How can I change a scanf(“%d”,&variable) to receive an argv from the command line in C

Question

I'm trying to modify a portion of a program which currently takes user input during runtime and saves the response to a reference to a variable and instead make it fetch the response as an argv[] when compiling. I can fetch the response from argv, but i'm not sure how to retain the variable reference functionality.

Here is the original code:

printf("\n Enter Variable : ");
scanf("%d",&variable);

Output:

PID BURST ARRIVAL 0 0 12
1 2 4
2 3 1
3 4 2
Enter quantum time : 5

0 2 3 3 7 1 9

Average waiting time = 0.00

Average turn-around = 2.25.

Here is what I have attempted:

variable = argv[1];

Output:

0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1 0 2 0 3 0 1

And:

&variable = argv[1];

Also:

(int*)&variable = argv[1];

The first attempt compiles and runs with the command line input, but the output from the program is broken/different from the original. The second and third attempts give gcc errors about using & as the left operand. Does anyone know how to mimic the original functionality with argv[] instead of scanf()?

Answer 1

scanf is both reading the string and converting it to an integer (the %d ). When you use argv[1] , you only have the string, but haven't converted it to an integer. You need to use a function like atoi or strtol , like:

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

Or, as @JohnBollinger correctly points out, sscanf also works:

sscanf(argv[1], "%d", &variable);

Answer 2

If you're trying to convert the command line argument to an integer, then I think the functionality you're looking for is in atoi or strtol . But a near drop-in replacement for scanf is sscanf , which reads from a string instead of the standard input.

sscanf(argv[1], "%d", &variable);

The reason your first attempt didn't work is because what you're really doing is converting the char* pointer to an integer. It's not actually reading the content of the string. You were seeing the value of the pointer converted to an int .

Your other two didn't work because &variable is not an lvalue and cannot be assigned to.