Why can a subprocess hang when both stdout and stderr are piped?

A child can fill one pipe while the parent waits for data or completion elsewhere. The child then blocks before it can finish. For bounded output use subprocess.run(..., capture_output=True, timeout=...) or communicate() so both streams are drained. For large output, stream both channels concurrently or redirect them to controlled files instead of accumulating everything in memory. Reproduce with a child that writes more than a pipe buffer to stderr and also writes stdout. Setting a timeout alone does not design cleanup or preserve partial output. After termination, reap the child and account for subprocesses it may have started.