Question

First,I have to say that I am new to C programming. What I’m trying to do is write a program that takes an argument input and converts it into an integer and then returns its value. My code looks like this:

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

int main (int argc, char *argv[])
{
   int fromArgv = NULL;  /* holds value from argv[1] */
   fromArgv = atoi (argv[1]);    /* convert argv[1] to int */

   /* if incorrect no. of arguments entered */
   if (argc != 2) {
   fprintf (stderr, "error: wrong number of arguments\n");
   exit (EXIT_FAILURE);
   } 

   return fromArgv;
}

I get the following error when trying to compile:

ex1.c: In function ‘main’:
ex1.c:6:18: error: initialization makes integer from pointer without a cast [-Werror]
cc1: all warnings being treated as errors

Answer 1

Your problem is the line

int fromArgv = NULL; 

I believe NULL is usually defined as (void *) 0, which is a pointer. You’re assigning this pointer to an int variable, which gives you the error.

However, there’s no need to initialize fromArgv in this case. You can just do:

int fromArgv;  /* holds value from argv[1] */
fromArgv = atoi (argv[1]);    /* convert argv[1] to int */

or even just

int fromArgv = atoi (argv[1]);    /* convert argv[1] to int */

What you do have to do, however, is make sure argv[1] exists before you access it so you don’t get a segfault if the user does not enter any command-line arguments. You should move your if (argc != 2) test before the assignment of fromArgv.