implicit declaration of function 'GetInt' is invalid in C99

Question

I am trying to make this simple code for practice in C. It asks the user to give a positive number, checks if it's positive or not, then returns just positive numbers.

I am getting this error:

positive.c:28:7: warning: implicit declaration of function 'GetInt' is invalid
  in C99 [-Wimplicit-function-declaration]
            n = GetInt();

I would have thought this meant I didn't declare one of my functions or that I didn't call in some library. To the best of my knowledge, I did all of this. Here is my code:

#include <stdio.h>

int GetPositiveInt(void);

int main(void)
{
    int n = GetPositiveInt();
    printf("Thanks for the %i\n", n);
}


/*This all gets called into the above bit*/
int GetPositiveInt(void)
{
    int n; /*declare the variable*/
    do
    {
        printf("Give me a positive integer: ");
        n = GetInt();
    }
    while (n <= 0);
    return n; /*return variable to above*/
}

Does anyone have any ideas on why this is giving me an error?

Answer 1

That's because this function, GetInt does not exist or you forgot to include the correct header.

You can replace the call to the function GetInt by:

scanf("%d", &n);

Answer 2

You haven't declared GetInt() . Compiler don't know what it returns and what arguments it receives. Implicit declarations was forbidden in C99 (was valid before - would only produce warning if you'll enable C89).

Of course if you'll have only declaration and no implementation - you'll get an error on linking phase.

Answer 3

There's a library created for the CS50 edX class you need to include at the top of your.c file. Otherwise the compiler doesn't know about the GetInt() function.

#include <cs50.h>

Answer 4

Worked for me: You may use get_int(); in the CS50 IDE and compile it using make or simply put -lcs50 at the end of clang.

Answer 5

The CS-50 library has been updated. Now you must use:

$ int x = get_int("Prompt: ");

instead of GetInt()

Your code will look like this:

#include <stdio.h>


int main(void)

{
    int n; /*declare the variable*/
    do
    {
        n = get_int(“Give me a positive integer: “);
    }
    while (n <= 0);
    return n; /*return variable to above*/
}

Answer 6

This worked for me. I'm also doing the CS50x 2023 Introduction to Computer Science.

Taken from the one of the "Shorts" Video, (Week 1: C). Conditional Statements

#include <stdio.h>
#include <cs50.h>

int main(void)
{
int x = get_int("Prompt: ");
switch(x)
  {
    case 5:
        printf("Five, ");
    case 4:
        printf("Four, ");
    case 3:
        printf("Three, ");
    case 2:
        printf("Two, ");
    case 1:
        printf("One, ");
    default:
        printf("Blast-off!\n");
  }
}