How to create global functions in Objective-C

First note that Objective-C language is a superset of C language (meaning there is absolutely nothing wrong with mixing them).

There are two approaches.

#1 Real global function:

Declare a global C-style function, which can have ObjC logic (in definetion instead of just C-style logic).

Header:

void GSPrintTest();

Implementation:

#import "functions.h"

#import <Foundation/Foundation.h>

void GSPrintTest() {
  NSLog(@"test");
}

Call using:

#import "functions.h"
...
GSPrintTest();

A third (bad, but possible) option would be adding a category to NSObject for your methods:

Header:

#import <Foundation/Foundation.h>

@interface NSObject(GlobalStuff)
- (void) printTest;
@end

Implementation:

#import "functions.h"

@implementation NSObject(GlobalStuff)
- (void) printTest {
  NSLog(@"test");
}
@end

Call using:

#import "functions.h"
...
[self printTest];

Create a class method with + sign, in helper class (instead of instance method with - sign).

Header:

#import <Foundation/Foundation.h>

@interface GlobalStuff : NSObject {}

+ (void)printTest;

@end

Implementation:

#import "functions.h"

@implementation GlobalStuff

+ (void) printTest {
  NSLog(@"test");
}

@end

Call using:

#import "functions.h"

...
[GlobalStuff printTest];