Unix Pipes & IPC - Key Functions & Syntax

1. pipe()

int pipe(int pfd[2]);

• Purpose: Creates an anonymous pipe for Inter-Process Communication (IPC).
• Usage: Typically used between a parent and child process after fork().
• Parameters:
  • pfd[2]: An integer array storing two file descriptors:
    • pfd[0] → Read end (used for reading from the pipe)
    • pfd[1] → Write end (used for writing to the pipe)
• Return Value:
  • 0 on success.
  • 1 on failure (sets errno).
• Common Errors:
  • EMFILE: Too many file descriptors open.
  • ENFILE: System-wide file descriptor limit reached.
  • EFAULT: pfd points to an invalid memory address.

2. read()

ssize_t read(int fd, void *buf, size_t count);

• Purpose: Reads data from a file descriptor (fd), typically from pfd[0] (pipe read end).
• Usage: The process calls read() to retrieve data from the pipe.
• Parameters:
  • fd: File descriptor (must be pfd[0] for reading from a pipe).
  • buf: Pointer to a buffer where the read data will be stored.
  • count: The maximum number of bytes to read.
• Return Value:
  • >0 → Number of bytes read successfully.
  • 0 → End of file (EOF) or all write ends of the pipe are closed.
  • 1 → Error occurred (errno set).
• Blocking Behavior:
  • If there’s no data in the pipe, read() blocks (waits) until data is available.
  • If all write ends are closed, read() returns 0 (indicating EOF).
• Non-Blocking Mode:
  • Set file descriptor as non-blocking using fcntl(fd, F_SETFL, O_NONBLOCK).
  • In non-blocking mode, read() returns 1 with errno = EAGAIN if no data is available.

3. write()