c popen won't catch stderr

Question

I'm trying to use popen() to catch the stderr of a call, but of course it doesn't seem to be doing that. Any ideas?

My code looks more or less like this:

popen("nedit", "r");

But I'm getting all this garbage about non-utf8 on my screen...

Answer 1

popen gives you a file handle on a process' stdout, not its stderr. Its first argument is interpreted as a shell command, so you can do redirections in it:

FILE *p = popen("prog 2>&1", "r");

or, if you don't want the stdout at all,

FILE *p = popen("prog 2>&1 >/dev/null", "r");

(Any other file besides /dev/null is acceptable as well.)

Answer 2

If you want to discard all of the error messages, then you can use:

popen("nedit 2>/dev/null", "r");

Answer 3

if you need both stdout and stderr streams, you can open an extra pipe and redirect child stderr to it:

if (pipe(pfd) < 0)
    return -1;
    
perr = fdopen(pfd[0], "r");

snprintf(command, LINE_LEN, "... 2>&%d", pfd[1]);

if (pout = popen(command, "r")) {
    close(pfd[1]);

    while (fgets(line, LINE_LEN, pout) != NULL)
        ...
    
    while (fgets(line, LINE_LEN, perr) != NULL)
        ...

    pclose(pout);
}

fclose(perr);
close(pfd[0]), close(pfd[1]);

As Andrew pointed out, if command generates lots of output/errors, you must handle the streams more carefully (see comments below).