http://blog.csdn.net/jasonblog/article/details/49909209这里对上个月做的一个技术项目做部分技术小结，这篇文章描述的功能和我们在使用Xcode进行调试时点击暂停的效果类似。

一、获取任意一个线程的Call Stack

如果要获取当前线程的调用栈，可以直接使用现有API：[NSThread callStackSymbols]。

但是并没有相关API支持获取任意线程的调用栈，所以只能自己编码实现。

1. 基础结构

一个线程的调用栈是什么样的呢？

我的理解是应该包含当前线程的执行地址，并且从这个地址可以一级一级回溯到线程的入口地址，这样就反向构成了一条链：线程入口执行某个方法，然后逐级嵌套调用到当前现场。

（图片来源于维基百科）

如图所示，每一级的方法调用，都对应了一张活动记录，也称为活动帧。也就是说，调用栈是由一张张帧结构组成的，可以称之为栈帧。

我们可以看到，一张栈帧结构中包含着Return Address，也就是当前活动记录执行结束后要返回的地址（展开）。

那么，在我们获取到栈帧后，就可以通过返回地址来进行回溯了。

2. 指令指针和基址指针

我们明确了两个目标：(1)当前执行的指令，(2)当前栈帧结构。

以x86为例，寄存器用途如下：

SP/ESP/RSP: Stack pointer for top address of the stack.
BP/EBP/RBP: Stack base pointer for holding the address of the current stack frame.
IP/EIP/RIP: Instruction pointer. Holds the program counter, the current instruction address.

可以看到，我们可以通过指令指针来获取当前指令地址，以及通过栈基址指针获取当前栈帧地址。

那么问题来了，我们怎么获取到相关寄存器呢？

3. 线程执行状态

考虑到一个线程被挂起时，后续继续执行需要恢复现场，所以在挂起时相关现场需要被保存起来，比如当前执行到哪条指令了。

那么就要有相关的结构体来为线程保存运行时的状态，经过一番查阅，得到如下信息：

The function thread_get_state returns the execution state (e.g. the machine registers) of target_thread as specified by flavor.

Function - Return the execution state for a thread.

SYNOPSIS

kern_return_t   thread_get_state
                (thread_act_t                     target_thread,
                 thread_state_flavor_t                   flavor,
                 thread_state_t                       old_state,
                 mach_msg_type_number_t         old_state_count);
/*
 * THREAD_STATE_FLAVOR_LIST 0
 *  these are the supported flavors
 */
#define x86_THREAD_STATE32      1
#define x86_FLOAT_STATE32       2
#define x86_EXCEPTION_STATE32       3
#define x86_THREAD_STATE64      4
#define x86_FLOAT_STATE64       5
#define x86_EXCEPTION_STATE64       6
#define x86_THREAD_STATE        7
#define x86_FLOAT_STATE         8
#define x86_EXCEPTION_STATE     9
#define x86_DEBUG_STATE32       10
#define x86_DEBUG_STATE64       11
#define x86_DEBUG_STATE         12
#define THREAD_STATE_NONE       13
/* 14 and 15 are used for the internal x86_SAVED_STATE flavours */
#define x86_AVX_STATE32         16
#define x86_AVX_STATE64         17
#define x86_AVX_STATE           18

所以我们可以通过这个API搭配相关参数来获得想要的寄存器信息：

bool jdy_fillThreadStateIntoMachineContext(thread_t thread, _STRUCT_MCONTEXT *machineContext) {
    mach_msg_type_number_t state_count = x86_THREAD_STATE64_COUNT;
    kern_return_t kr = thread_get_state(thread, x86_THREAD_STATE64, (thread_state_t)&machineContext->__ss, &state_count);
    return (kr == KERN_SUCCESS);
}

这里引入了一个结构体叫_STRUCT_MCONTEXT。

4. 不同平台的寄存器

_STRUCT_MCONTEXT在不同平台上的结构不同：

x86_64，如iPhone 6模拟器：

_STRUCT_MCONTEXT64
{
    _STRUCT_X86_EXCEPTION_STATE64   __es;
    _STRUCT_X86_THREAD_STATE64  __ss;
    _STRUCT_X86_FLOAT_STATE64   __fs;
};

_STRUCT_X86_THREAD_STATE64
{
    __uint64_t  __rax;
    __uint64_t  __rbx;
    __uint64_t  __rcx;
    __uint64_t  __rdx;
    __uint64_t  __rdi;
    __uint64_t  __rsi;
    __uint64_t  __rbp;
    __uint64_t  __rsp;
    __uint64_t  __r8;
    __uint64_t  __r9;
    __uint64_t  __r10;
    __uint64_t  __r11;
    __uint64_t  __r12;
    __uint64_t  __r13;
    __uint64_t  __r14;
    __uint64_t  __r15;
    __uint64_t  __rip;
    __uint64_t  __rflags;
    __uint64_t  __cs;
    __uint64_t  __fs;
    __uint64_t  __gs;
};

x86_32，如iPhone 4s模拟器：

_STRUCT_MCONTEXT32
{
    _STRUCT_X86_EXCEPTION_STATE32   __es;
    _STRUCT_X86_THREAD_STATE32  __ss;
    _STRUCT_X86_FLOAT_STATE32   __fs;
};

_STRUCT_X86_THREAD_STATE32
{
    unsigned int    __eax;
    unsigned int    __ebx;
    unsigned int    __ecx;
    unsigned int    __edx;
    unsigned int    __edi;
    unsigned int    __esi;
    unsigned int    __ebp;
    unsigned int    __esp;
    unsigned int    __ss;
    unsigned int    __eflags;
    unsigned int    __eip;
    unsigned int    __cs;
    unsigned int    __ds;
    unsigned int    __es;
    unsigned int    __fs;
    unsigned int    __gs;
};

ARM64，如iPhone 5s：

_STRUCT_MCONTEXT64
{
    _STRUCT_ARM_EXCEPTION_STATE64   __es;
    _STRUCT_ARM_THREAD_STATE64  __ss;
    _STRUCT_ARM_NEON_STATE64    __ns;
};

_STRUCT_ARM_THREAD_STATE64
{
    __uint64_t    __x[29];  /* General purpose registers x0-x28 */
    __uint64_t    __fp;     /* Frame pointer x29 */
    __uint64_t    __lr;     /* Link register x30 */
    __uint64_t    __sp;     /* Stack pointer x31 */
    __uint64_t    __pc;     /* Program counter */
    __uint32_t    __cpsr;   /* Current program status register */
    __uint32_t    __pad;    /* Same size for 32-bit or 64-bit clients */
};

ARMv7/v6，如iPhone 4s：

_STRUCT_MCONTEXT32
{
    _STRUCT_ARM_EXCEPTION_STATE __es;
    _STRUCT_ARM_THREAD_STATE    __ss;
    _STRUCT_ARM_VFP_STATE       __fs;
};

_STRUCT_ARM_THREAD_STATE
{
    __uint32_t  __r[13];    /* General purpose register r0-r12 */
    __uint32_t  __sp;       /* Stack pointer r13 */
    __uint32_t  __lr;       /* Link register r14 */
    __uint32_t  __pc;       /* Program counter r15 */
    __uint32_t  __cpsr;     /* Current program status register */
};

可以对照《iOS ABI Function Call Guide》，其中在ARM64相关章节中描述到：

The frame pointer register (x29) must always address a valid frame record, although some functions–such as leaf functions or tail calls–may elect not to create an entry in this list. As a result, stack traces will always be meaningful, even without debug information

而在ARMv7/v6上描述到：

The function calling conventions used in the ARMv6 environment are the same as those used in the Procedure Call Standard for the ARM Architecture (release 1.07), with the following exceptions:

*The stack is 4-byte aligned at the point of function calls.
Large data types (larger than 4 bytes) are 4-byte aligned.
Register R7 is used as a frame pointer
Register R9 has special usage.*

所以，通过了解以上不同平台的寄存器结构，我们可以编写出比较通用的回溯功能。

5. 算法实现

/**
 * 关于栈帧的布局可以参考：
 * https://en.wikipedia.org/wiki/Call_stack
 * http://www.cs.cornell.edu/courses/cs412/2008sp/lectures/lec20.pdf
 * http://eli.thegreenplace.net/2011/09/06/stack-frame-layout-on-x86-64/
 */
typedef struct JDYStackFrame {
    const struct JDYStackFrame* const previous;
    const uintptr_t returnAddress;
} JDYStackFrame;

//

int jdy_backtraceThread(thread_t thread, uintptr_t *backtraceBuffer, int limit) {
    if (limit <= 0) return 0;

    _STRUCT_MCONTEXT mcontext;
    if (!jdy_fillThreadStateIntoMachineContext(thread, &mcontext)) {
        return 0;
    }

    int i = 0;
    uintptr_t pc = jdy_programCounterOfMachineContext(&mcontext);
    backtraceBuffer[i++] = pc;
    if (i == limit) return i;

    uintptr_t lr = jdy_linkRegisterOfMachineContext(&mcontext);
    if (lr != 0) {
        /* 由于lr保存的也是返回地址，所以在lr有效时，应该会产生重复的地址项 */
        backtraceBuffer[i++] = lr;
        if (i == limit) return i;
    }

    JDYStackFrame frame = {0};
    uintptr_t fp = jdy_framePointerOfMachineContext(&mcontext);
    if (fp == 0 || jdy_copyMemory((void *)fp, &frame, sizeof(frame)) != KERN_SUCCESS) {
        return i;
    }

    while (i < limit) {
        backtraceBuffer[i++] = frame.returnAddress;
        if (frame.returnAddress == 0
            || frame.previous == NULL
            || jdy_copyMemory((void *)frame.previous, &frame, sizeof(frame)) != KERN_SUCCESS) {
            break;
        }
    }

    return i;
}

如上。

二、编码实现对一个地址进行符号化解析

后续iOS中线程Call Stack的捕获和解析（二）。