C Interview Questions and Answers

 

How can I invoke another program or command and trap its output?

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 calling program
can read the output (or alternatively supply the input). Using popen,



extern FILE *popen();



sprintf(cmdbuf, "sort < %s", datafile);





fp = popen(cmdbuf, "r");





/* ...now read sorted data from fp... */





pclose(fp);





(Do be sure to call pclose, as shown; leaving it out will seem to work at first
but may eventually run you out of processes or file descriptors.)



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.)

Posted by:Richards