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LLDB: How to Set a Breakpoint at a Function's returnLLDB：如何在函数 return 处下断点

2020-04-23 · Debugging & reverse engineering

Translated from Chinese — .

You have a complex function with lots of lines and many returns inside. Single-stepping through it is slow — how can you quickly find which line it returns from?

For example, this code:

void foo() {
    int i = arc4random() %100;
    
    if (i > 30) {
        if (i < 40) {
            return;
        }
        if (i > 77) {
            return;
        }
        if (i < 66) {
            return;
        }
    }
    
    switch (i) {
        case 0:
            return;
        case 1:
            return;
        case 2:
            return;
        case 3:
            return;
        case 4:
            return;
        default:
            return;
    }
}

int main(int argc, const char * argv[]) {
    foo();
    return 0;
}

Suppose foo is a very long, very complex function with many returns. How do you know which line it returned from?

You can use an lldb breakpoint:

breakpoint set -p return
或者
br set -p return

First add a breakpoint on the first line of foo.

Once the breakpoint triggers, type br set -p return in the console.

Then continue, and it will break at the line where the function returns.

Pretty interesting~

有一个代码行很多的复杂函数，内部有很多return，单步调试很慢，如何快速找到哪一行return了？

例如代码：

void foo() {
    int i = arc4random() %100;
    
    if (i > 30) {
        if (i < 40) {
            return;
        }
        if (i > 77) {
            return;
        }
        if (i < 66) {
            return;
        }
    }
    
    switch (i) {
        case 0:
            return;
        case 1:
            return;
        case 2:
            return;
        case 3:
            return;
        case 4:
            return;
        default:
            return;
    }
}

int main(int argc, const char * argv[]) {
    foo();
    return 0;
}

假设foo是个很长很复杂有很多return的函数，如何知道是通过哪一行return的呢？

可以使用lldb的断点

breakpoint set -p return
或者
br set -p return

首先在foo第一行加断点

断点触发后，控制台输入 br set -p return

然后continue，就会断点到函数return的那一行了。

很有趣~