C compile time error

[entellus]

I'm using gcc to compile the code listed below when I get the following error

error: expected '=', ',', ';', 'asm' or '__attribute__' before ' is_prime_number

/*
 * What is the largest prime factor of the number 317584931803?
 */

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

bool is_prime_number(unsigned long number){
    if(number < 2){
        return false;
    }

    bool is_a_prime = true;
    unsigned long i;
    unsigned long factor = number/2;
    
    for(i=factor;i>1;i--){
        if(number%i == 0){
           is_a_prime = false; 
           break;
        }
    }
    
    return is_a_prime;
}

int main(int argc, char* argv[]){
    if(argc != 2){
        fprintf(stderr, "Usage: ./euler_3 <number>\n");
        exit(1);
    }

    unsigned long max_val, biggest, result, i; 
    max_val = strtoul(argv[1], NULL, 10);
    biggest = max_val/2;

    //check in the reverse direction if the number is a factor of max_val
    for(i=biggest; i>1; i--){
        if(max_val%i == 0){
            //check if the number is a prime number
            if(is_prime_number(i)){
                result = i;
                break; 
            }
        } 
    }

    printf("Largest prime factor: %d", result);
    exit(0);
}

Any pointers will be greatly appreciated. Thanks in advance.

[entellus]

I'm using gcc to compile the code listed below when I get the following error

error: expected '=', ',', ';', 'asm' or '__attribute__' before ' is_prime_number

compiling with g++ -o file file.c works just fine. strange!

thanks

[weaknessforcats]

Not strange at all.

bool does not exist in C.