The Linux Programming Interface

918 Chapter 44

The impact of the O_NONBLOCK flag when writing to a pipe or FIFO is made complex

by interactions with the PIPE_BUF limit. The write() behavior is summarized in

Table 44-3.

The O_NONBLOCK flag causes a write() on a pipe or FIFO to fail (with the error EAGAIN)

in any case where data can’t be transferred immediately. This means that if we are

writing up to PIPE_BUF bytes, then the write() will fail if there is not sufficient space in

the pipe or FIFO, because the kernel can’t complete the operation immediately

and can’t perform a partial write, since that would break the requirement that

writes of up to PIPE_BUF bytes are atomic.

When writing more than PIPE_BUF bytes at a time, a write is not required to be

atomic. For this reason, write() transfers as many bytes as possible (a partial write)

to fill up the pipe or FIFO. In this case, the return value from write() is the number

of bytes actually transferred, and the caller must retry later in order to write the

remaining bytes. However, if the pipe or FIFO is full, so that not even one byte can

be transferred, then write() fails with the error EAGAIN.

44.11 Summary

Pipes were the first method of IPC under the UNIX system, and they are used fre-

quently by the shell, as well as in other applications. A pipe is a unidirectional, limited-

capacity byte stream that can be used for communication between related processes.

Although blocks of data of any size can be written to a pipe, only writes that do not

exceed PIPE_BUF bytes are guaranteed to be atomic. As well as being used as a

method of IPC, pipes can also be used for process synchronization.

Table 44-2: Semantics of reading n bytes from a pipe or FIFO containing p bytes

O_NONBLOCK

enabled?

Data bytes available in pipe or FIFO (p)

p = 0, write end open p = 0, write end closed p < n p >= n

No block return 0 (EOF) read p bytes read n bytes

Yes fail (EAGAIN)return 0 (EOF)read p bytes read n bytes

Table 44-3: Semantics of writing n bytes to a pipe or FIFO

O_NONBLOCK

enabled?

Read end open Read end

n <= PIPE_BUF n > PIPE_BUF closed

No

Atomically write n bytes;

may block until sufficient

data is read for write() to

be performed

Write n bytes; may block until

sufficient data read for write() to

complete; data may be interleaved

with writes by other processes

SIGPIPE

+

EPIPE

Yes

If sufficient space is

available to immediately

write n bytes, then write()

succeeds atomically;

otherwise, it fails (EAGAIN)

If there is sufficient space to

immediately write some bytes,

then write between 1 and n bytes

(which may be interleaved with

data written by other processes);

otherwise, write() fails (EAGAIN)