iOS block调用为啥要判空
0x1 前言
在iOS中，使用nil指针调用OC的方法是安全的，但是使用nil指针调用block却会产生崩溃。本篇文章，将会从汇编的角度解释该现象。
0x2 block的结构
Block 的结构可以在 Runtime 的开源代码Objc4-706 中找到，它位于 Block-private.h 中： ``` struct Block_layout { void isa; volatile int32_t flags; // contains ref count int32_t reserved; void (invoke)(void , ...); struct Block_descriptor_1 descriptor; // imported variables };
```
在arm64中，一个指针占8字节，int32_t占4个字节，所以一个block的内存基本布局如下图：
0x3 测试代码
1.首先定义Helper类辅助测试，代码如下：
``` @interface Helper : NSObject
@property (nonatomic, copy) dispatch_block_t block;
@end
@implementation Helper
- (void)triger {}
@end ```
2.测试用例1：调用一个正常对象的block
- (void)testBlock {
Helper *helper = [Helper new];
helper.block = ^{
NSLog(@"test");
};
helper.block();
}
在testBlock函数入口处打上断点
然后在Xcode菜单栏找到Debug -> Debug Workflow，勾选Always Show Disassembly
运行代码，触发断点，会自动进入Xcode汇编，如图所示。
3.分析汇编
TestBlock`-[ViewController testBlock]:
0x1001a5d1c <+0>: sub sp, sp, #0x40
0x1001a5d20 <+4>: stp x29, x30, [sp, #0x30]
0x1001a5d24 <+8>: add x29, sp, #0x30
0x1001a5d28 <+12>: stur x0, [x29, #-0x8]
0x1001a5d2c <+16>: stur x1, [x29, #-0x10]
0x1001a5d30 <+20>: adrp x8, 8
-> 0x1001a5d34 <+24>: ldr x0, [x8, #0x428]
0x1001a5d38 <+28>: bl 0x1001a634c ; symbol stub for: objc_opt_new
0x1001a5d3c <+32>: ldr x1, [sp]
0x1001a5d40 <+36>: add x8, sp, #0x18
0x1001a5d44 <+40>: str x8, [sp, #0x10]
0x1001a5d48 <+44>: str x0, [sp, #0x18]
0x1001a5d4c <+48>: ldr x0, [sp, #0x18]
0x1001a5d50 <+52>: adrp x2, 3
0x1001a5d54 <+56>: add x2, x2, #0x50 ; __block_literal_global.13
0x1001a5d58 <+60>: bl 0x1001a64c0 ; objc_msgSend$setBlock:
0x1001a5d5c <+64>: ldr x1, [sp]
0x1001a5d60 <+68>: ldr x0, [sp, #0x18]
0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block
0x1001a5d68 <+76>: mov x29, x29
0x1001a5d6c <+80>: bl 0x1001a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1001a5d70 <+84>: str x0, [sp, #0x8]
0x1001a5d74 <+88>: ldr x8, [x0, #0x10]
0x1001a5d78 <+92>: blr x8
0x1001a5d7c <+96>: ldr x0, [sp, #0x8]
0x1001a5d80 <+100>: bl 0x1001a6358 ; symbol stub for: objc_release
0x1001a5d84 <+104>: ldr x0, [sp, #0x10]
0x1001a5d88 <+108>: mov x1, #0x0
0x1001a5d8c <+112>: bl 0x1001a6388 ; symbol stub for: objc_storeStrong
0x1001a5d90 <+116>: ldp x29, x30, [sp, #0x30]
0x1001a5d94 <+120>: add sp, sp, #0x40
0x1001a5d98 <+124>: ret
打个断点在0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block指令处，此时x0为helper对象，bl指令调用的是helper对象的block属性的get方法，即[helper block]函数。
```
-> 0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block
0x1001a5d68 <+76>: mov x29, x29
0x1001a5d6c <+80>: bl 0x1001a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1001a5d70 <+84>: str x0, [sp, #0x8]
0x1001a5d74 <+88>: ldr x8, [x0, #0x10]
0x1001a5d78 <+92>: blr x8
(lldb) register read x0
x0 = 0x0000000282b80a60
(lldb) po 0x0000000282b80a60
```
单步断点下一个指令，断点到0x1001a5d68 <+76>: mov x29, x29处，此时执行完[helper block]函数，返回了block的指针，放于寄存器x0中，可以简单的理解为x0 = [helper block]。
```
0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block
-> 0x1001a5d68 <+76>: mov x29, x29
0x1001a5d6c <+80>: bl 0x1001a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1001a5d70 <+84>: str x0, [sp, #0x8]
0x1001a5d74 <+88>: ldr x8, [x0, #0x10]
0x1001a5d78 <+92>: blr x8
(lldb) register read x0 x0 = 0x00000001001a8050 TestBlock`__block_literal_global.13 ```
断点打在0x1001a5d70 <+84>: str x0, [sp, #0x8]指令处，执行完objc_retainAutoreleasedReturnValue函数后，x0仍然是block指针。
``` 0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block 0x1001a5d68 <+76>: mov x29, x29 0x1001a5d6c <+80>: bl 0x1001a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue -> 0x1001a5d70 <+84>: str x0, [sp, #0x8] 0x1001a5d74 <+88>: ldr x8, [x0, #0x10] 0x1001a5d78 <+92>: blr x8
(lldb) register read x0 x0 = 0x00000001001a8050 TestBlock`__block_literal_global.13 ```
断点打在0x1001a5d78 <+92>: blr x8指令处，0x1001a5d74 <+88>: ldr x8, [x0, #0x10]这句指令的伪代码：x8 = x0 + 0x10, 即 0x00000001001a8060 = 0x00000001001a8050 + 0x10，在地址0x00000001001a8060处内存存放的地址就是block对象的invoke指针0x00000001001a5d9c。
```
0x1001a5d64 <+72>: bl 0x1001a6460 ; objc_msgSend$block
0x1001a5d68 <+76>: mov x29, x29
0x1001a5d6c <+80>: bl 0x1001a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1001a5d70 <+84>: str x0, [sp, #0x8]
0x1001a5d74 <+88>: ldr x8, [x0, #0x10]
-> 0x1001a5d78 <+92>: blr x8
(lldb) register read x0
x0 = 0x00000001001a8050 TestBlock__block_literal_global.13
(lldb) memory read 0x00000001001a8060
0x1001a8060: 9c 5d 1a 00 01 00 00 00 10 80 1a 00 01 00 00 00 .]..............
0x1001a8070: b8 2f 97 f6 01 00 00 00 c8 07 00 00 00 00 00 00 ./..............
(lldb) register read x8
x8 = 0x00000001001a5d9c TestBlock__27-[ViewController testBlock]_block_invoke at ViewController.m:71
``
根据block的内存布局图可以知道在block的isa + 0x10处的内存就是block的invoke指针地址。指令0x1001a5d78 <+92>: blr x8是调用block的invoke指针进行函数调用，即调用的是helper block`，执行block的调用。这是一个正常oc对象的block的调用汇编分析，现在来看一下下面两种测试用例。
4.测试用例2：调用一个对象的nil block，重复2步骤，进入Xcode汇编
- (void)testBlockNilBlock {
Helper *helper = [Helper new];
helper.block();
}
将断点打到0x100d09c14 <+52>: bl 0x100d0a460 ; objc_msgSend$block指令处，获取block指针的指令调用之后。查看此时的x0，发现获取的值为0，也就是nil，取到一个为nil的block指针。
``
TestBlock-[ViewController testBlockNilBlock]:
0x100d09be0 <+0>: sub sp, sp, #0x40
0x100d09be4 <+4>: stp x29, x30, [sp, #0x30]
0x100d09be8 <+8>: add x29, sp, #0x30
0x100d09bec <+12>: stur x0, [x29, #-0x8]
0x100d09bf0 <+16>: stur x1, [x29, #-0x10]
0x100d09bf4 <+20>: adrp x8, 8
0x100d09bf8 <+24>: ldr x0, [x8, #0x428]
0x100d09bfc <+28>: bl 0x100d0a34c ; symbol stub for: objc_opt_new
0x100d09c00 <+32>: ldr x1, [sp]
0x100d09c04 <+36>: add x8, sp, #0x18
0x100d09c08 <+40>: str x8, [sp, #0x10]
0x100d09c0c <+44>: str x0, [sp, #0x18]
0x100d09c10 <+48>: ldr x0, [sp, #0x18]
0x100d09c14 <+52>: bl 0x100d0a460 ; objc_msgSend$block
-> 0x100d09c18 <+56>: mov x29, x29
0x100d09c1c <+60>: bl 0x100d0a364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x100d09c20 <+64>: str x0, [sp, #0x8]
0x100d09c24 <+68>: ldr x8, [x0, #0x10]
0x100d09c28 <+72>: blr x8
0x100d09c2c <+76>: ldr x0, [sp, #0x8]
0x100d09c30 <+80>: bl 0x100d0a358 ; symbol stub for: objc_release
0x100d09c34 <+84>: ldr x0, [sp, #0x10]
0x100d09c38 <+88>: mov x1, #0x0
0x100d09c3c <+92>: bl 0x100d0a388 ; symbol stub for: objc_storeStrong
0x100d09c40 <+96>: ldp x29, x30, [sp, #0x30]
0x100d09c44 <+100>: add sp, sp, #0x40
0x100d09c48 <+104>: ret
(lldb) register read x0
x0 = 0x0000000000000000
将断点打在`0x100d09c24 <+68>: ldr x8, [x0, #0x10]`指令处，该指令等价于x8 = x0 + 0x10，由于此时x0为0x0000000000000000，所以 0x0000000000000010 = 0x0000000000000000 + 0x10，该地址0x0000000000000010为非法地址，所以会触发非法地址异常。
0x100d09c14 <+52>: bl 0x100d0a460 ; objc_msgSend$block
0x100d09c18 <+56>: mov x29, x29
0x100d09c1c <+60>: bl 0x100d0a364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x100d09c20 <+64>: str x0, [sp, #0x8]
-> 0x100d09c24 <+68>: ldr x8, [x0, #0x10]
0x100d09c28 <+72>: blr x8
```
放开断点，继续执行，触发EXC_BAD_ACCESS异常，异常信息中address=0x10，如下图：
从这个用例中可以得出结论，当对象的block为nil时，在汇编层，仍然会按照正常的block调用逻辑去取block的invoke指针去执行，当寄存器进行计算获取invoke指针时，由于block为nil，寄存器计算出的地址为0x10，触发非法地址异常。
5.测试用例3：调用一个nil对象的block，重复2步骤，进入Xcode汇编
- (void)testBlockNilObj {
Helper *helper = nil;
helper.block();
}
TestBlock`-[ViewController testBlockNilObj]:
0x1025a5b78 <+0>: sub sp, sp, #0x40
0x1025a5b7c <+4>: stp x29, x30, [sp, #0x30]
0x1025a5b80 <+8>: add x29, sp, #0x30
0x1025a5b84 <+12>: mov x8, x1
0x1025a5b88 <+16>: stur x0, [x29, #-0x8]
0x1025a5b8c <+20>: stur x8, [x29, #-0x10]
0x1025a5b90 <+24>: add x8, sp, #0x18
0x1025a5b94 <+28>: str x8, [sp, #0x8]
0x1025a5b98 <+32>: mov x8, #0x0
0x1025a5b9c <+36>: str x8, [sp, #0x10]
-> 0x1025a5ba0 <+40>: str xzr, [sp, #0x18]
0x1025a5ba4 <+44>: ldr x0, [sp, #0x18]
0x1025a5ba8 <+48>: bl 0x1025a6460 ; objc_msgSend$block
0x1025a5bac <+52>: mov x29, x29
0x1025a5bb0 <+56>: bl 0x1025a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1025a5bb4 <+60>: str x0, [sp]
0x1025a5bb8 <+64>: ldr x8, [x0, #0x10]
0x1025a5bbc <+68>: blr x8
0x1025a5bc0 <+72>: ldr x0, [sp]
0x1025a5bc4 <+76>: bl 0x1025a6358 ; symbol stub for: objc_release
0x1025a5bc8 <+80>: ldr x0, [sp, #0x8]
0x1025a5bcc <+84>: ldr x1, [sp, #0x10]
0x1025a5bd0 <+88>: bl 0x1025a6388 ; symbol stub for: objc_storeStrong
0x1025a5bd4 <+92>: ldp x29, x30, [sp, #0x30]
0x1025a5bd8 <+96>: add sp, sp, #0x40
0x1025a5bdc <+100>: ret
对比其获取block指针到取invoke指针去执行这一过程，与测试用例2并无区别：
0x1025a5ba8 <+48>: bl 0x1025a6460 ; objc_msgSend$block
0x1025a5bac <+52>: mov x29, x29
0x1025a5bb0 <+56>: bl 0x1025a6364 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x1025a5bb4 <+60>: str x0, [sp]
0x1025a5bb8 <+64>: ldr x8, [x0, #0x10]
0x1025a5bbc <+68>: blr x8
所以，不管是调用nil对象的block还是正常对象的一个为nil的block指针最终都会触发到非法地址异常上。
6.测试用例4: 调用一个nil对象的函数，重复2步骤，进入Xcode汇编
- (void)test {
Helper *helper = nil;
[helper triger];
}
TestBlock`-[ViewController test]:
0x102635c4c <+0>: sub sp, sp, #0x40
0x102635c50 <+4>: stp x29, x30, [sp, #0x30]
0x102635c54 <+8>: add x29, sp, #0x30
0x102635c58 <+12>: mov x8, x1
0x102635c5c <+16>: stur x0, [x29, #-0x8]
0x102635c60 <+20>: stur x8, [x29, #-0x10]
0x102635c64 <+24>: add x8, sp, #0x18
0x102635c68 <+28>: str x8, [sp, #0x8]
0x102635c6c <+32>: mov x8, #0x0
0x102635c70 <+36>: str x8, [sp, #0x10]
-> 0x102635c74 <+40>: str xzr, [sp, #0x18]
0x102635c78 <+44>: ldr x0, [sp, #0x18]
0x102635c7c <+48>: bl 0x102636500 ; objc_msgSend$triger
0x102635c80 <+52>: ldr x0, [sp, #0x8]
0x102635c84 <+56>: ldr x1, [sp, #0x10]
0x102635c88 <+60>: bl 0x102636388 ; symbol stub for: objc_storeStrong
0x102635c8c <+64>: ldp x29, x30, [sp, #0x30]
0x102635c90 <+68>: add sp, sp, #0x40
0x102635c94 <+72>: ret
对于OC函数调用最终都会转换成objc_msgSend的调用
0x102635c7c <+48>: bl 0x102636500 ; objc_msgSend$triger
查看objc_msgSend的实现可知，指令cbz r0, LNilReceiver_f先判断x0是否为nil，如果为nil，清空寄存器，消息发送返回nil。所以对于nil对象的方法调用，是安全的。并不会像block调用一样对寄存器中的内容（即使内存为0，没有作判空）进行偏移计算获取invoke指针进行调用，进而导致取到非法地址，触发异常。
0x4 总结
本篇文章通过分析以上几个测试用例的汇编代码，分析了OC对象函数与block调用在汇编层面上的区别，这种区别导致了对于block的调用需要进行判空后才能确保安全。
!block ?: block();
值得注意的是，调用多层对象的block时，也需要进行判空，即使d对象与其block必然存在，也可能因为a、b、c对象中任意一个为nil，导致出现测试用例3的场景，调用一个nil对象的block产生崩溃，比如：
```
//不安全调用
a.b.c.d.block();
//安全调用
!a.b.c.d.block ?: a.b.c.d.block();
对于这种情况，可以对将该block进行一层函数封装，可以避免过长的判断逻辑。
//d类
- (void)callBlock {
!self.block ?: self.block();
}
//调用 [a.b.c.d callBlock]; ```
总而言之，block调用之前需要进行判空。