implicit declaration of function is invalid in C99

Question

I'm getting an error. Here's my info from Terminal:

make: *** [tests/myFunction] Error 1
~/workspace/ $ make ./tests/myFunction
clang -fsanitize=integer -fsanitize=undefined -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wshadow    tests/myFunction.c  -lcrypt -lcs50 -lm -o tests/myFunction
tests/myFunction.c:7:5: error: implicit declaration of function 'PrintName' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    PrintName(s);
    ^

The code I used is:

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

int main(void)
{
    string s = get_string();
    PrintName(s);
}

void PrintName(string name)
{
    printf("hello, %s\n", name);
}

The code I used is from here:

http://docs.cs50.net/2016/fall/notes/1/week1.html

Please, help to fix an error.

Answer 1

Sorry, beginner error:

But now if we try to print_name(s) in our main function, we still get an error. And that’s because the compiler reads from top to bottom, in order, so at the time main calls print_name, it doesn’t exist yet. So we need to declare it with something called a prototype first:

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

void print_name(string name);

int main(void)
{
    string s = get_string();
    print_name(s);
}

void print_name(string name)
{
    printf("hello, %s\n", name);
}