Reading From a Pipe or FIFO

read(2) or getmsg(2)) are used to read from a pipe or FIFO. Data can be read from either end of a pipe. On success, the read(2) returns the number of bytes read and buffered. When the end of the data is reached, read(2) returns 0.

When a user process attempts to read from an empty pipe (or FIFO), the following happens:

• If one end of the pipe is closed, 0 is returned, indicating the end of the file.

• If the write side of the FIFO has closed, read(2) returns 0 to indicate the end of the file.

• If some process has the FIFO open for writing, or both ends of the pipe are open, and O_NDELAY is set, read(2) returns 0.

• If some process has the FIFO open for writing, or both ends of the pipe are open, and O_NONBLOCK is set, read(2) returns -1 and sets errno to EAGAIN.

• If O_NDELAY and O_NONBLOCK are not set, the read call blocks until data is written to the pipe, until one end of the pipe is closed, or the FIFO is no longer open for writing.