Compare command line arguments in c

Question

Okay so, i need to write a C program which handles and compares command line arguments. For example, lets say the program is called test.c. I execute it like that : ./test load something store one two load store ...etc. What i mean by that is when there is a "load" command it will parse the argc+1 and do something with it and when there is a "store" command it's goind to be doing something else and parsing both argc+1 and argc+2. Heres is my approach so far :

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct Tooks {
    char s1[50];
    char s2[50];
    char s3[50];
} Test; /*use this struct to compare the command line arguments in 
         which in am interested at every time. */

int main (int argc, char *argv){
  int num = argc;
  int y,i;
  char args[num][50]; /*use this num-places array with each string 
       containing 50 chars. I did that because if i try directly to 
        strcpy( test.s1, argv[i]) i would get a seg fault. */
Test test;
strcpy( test.s1, "null");
strcpy( test.s2, "null");
strcpy( test.s3, "null");
for (y=0; y<num; y++){
    strcpy(args[y], "null");
    }//"initializing" the array as null

for (i=1; i<num;){
    //copy the 3 command line arguments.
    strcpy( test.s1, args[i]);
    strcpy( test.s2, args[i+1]);
    strcpy( test.s3, args[i+2]);
    printf("s1 %s, s2 %s, s3 %s, num %d\n", test.s1, test.s2, test.s3, num);//Just a test print

    if (strcmp(test.s1, "store")==0 && strcmp(test.s2, "null") != 0 && strcmp(test.s3, "null") != 0){
        printf("%s\n, %s\n", test.s2, test.s3);
            i=i+3;
            }
    else if (strcmp(test.s1, "load")==0 && strcmp(test.s2, "null") != 0){
            printf("%s\n", test.s2);
            i=i+2;
            }
    else {
            printf("nothing\n");
            printf("s1 %s, s2 %s, s3 %s\n", test.s1, test.s2, test.s3);
            i++;    
}
}

printf("end %d\n", argc);
return 0;
}

And here is the output:

s1 null, s2 null, s3 null, num 6
nothing
s1 null, s2 null, s3 null
s1 null, s2 null, s3 null, num 6
nothing
s1 null, s2 null, s3 null
s1 null, s2 null, s3 null, num 6
nothing
s1 null, s2 null, s3 null
s1 null, s2 null, s3 �, num 6
nothing
s1 null, s2 null, s3 �
s1 null, s2 �, s3 , num 6
nothing
s1 null, s2 �, s3 
end 6

for the command ./test load something store one two

It seems that the command line arguments are not passing neither to the struct nor to the array. Any ideas? :)

You never actually use argv, so there is no way you will get the command line arguments. — AShelly
– AShelly, Commented Apr 27, 2017 at 22:30
What happens if there's no arguments? argv[0] refers to the executable name itself. You are not really passing arguments into the app itself — t0mm13b
– t0mm13b, Commented Apr 27, 2017 at 22:31
very strange way of incrementing the loop control variable i. Within that loop, some of the three strcpy statements will break the args array. — Weather Vane
– Weather Vane, Commented Apr 27, 2017 at 22:37
... for example args[i+2] in the final iteration when i == num-1 will be args[num+1] which clearly exceeds the array bounds. — Weather Vane
– Weather Vane, Commented Apr 27, 2017 at 22:47

Jonathan Leffler · Accepted Answer · 2017-04-27 22:41:41Z

1

There are several things wrong with your code.

You never fill args with anything else than "null". You proceed to copy the "null" strings into your struct and that's what it prints.
You're never referring to argv, which is the array which stores the executable name in index 0 and fills the rest with whatever you feed it in the command line.
Argv is supposed to be
```
char* argv[] 
```
And not
```
char* argv
```

edited Apr 27, 2017 at 22:41

Jonathan Leffler

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answered Apr 27, 2017 at 22:38

Arrrow

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5 Comments

t0mm13b Over a year ago

or use char **argv .

Arrrow Over a year ago

It's identical as far as I know

t0mm13b Over a year ago

As far as I know says it all really.

Pantelos Papado Over a year ago

Well damn just a simple mistake on number 3 that you pointed out solved the issue.

Arrrow Over a year ago

@t0mm13b then enlighten me instead of being condescending

AShelly · Accepted Answer · 2017-04-27 22:44:34Z

0

There are libraries for parsing command lines, like getopt.

If you want to do it yourself, it would look something like

for (int i = 1; i<argc; i++){
  if (!strcmp(argv[i], "load")) {
    if (i+1<argc) {
      handle_load(argv[i+1]);
      i+=1; 
    }
    else { 
      show_error("`load` needs 1 value"); 
    }
  }
  else if (!!strcmp(argv[i],"store")) {
    //similar, but with 2 next aruments
  }
  else {
     show_error("invalid option %s", argv[i]);
  }     
}

answered Apr 27, 2017 at 22:44

AShelly

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