pread; threadsafe or not?

Question

Is there a problem with using pread on the same file descriptor from 2 or more different threads at the same time?

Answer 1

pread itself is thread-safe, since it is not on the list of unsafe functions. So it is safe to call it.

The real question is: what happens if you read from the same file concurrently (not necessarily from two threads, but also from two processes).

Regarding this, the specification says:

• The behavior of multiple concurrent reads on the same pipe, FIFO, or terminal device is unspecified.

  Note that it doesn't mention ordinary files. This bit relates only to read anyway, because pread cannot be used on unseekable files.

• I/O is intended to be atomic to ordinary files and pipes and FIFOs.

  But this is from the non-normative section, so your OS might do it differently. E.g., if you read from two threads and there is a concurrent write, you might get different pieces of the write in your two read buffers. But this kind of problem is not specific to multithreading.

Also nice to know that in some cases

read() shall block the calling thread

Not the process, just the thread. And

A thread that has blocked shall not prevent any unblocked thread [...] from eventually making forward progress