printf

Key idea: printf is usually user-space buffered I/O (stdio), not a syscall per character.

• printf writes into a FILE buffer* in user space.
• The actual syscall is typically write(fd, ...) when the buffer flushes.
• Flushing happens when:
  • buffer is full
  • you print a newline to a terminal (line-buffered stdout)
  • you call fflush(stdout)
  • program exits normally (flushes stdio)
  • you print to stderr (often unbuffered by default)

The fork interaction gotcha (very interview-y):

• After fork, the child gets a copy of the parent’s user-space buffers.
• If stdout had buffered data not yet flushed, both processes may flush it, causing duplicated output.
• Fix: fflush(stdout) before fork(), or use write() (unbuffered syscall) when you need strict behavior.