Segmentation fault with strcmp()

Why segmentation fault?

Because of code if(strcmp(argv[2], NULL) == 0), you are passing NULL as string pointer to strcmp() function; that try to deference at NULL to compare chars codes (e.g. acsii code) this cause undefined behavior at run time.

You should compare a string pointer with NULL using == as if(argv[2] == NULL)

I'm passing 3 command line arguments but I also want to run it with only 2 command line arguments with the above statement.

You can implement this in two ways:

1. The main syntax is:

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

   The first argument argc is argument counter that is total number of arguments passed to your process including process name.

   So when you pass no extra argument then argc == 1 e.g. ./exe

   Suppose if you pass three arguments as follows:

   ./exe firstname lastname    
   

   Then argc == 3, it looks like you are passing two arguments but including executable name you are actually passing three arguments to process.

   So you can use of argc value to iterate in a loop to print arguments passed (other then executable)

    printf("Process name is: %s", argv[0]);
    for(i = 1; i < argc; i++ ){
         printf("argv[%d] %s\n", argv[i]);
    }
   

2. Second technique is using second argument: argv[] is NULL terminated array of string strings so argv[argc] is always equals to NULL. You can use this information in loop to iterate and process of arguments passed.

   To understand this suppose you are executing function as:

   ./exe firstname lastname    
   

   then argv[0] == ./exe, argv[1] == firstname and argv[2] == lastname and argv[3] == NULL, Note this time argc == 3 (argv[argc] means argv[3] == NULL).

   For example to print all arguments, you can write you code like:

     int i = 1;
     printf("Process name is: %s", argv[0]);
     while(argv[i]){// terminates when argv[i] == NULL
       printf("argv[%d] %s\n", argv[i]);   
       i++;
     }
   

Do you notice argv[0] is always your executable name! this means whenever you need to print your executable name use argv[0] instead of hard code name of your executable while writing code, so that if you recompile and give new name to your executable then argv[0] always prints correct name. You should write code as follows:

int main(int argc, char* argv[]){
  :
  :// some other code
  if(argc < min_number_of_arguments){
      fprintf(stderr, "Error: wrong number of arguments passed!\n");
      fprintf(stderr, "Usage: %s [first] [second] \n", argv[0]);
      exit(EXIT_FAILURE);
  }
  :
  :// some other code 
   return EXIT_SUCCESS;
}