What happens by default when a process attempts to read() from an empty pipe, assuming the write end is still open by another process?

Question

BlackTom AI · Accepted Answer

When a process calls read() on a pipe and there is no data available but the write end is still open, the read blocks by default until data becomes available.

1. Relevant knowledge points and concepts
- Pipes are unidirectional kernel buffers used for IPC.  
- By default file descriptors are in blocking mode.  
- POSIX read() semantics: if no data is available on a blocking file descriptor, the call sleeps until data arrives or an error/EOF occurs.  
- EOF on a pipe is indicated when all write ends are closed; read() then returns 0.  
- SIGPIPE is delivered to a process that attempts to write to a pipe with no readers; it is not sent to readers.  
- EAGAIN/EWOULDBLOCK is returned by read/write only when the descriptor has been opened or set non-blocking (O_NONBLOCK).

2. Step-by-step reasoning
- A process issues read(fd, buf, n) on a pipe.  
- Kernel inspects the pipe buffer: no bytes available.  
- Kernel checks whether any process still holds the write end open. In this scenario, yes.  
- Since the fd is blocking by default and there are writers, the kernel blocks the reader (puts it to sleep) until data is written into the pipe or the write end is closed or a signal interrupts the call.  
- Once another process writes data, the kernel wakes the reader and read() returns the number of bytes read.

3. Intermediate steps / conditions shown
- Condition A: No data + write end open → read() blocks (default).  
- Condition B: No data + no writers → read() returns 0 (EOF).  
- Condition C: Non-blocking mode (O_NONBLOCK) + no data → read() returns -1 and errno = EAGAIN/EWOULDBLOCK.  
- Condition D: Writer attempts to write with no readers → writer gets SIGPIPE or write() returns -1 with errno = EPIPE.

4. Why the correct answer is correct
Because default file descriptors are blocking and the presence of an open write end signals that data may arrive in the future, the kernel blocks the reader until data is written.

5. Why the other options are incorrect
- "Receives a SIGPIPE": SIGPIPE is delivered to writers when writing to a pipe with no readers, not to readers.  
- "read() returns 0 immediately": Returning 0 indicates EOF, which only happens when all write ends are closed. Here a writer is still open.  
- "read() returns -1 and sets errno to EAGAIN": That happens only if the descriptor is in non-blocking mode (O_NONBLOCK); it is not the default.

6. Conclusion
The correct behavior is: The process blocks until data is written to the pipe. This matches the provided answer.

What happens by default when a process attempts to read() from an empty pipe, assuming the write end is still open by another process?单项选择题

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