How to get string from system command output

archieval · 05-15-2010, 08:23 AM

I have found this from a site http://stackoverflow.com/questions/2...ut-as-a-string

Code:

#define _GNU_SOURCE
#include <stdio.h>
int main() {
    char *s = NULL;
    FILE *p = popen("ls", "r");
    getdelim(&s, NULL, '\0', p);
    pclose(p);
    printf("%s", s);
    return 0;
}

But it does not work for me, segfault. Any suggestions?

Regards,
archieval

grail · 05-15-2010, 08:28 AM

Compiled and ran on my machine, although output was - (null)

jlinkels · 05-15-2010, 08:38 AM

You should check the result of popen and getdelim. p can be null or getdelim could have returned 0, most likely resulting in s remaining null as well. Those functions do not return a value for nothing, it is for checking.

A wild guess: "ls" is not found as executable.

jlinkels

archieval · 05-15-2010, 08:44 AM

I checked the return value, and it's Invalid argument. But "ls" is a valid unix command..

catkin · 05-15-2010, 08:48 AM

Try popen("/bin/ls", "r"); in case $PATH does not include /bin in the sh shell created to run popen()'s first argument.

archieval · 05-15-2010, 08:53 AM

I did /bin/ls, but still invalid argument. And the getdelim returns (null).

catkin · 05-15-2010, 09:18 AM

I'm only butting in here because progress seems slow not because I'm a C-spert. So, for what it is worth ...

Given that "/bin/ls" and "r" are definitely valid for popen it is unlikely that popen is failing but always good to test. How about adding error traps as jinkels suggested? Here's your code snippet modified with lines cribbed from the getdelim man page

Code:

#define _GNU_SOURCE
#include <stdio.h>
int main() {
    char *s = NULL;
    FILE *p;
    ssize_t read;
    p = popen("ls", "r");
    if (p == NULL)
        exit(10);
    read = getdelim(&s, NULL, '\0', p);
    if (read = -1)
        exit(20);
    pclose(p);
    printf("%s", s);
    return 0;
}

The exit code will let you know if either popen or getdelim are failing (not pretty but functional).

My un-ediccated guess is that the problem is the NULL argument to getdelim(); I think it should be a pointer to an initialised size_t variable indicating the maximum number of bytes that getdelim() should transfer to buffer.

archieval · 05-15-2010, 10:05 AM

Yes, I have done that error checkings, the result is Invalid argument for popen, and (null) was printed after getdelim. Anyway, I just used fgets, to read from the stream. Thanks!