You want this:

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

int main(int argc, char *argv[])
{
  if (argc < 2)
  {
    printf("missing command line argument\n");
    return 1;
  }

  int decimal = (int)strtol(argv[1], NULL, 10);

  ...
}

• you need a minimal understanding of strings.
• The (int) cast is for making clear that we explicitely convert the long int returned by strtol to int which may or may not be a smaller type than long int, depending on your platform.
• Documentation for strtol.
• Explanation about argc and argv

Answer:

#include <stdio.h>
#include<stdlib.h>
int main(int argc, char *argv[]) 
{
    int a[10], decimal, i, j;
    decimal = atoi(argv[1]); 
    for(i = 0; decimal > 0; i++)
    {
        a[i] = decimal % 2;
        decimal = decimal / 2;
    } 
    printf("\nNumarul binar este: ");
    for(j = i - 1; j >= 0; j--)  {
        printf("%d", a[j]);
    }
    printf("\n");
    return 0;
}

Here gcc is the c-compiler, and demo.c is the file name After your file gets successfully compiled then ./a.out is the command to run that build file, and append the command-line argument with it, as shown in the image. atoi(argv1) see argv[0] is the file name, argv1 is the argument, and atoi is used to convert string to int, as we can take only string input from the command line, and argc is the total number of argument user have given.