Why does the message sending program to nil in Objective-C not crash?

We know that the message sending program to nil in Objective-C will not crash,

Objective-C is based on the C language,

On PC, null pointer operations are performed in C language,

The program crashes due to a protection error due to out-of-bounds access,

But why won't Objective-C crash?

The reason must be found in the source code,

The following is the arm assembly code snippet of objc_msgSend:

In the arm function call process,

Generally, parameters are transmitted by r0-r4,

Use r0 to pass the returned value.

Corresponding to objc_msgSend. The first parameter is self, and the returned value is self, which is put in r0 (a1.

 

 

/*************************************** *****************************

* Idobjc_msgSend (idself, SELop ,...)

* On entry:A1 is the message sender er,

*A2 is the selector

**************************************** ****************************/

 

ENTRY objc_msgSend

# Check whether receiver is nil

Teq a1, #0

Moveq a2, #0

Bxeq lr

 

Teq instructions:

TEQ Rn,Operand2 The TEQ instruction performs a bitwise Exclusive OR operation on the value in Rn and the value of Operand2.

Test whether self is empty.

Moveq instructions:

If self is empty, the selector is also set to null.

Bx instructions:

In arm, bx lr is used to return to the place where the subroutine is called (that is, return to the caller). If self is empty, it is returned to the place where objc_msgSend is called for further execution.

In short:

If the self parameter passed to objc_msgSend is nil, this function will not perform meaningful operations and will return directly.