19.30: How can I invoke another program or command and trap its output?Esto te debe de servir. (http://www.iu.hio.no/~mark/unix/unix.html#SEC177)
A: Unix and some other systems provide a popen() function, which
sets up a stdio stream on a pipe connected to the process
running a command, so that the output can be read (or the input
supplied). (Also, remember to call pclose().)
If you can't use popen(), you may be able to use system(), with
the output going to a file which you then open and read.
If you're using Unix and popen() isn't sufficient, you can learn
about pipe(), dup(), fork(), and exec().
(One thing that probably would *not* work, by the way, would be
to use freopen().)
References: PCS Sec. 11 p. 169.