How to fix implicit declaration of function? [duplicate]

Question

I am a complete beginner and have been taking youtube lessons on C. However I am stuck with a very basic error (I assume) and would appreciate an explanation of why I am getting this error:

I haven't tried any fixes as I have no clue - being a complete beginner.

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

int main()
{
  sayHi();
  return 0;
}   


void sayHi()
{
 printf("Hello User");
}

f.c:6:5: error: implicit declaration of function 'sayHi' is invalid in C99 [-Werror,-Wimplicit-function-declaration] sayHi(); ^ f.c:11:6: error: conflicting types for 'sayHi' void sayHi() ^ f.c:6:5: note: previous implicit declaration is here sayHi(); ^

Add a forward declaration of the function prior to main: void sayHi(); — Chris White
– Chris White, Commented May 14, 2019 at 17:40
...or move the function to the top of the file, before main(). — Lee Daniel Crocker
– Lee Daniel Crocker, Commented May 14, 2019 at 17:47
Empty parentheses () on a function declaration mean that it takes an unspecified number and type(s) of arguments. That's an obsolescent feature. To specify that the function takes no arguments (and get better compile-time checking), use int main(void) and void sayHi(void). (Note that in C++, empty parentheses do mean that the function takes no arguments.) — Keith Thompson
– Keith Thompson, Commented May 14, 2019 at 18:11

Yoni Newman · Accepted Answer · 2019-05-14 17:42:10Z

3

Declare SayHi function before you call it.

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

void sayHi(); //declartion of the function

int main()
{
sayHi();
return 0;
}   


void sayHi()
{
printf("Hello User");
}

answered May 14, 2019 at 17:42

Yoni Newman

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