How to get the execl() output?

estratos · 06-06-2006, 03:44 AM

I need to get the output from an execl() or a system() call. How can I do it? Is there any way of redirecting the output to a text file? Any ideas?

Thanks,

Daniel.

Guttorm · 06-06-2006, 06:18 AM

Hi

If you want to write to a text file, just pipe it, e.g.
system("ls >/tmp/ls.txt");

If you want to read the output in your program, you can use popen and pclose which acts a bit like fopen and fclose, e.g.
FILE fp = popen("ls","r");
Then read from fp as it was a regular file, and when you're done, use pclose instead of fclose.

Hope this helps,
Guttorm

tuxdev · 06-06-2006, 01:23 PM

Use pipes cause they are faster since it doesn't have to deal with the disk.

Code:

int pfd[2];
pipe(pfd);
pid_t pid=fork();
if(pid==-1)
{
   perror("fork");
   exit(-1);
}
else if(pid==0)
{
   close(pfd[0]);
   dup2(pfd[1],STDOUT_FILENO)
   close(pfd[1]);
   execl("foo","foo",...);
   perror("exec");
   exit(-1);
}
waitpid(pid,0,0);
close(pfd[0]);

estratos · 06-07-2006, 01:23 AM

And what about using freopen()?

FILE* fpStdout = freopen(STDOUTFILE, "w+", stdout);
system("ls");

Then I can access fpStdout with fgets, as any other file.
Is this method slower than those proposed by you? Does freopen use pipes too?

Thanks,

Daniel

tuxdev · 06-07-2006, 05:18 PM

That redefines stdout in such a way that it cannot be recovered as true stdout later, as well as involving the hard disk when you probably don't need to.

popen and pclose use pipes and will work, but it may not have the level of control you may need/want.