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comp.lang.c FAQ list · Question 19.30

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

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 calling program can read the output (or alternatively supply the input). Using popen, the last example from question 19.28 would look like

	extern FILE *popen();

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

	fp = popen(cmdbuf, "r");

	/* read sorted data from 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, as the code in question 19.28 was doing already. [footnote]

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

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