CODEWITHSUNDEEP
linuxpipeposix
pjkozlowski
pjkozlowski

Reputation: 123

Can posix read() receive less than requested 4 bytes from a pipe?

A program from the answer https://stackoverflow.com/a/1586277/6362199 uses the system call read() to receive exactly 4 bytes from a pipe. It assumes that the function read() returns -1, 0 or 4. Can the read() function return 1, 2 or 3 for example if it was interrupted by a signal?

In the man page read(2) there is:

On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal.

Does this mean that the read() function can be interrupted during receiving such a small amount of data as 4 bytes? Should the source code from this answer be corrected?

In the man page pipe(7) there is:

POSIX.1-2001 says that write(2)s of less than PIPE_BUF bytes must be atomic: the output data is written to the pipe as a contiguous sequence.

but there is nothing similar about read().

Upvotes: 9

Views: 1063

Answers (1)

Jim Driscoll
Jim Driscoll

Reputation: 904

If the write is atomic, that means that the entire content is already present in the buffer when the read happens so the only way to have an incomplete read is if the kernel thread decides to yield before it's finished - which wouldn't happen here.

In general you can rely on small write()s on pipes on the same system mapping to identical read()s. 4 bytes is unquestionably far smaller than any buffer would ever be, so it will definitely be atomic.

Upvotes: 2

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