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）。