Pipe is not receiving all output from child process

Question

I wanted to open up a pipe to a program and read output from it. My initial inclination was to use popen(), but the program takes a number of options, and rather that fighting with shell quoting/escaping, I decided to use a combination of pipe(), fork(), dup() to tie the ends of the pipe to stdin/stdout in the parent/child, and execv() to replace the child with an invocation of the program passed all of the options it expects as an array.

The program outputs many lines of data (and flushes stdout after each line). The parent code sets stdin to non-blocking and reads from it in a loop using fgets(). The loop runs while fgets() return non-NULL or stdin has an error condition that is EAGAIN or EWOULDBLOCK.

It receives most of the lines successfully, but towards the end it seems to drop off, with the last fgets() failing with an odd error of "No such file or directory."

Does anyone know what I might have done wrong here?

Answer 1

not sure, there is a cool function on linux called posix_spawn (example here http://www.opengroup.org/onlinepubs/000095399/xrat/xsh_chap03.html#tag_03_03_01_02) sometimes it makes it easier to do pipes... but sounds like a possible blocking issue or pipe....

Answer 2

Make sure you open a pipe to STDERR. Most programs write error data there instead of STDIN.

Answer 3

I found the problem. I stupidly was not resetting errno to zero each iteration. I guess I just assumed fgets() would take care of it or something... My stupid mistake. Now it is working fine. Always reset errno!

Thanks for the help anyway.