C command line argument check

Question

I have the below part of code in which I noticed that if I change the 0 to 1 the result is the same. I get the STACKprint(); with "on" as the second argument, nothing with anything else and if there is no argument I get a segmentation fault. I guess for the segmentation fault I need to check if the argument is NULL but I am not sure how to do that with the second parameter and it really bugs me out why (strcmp (argv[2],"on") == 1) has no effect. Is it not supposed to take a TRUE value?

 if (strcmp (argv[2],"on") == 0) {
            STACKprint();
            printf("\n");
 }

Solution 1

First of all, to check if there is an argument, you should use the argc variable of the main(int argc, char** argv), which indicates the length of your argv array.

if (argc < 3) {
  printf("missing argument\n");
  exit(-1);
}

As for strcmp, the man page only states that it returns 0 if the two strings in argument are equal, else non-zero... but not necessarily 1. In fact it is implementation dependent. The way to use it to check for string equality is therefore :

if (0 == strcmp(argv[2], "on")) {
   // do something
} else {
   // do something else
}

Solution 2

If your program is something like this

#include <stdio.h>


  int main (int argc, char**argv)
  {
     if (argc >= 3 && strcmp (argv[2],"on") == 1){
  //        STACKprint();
          printf("\n");
      }
  }

and you try to run it with myexe 1 on, It will never go into the if block and if you change the 1 to 0, it will go.

Something else is wrong.

It will be nice if you can post your code and the way you are calling it.

Comments

[chux - Reinstate Monica]

Sure you want strcmp (argv[2],"on") == 1 instead of strcmp (argv[2],"on") == 0 or strcmp (argv[2],"on") != 0?

[Jay]

You mean argc<3, since he's using argv[2], right?

[goodfellow]

Good point @chux. This is not going to work as expected (programiz.com/c-programming/library-function/string.h/strcm‌​p0