answer 1

The problem here is that printf is buffered. The underlying file descriptors are not buffered in that way (they are buffered in the kernel, but the other end can read from the same kernel buffer). You can change the buffering using setvbuf as mentioned in a comment which should have been an answer.

setvbuf(stdout, NULL, _IONBF, 0);

You do not need to do this for stdin or stderr.

You can’t do this from the parent process. This is because the buffers are created by the child process. The parent process can only manipulate the underlying file descriptors (which are in the kernel), not stdout (which is part of the C library).

P.S. You mean fflush, not flush.