objc_msgSend is the Objective-C message dispatcher. It's the function-calling function, using selector and the receiver object's class to decide where to jump to. objc_msgSend_stret is exactly the same, but for methods that return values of struct types. Why does objc_msgSend_stret exist? Because of the machine-level guts of the C language require it, and Objective-C methods are C functions if you tilt your head and squint a bit.

On most processors, the first few parameters to a function are passed in CPU registers, and return values are handed back in CPU registers. Objective-C methods do the same, but with id self and SEL _cmd as the first two parameters. Here's a PowerPC example:

    -(int) method:(id)arg;
        r3 = self
        r4 = _cmd, @selector(method:)
        r5 = arg
	(on exit) r3 = returned int

CPU registers work fine for small return values like ints and pointers, but structure values can be too big to fit. For structs, the caller allocates stack space for the returned struct, passes the address of that storage to the function, and the function writes its return value into that space. The address of the struct is an implicit first parameter just like self and _cmd:

    -(struct st) method:(id)arg;
        r3 = &struct_var (in caller's stack frame)
        r4 = self
        r5 = _cmd, @selector(method:)
        r6 = arg
        (on exit) return value written into struct_var

Now consider objc_msgSend's task. It uses _cmd and self->isa to choose the destination. But self and _cmd are in different registers if the method will return a struct, and objc_msgSend can't tell that in advance. Thus objc_msgSend_stret: just like objc_msgSend, but reading its values from different registers.

But there's a catch.

On most architectures, some small C structs are returned in registers after all, instead of using the struct-address first parameter that objc_msgSend_stret expects. If the struct type falls into this category, then objc_msgSend is used instead. So the "struct return" part of objc_msgSend_stret refers to the architecture's definition of a stack-returned struct, which may not match C struct.

The rules for which struct types return in registers are always arcane, sometimes insane. ppc32 is trivial: structs never return in registers. i386 is straightforward: structs with sizeof exactly equal to 1, 2, 4, or 8 return in registers. x86_64 is more complicated, including rules for returning floating-point struct fields in FPU registers, and ppc64's rules and exceptions will make your head spin. The gory details are documented in the Mac OS X ABI Guide, though as usual if the documentation and the compiler disagree then the documentation is wrong.

If you're calling objc_msgSend directly and need to know whether to use objc_msgSend_stret for a particular struct type, I recommend the empirical approach: write a line of code that calls your method, compile it on each architecture you care about, and look at the assembly code to see which dispatch function the compiler uses.