Passing variables between functions in c with out using global variables

You need to call the function passing the value (or variable) required.

int function1(int);

int main(void) 
{
  int hello=10;
  function1(hello);
  function1(130);
}

int function1(int ip)
{
  return printf("hello - %d\n",ip); 
}

https://godbolt.org/z/97o3dPWzj

change

int function1();

to

int function1(int);

and

 printf(function1());

to

 function1(hello);

"I am expecting the 10 to be printed next to the "hello" but instead get a 0."

There are already examples of working versions of your code in the other answers, but little is said in the way of explanation. I will try to do that here, and offer some general suggestions.

Although your code compiles, it does so with warnings and when run, produces run-time errors. The information resulting from both of these, if heeded would likely provide a guide to addressing most of your issues. (If using GCC for example, you can set warnings to -Wall). To illustrate, here is an image showing line numbers and the warning that correspond with them on my system (not GCC, but warning are turned on)

Build warnings:

And finally, run-time error:

From these you can begin to derive some of the following observations (others listed are my own.)

• The signature of a function prototype must match its implementation. (for function1, they do not)
• If the prototype of a function is non-void, then it should return a value.
• If a function returns a value, its return value should not be ignored.
• The warning of an unused variable pointed out that you probably intended to use it as an argument here: function1()

See comments below for other clarifications:

int function1();//this prototype is incomplete, and is inconsistent
                //with the signature of its implementation

int main(void) {
  int hello=10;
  printf(function1());//because function1 is not returning a value, your code outputs a 0
                      //Function1 prototype requires it to pass an argument
  // non-void function should return a value here
}

int function1(int ip){//signature of this implementation disagrees with its prototype
                      //because there is no value passed, ip can be any value 
  printf("hello%d",ip); 
  // non-void function should return a value here
}