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");
 }

To prevent the segmentation fault, you need to check `argc`. — aschepler, Jul 28 '15 at 16:19
You would use `argc` (a count) to determine if `argv[2]` exists. — donjuedo, Jul 28 '15 at 16:20
@aschepler Sorry for stupid question but what is `STACKprint()` ? — ameyCU, Jul 28 '15 at 16:20
Changing 0 to 1 should change the outcome. Something else is wrong. Could you edit your post to include the entire `main()` function, with the 1 in place, and show the output, too? — donjuedo, Jul 28 '15 at 16:23

score 4 · Answer 1 · 2015-07-28T16:44:55.960

4

To avoid the segfault, check the value of argc to discover whether argv[2] exists. If argc < 3, argv[2] wasn't supplied.

strcmp() doesn't return true/false; it returns a value either less than, equal to, or greater than zero depending on the relative values of its arguments.

edited Jul 28 '15 at 16:44

answered Jul 28 '15 at 16:22

Pierre-Rudolf Gerlach · Accepted Answer · 2015-07-29T18:06:40.273

3

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
}

edited Jul 29 '15 at 18:06

answered Jul 28 '15 at 16:31

Pierre-Rudolf Gerlach

81
6

You mean `argc<3`, since he's using `argv[2]`, right? – Jay Jul 28 '15 at 16:44

score 1 · Answer 3 · edited Jul 28 '15 at 16:42

1

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.

edited Jul 28 '15 at 16:42

Jay

9,585
6
49
72

answered Jul 28 '15 at 16:29

Ritesh

1,809
1
14
16

1

Sure you want `strcmp (argv[2],"on") == 1` instead of `strcmp (argv[2],"on") == 0` or `strcmp (argv[2],"on") != 0`? – chux - Reinstate Monica Jul 28 '15 at 16:34
Good point @chux. This is not going to work as expected (https://www.programiz.com/c-programming/library-function/string.h/strcmp0 – goodfellow Nov 15 '20 at 07:02

C command line argument check

3 Answers3