Calculator program in C using Command Line Arguments (argc, argv)

I've written a program to take input arguments from command line and create a basic calculator in C. Codebelow:

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


void CheckArgumentCount(int argc);
int CheckOperands(char *argv[]);
long Calculate(long nr1, long nr2, char operation);

int main(int argc, char *argv[])
{
    long nr1 = CheckOperands(&argv[1]);
    long nr2 = CheckOperands(&argv[3]);
    int result;

    CheckArgumentCount(argc);
    result = Calculate(nr1, nr2, *argv[2]);
    printf("Result: %d", result);

    return 0;
}


void CheckArgumentCount(int argc)
{
    if (argc > 3 || argc < 3)
    {
        puts("Wrong number of arguments to perform the calculation.\n");
    }
}

int CheckOperands(char *argv[])
{
        int i = 1;
        
        while (*argv[i] != '\0')
        {
            if (*argv[i] < '0' || *argv[i] > '9')
            {
                return *argv[i];
            }
            argv++;
        }
        return atoi(argv[i]);
}


long Calculate(long nr1, long nr2, char operation)
{
    long result = 0;

    switch (operation)
    {
        case '+':
            result = nr1 + nr2;
            break;
        case '-':
            result = nr1 - nr2;
            break;
        case '*':
            result = nr1 * nr2;
            break;
        case '/':
            if (nr2 == 0)
            {
                puts("Error! Divion by zero.");
            }
            else
            {
                result = nr1 / nr2;
            }
            break;
        default:
            puts("Operator invalid.");
            break;
    }
    return result;
}

Which compiles fine, as the compiler does not show any errors. However, I get this error on zsh terminal on run when I provide 3 arguments, e.g.:

zsh: segmentation fault ./calc 1 + 3

I assume it might have to do something with my CheckOperands function? Any advice will be appreciated.

You need to check the argument count before you use CheckOperands, or rather before you dereference any argc[n] with n > 1.

You want this:

int main(int argc, char* argv[])
{
  if (!CheckArgumentCount(argc))
    exit(1);   // if argument count differenmt from 3 exit program
  ...

and this:

int CheckArgumentCount(int argc)
{
  if (argc != 4)   // argc is one more because if the command 
                      invocation which is argv[0]
  {
    puts("Wrong number of arguments to perform the calculation.\n");
    return 0;
  }

  return 1;
}

And there are more errors in CheckOperands.

Overall corrected code:

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

int CheckArgumentCount(int argc);
int CheckOperands(char* operand);
long Calculate(long nr1, long nr2, char operation);

int main(int argc, char* argv[])
{
  if (!CheckArgumentCount(argc))
    exit(1);

  long nr1 = CheckOperands(argv[1]);
  long nr2 = CheckOperands(argv[3]);
  int result;


  result = Calculate(nr1, nr2, argv[2][0]);
  printf("Result: %d", result);

  return 0;
}


int CheckArgumentCount(int argc)
{
  if (argc != 4)
  {
    puts("Wrong number of arguments to perform the calculation.\n");
    return 0;
  }

  return 1;
}

int CheckOperands(char* operand)
{
  int i = 0;

  while (operand[i] != '\0')
  {
    if (operand[i] < '0' || operand[i] > '9')
    {
      return operand[i];
    }

    i++;
  }
  return atoi(operand);
}


long Calculate(long nr1, long nr2, char operation)
{
  long result = 0;

  switch (operation)
  {
  case '+':
    result = nr1 + nr2;
    break;
  case '-':
    result = nr1 - nr2;
    break;
  case '*':
    result = nr1 * nr2;
    break;
  case '/':
    if (nr2 == 0)
    {
      puts("Error! Divion by zero.");
    }
    else
    {
      result = nr1 / nr2;
    }
    break;
  default:
    puts("Operator invalid.");
    break;
  }
  return result;
}