system programing in unix

djgerbavore · 01-14-2005, 10:51 AM

i'm learning system programing for fun in unix. I'm trying to implment the redirection (>) arrow, like in unix the command ls -l > foo.txt writings the ouptut to the file names foo.txt

Code:

         first fork to make child process...
         wait of child process to end
     in child:
        go through the arguments untill get to >(arrow)
        /* get the filename */
        freopen(args[i+1], "w", stout);
        /* the command line args */
         execvp(*args, args);
         fclose(stout);

when i run it, it just hangs. I read the man pages, and they say check the return values for errors. But the only error it gives me is 10, and i looked for what this number means, but no luck. If some could point me in the right direction and explain on how to go about redirecting a command to file.

thanks,

djgerbavor3

Hko · 01-14-2005, 01:00 PM

While it is probably a typo in your post, and not in your actual code, this line:

Code:

freopen(args[i+1], "w", stout);

should read:

Code:

freopen(args[i+1], "w", stdout);

To get a more understandable error-message, you can call the "perror()" function. It will print a error message about the last error that occurred in a library function.

An example for redirecting stdout of the program itself to a file given on its command line including checking for error,a nd printing a informative message with perror():

Code:

#include <stdio.h>

/*
 * This program redirects stdout to a file given as the first command line
 * argument, and then prints a message to stdout so it will appear in the file
 * given on the command line. 
 */

int main(int argc, char **argv)
{

    if (argc != 2) {    /* Check command line syntax. */
        fprintf(stderr, "Usage: %s <file name>\n", argv[0]);
        return 1;   /* exit program with error exit code */
    }

    /* Redirect my own stdout to file given and check for error. */
    if (freopen(argv[1], "w", stdout) == NULL) {
        perror("Redirecting stdout");   /* Will give information about the error */
        return 1;   /* exit program with error exit code */
    }

    /* Printing text to stdout, which will end up in the file */
    printf("Printing this to stdout, but it should appear in %s.\n", argv[1]);
    return 0;
}

djgerbavore · 01-14-2005, 03:36 PM

thank you, i'm making a little further. however freopen command isn't the one thats hanging up program, the command execvp is the one causing my program to do nothing

for example:

Code:

       
if(freopen(filename, "w", stdout) == NULL) {
      perror("ERROR");
      return 1;
}

execvp(*args, args);
fclose(stdout);

;

i put some printf statements to see what's happening and everything works fine untill the command execvp.

Then, I tried hard coded args array to:

Code:

args[0] = "ls";
args[1] = "-l";

i ran the man excevp page and i blieve the parametes are right, but i'm not sure.

Thanks

djgerbavor3

Inunu · 01-14-2005, 05:29 PM

Hi DJ.

The first parameter of execvp() is a constant string, indicating the file to execute. In your code it should be args[0].

execv() and execvp() need to have NULL-terminated array. Are you sure that your args will have NULL at the end? Otherwise it will keep on looking for more arguments endlessly.

Code:

const char* args[4];

args[0]="ls";
args[1]="-l";
/* Without this execvp() will go over args[3], args[4], args[5].... args[9999].... until it sees NULL */
args[2]=NULL; 

execvp(args[0], args);

djgerbavore · 01-15-2005, 01:26 PM

Inunu - you relied worked!!!! as soon as i put a NULL in my array the command worked.

also *args is the same as args[0], other then that you helped me out alot.

thanks again

djgerbavor3