NSString pointer passed to function... not keeping the value I set - cocoa

Question

I'm not sure what I'm doing wrong here. I've also tried setting s1..3 in foo by using:
s1 = [[NSString alloc] initWithString:[filepaths objectAtIndex:0]];
Context below:
void foo(NSString *s1, NSString *s2, NSString *s3){
//assign long string to NSString *fps
//...
//break fps into smaller bits
NSArray *filepaths = [fps componentsSeparatedByString:#"\n"];
//the above worked! now let's assign them to the pointers
s1 = [filepaths objectAtIndex:0];
//repeat for s2 and s3
NSLog(#"%#",s1); //it worked! we're done in this function
}
int main(int argc, const char * argv[]){
NSString *s1 = nil; //s2 and s3 as well
foo(s1,s2,s3); //this should work
NSLog(#"%#",s1); //UH OH, this is null!
return 0;
}

Answer 1

No.
You are passing in pointers to objects which can be mutated locally. You are not changing the original objects, as you might think from plain C.
If you want to use this method (which I would not recommend - it's really odd to see in Cocoa except in the case of NSError), you would have something like:
void foo(NSString **s1, NSString **s2, NSString **s3) {
*s1 = [filepaths objectAtIndex:0]; // etc.
}
You would then pass in &s1 as the argument.
This will, of course, clobber whatever was in s1, potentially cause memory leaks, thread unsafety, etc., unless you are really careful. Which is why I say you usually won't do this.

Answer 2

Functions take a local copy of their arguments, so you're modifying a copy of the NSString* not the original. This is called "pass by value". What you want is "pass by reference," which looks like this:
void foo(NSString** s1) {
if(s1) *s1 = #"Different string";
}
int main(int argc, const char* argv[]){
NSString* s1 = nil;
foo(&s1);
NSLog(#"%#", s1);
return EXIT_SUCCESS;
}
It boils down to how well you understand pointers in C. It would be a good idea to read up on the "address of" operator &, and the dereference operator *, and just C pointers in general.