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in C, Assigning output of system() to a variable

I'm new to C and Linux and I am confused on how things work. I tried to search but I don't really know if I'm searching for the right thing because lots of other things come up.

Currently, I figured out that I can use shell commands inside C by using the method system(). They display out on the screen fine, but my problem is that I need to assign them to a variable, and not have them display on the screen (yet).

I'm planning to put the output of "ls -l" to a variable. I'm thinking of an array of strings (I think there are no strings, but an array char, so I guess this is an array of array of chars?) which will contain every line of the "ls -l" command except for the first line. (I think I need it in an array because I'm supposed to scroll through it in a menu-like style where I'm following example 18 on this link:
http://www.tldp.org/HOWTO/NCURSES-Pr...tml#MENUBASICS
but I think it's another question for another time, hopefully I can figure it out and not have to though, hahah)

What I have is this, but it doesn't work.

Code:

#include <stdlib.h>

int main() {

   char output[] = system( "ls -l" );

   printf( "%s", output);

   return 0;

}

which gives this error.

Code:

system.c: In function 'main':

system.c:5: error: invalid initializer

I believe it is wrong, but I'm completely clueless. Perhaps you can help, thanks.

Two things. If a function returns a pointer-to-char (dynamically allocated or static) and you want to assign the return value to a variable you do:

Code:

char *ptr = some_func_returning_pointer_to_char();

Second, system() doesn't return char* it returns int. Do a:

Code:

$ man 3 system

for more details on the return value.

The system() call is part of standard C (and C++ of course) but how it operates and what you can do with the processes it spawns is platform-dependent.

man popen

I think this does what you are looking for.

popen example:

Code:

#include <stdio.h>               

#include <stdlib.h>

#include <string.h>

#include <errno.h>



void process(const char *filespec)

{

        char cmd[296]={0x0}; /* 40 + 256 */  

        char tmp[256]={0x0};

        long total=0;

        int retval=0;

        FILE *in=NULL;

        

    snprintf(cmd,sizeof(cmd)-40,"ll %s | /usr/bin/cut -c 35-45 2>/dev/null",filespec);

    in=popen(cmd, "r");

    if(in==NULL)

    {

         perror("Shell execution error");

         exit(EXIT_FAILURE);

    }           

    while(fgets(tmp,4095,in)!=NULL) 

    {

         total+=atol(tmp);

    }        

    if(!feof(in))

    {

        perror("Input stream error");

        exit(EXIT_FAILURE);

    }    

    retval=pclose(in);

    if(retval==EOF || retval==127)    

    {

        perror("Shell invocation error");

        exit(EXIT_FAILURE); 

    }    

    fprintf(stdout,"%-64s total: %d\n",filespec,total);

}



int main(int argc, char *argv[])

{

    if(argc<2)

    {

            char filespec[256]={0x0};

            while(fgets(filespec,sizeof(filespec),stdin)!=NULL)

            {

                char *p=memchr(filespec,'\n', sizeof(filespec));

                    if(p!=NULL) *p=0x0;

                process(filespec);

            }                

    }        

    else

    {              

        int i=1;

        while(i<argc)

        {

            process(argv[i]);

            i++;        

        }

    }

    return 0;

}

Thanks for the answers, I have gotten what I needed. Figured out char arrays (strings) and casting char to int in the process.

This is what I came up with:

Code:

#include <ncurses.h>

#include <stdio.h>



int main()

{

   int size=0;

   FILE *in=NULL;

   char temp[256];

   

   in=popen("ls -a -l | wc -l", "r");

   fgets(temp, 255, in);

   size = atoi(temp) - 1;



   char list[size][256];



   in=popen("ls -a -l", "r"); //for reading

   fgets(list[0], 255, in); // discard total: xxx, first line

  

   int i;

   for( i = 0; i < size; i++) {

      fgets(list[i], 255, in);   

   }



   for(i=0; i < size; i++) {

      printf("%s", list[i]);

   }

   return 0;

}

It does what I expect it to do (though it probably have hidden problems somewhere like memory things I don't really understand yet. Any comments on it?) , now to format and continue on with ncurses. Thanks for help!