multithreading - simultaneous read on file descriptor from two threads

Question

1. my question: in Linux (and in FreeBsd, and generally in UNIX) is it possible/legal to read single file descriptor simultaneously from two threads?

2. I did some search but found nothing, although a lot of people ask like question about reading/writing from/to socket fd at the same time (meaning reading when other thread is writing, not reading when other is reading). I also have read some man pages and got no clear answer on my question.

3. Why I ask it. I tried to implement simple program that counts lines in stdin, like wc -l. I actually was testing my home-made C++ io engine for overhead, and discovered that wc is 1.7 times faster. I trimmed down some C++ and came closer to wc speed but didn't reach it. Then I experimented with input buffer size, optimized it, but still wc is clearly a bit faster. Finally I created 2 threads which read same STDIN_FILENO in parallel, and this at last was faster than wc! But lines count became incorrect... so I suppose some junk comes from reads which is unexpected. Doesn't kernel care what process read?

Edit: I did some research and discovered just that calling read directly via syscall does not change anything. Kernel code seem to do some sync handling, but i didnt understand much (read_write.c)

Answer 1

That's undefined behavior, POSIX says:

The read() function shall attempt to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf. The behavior of multiple concurrent reads on the same pipe, FIFO, or terminal device is unspecified.