iOS中线程Call Stack的捕获和解析（一）

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.
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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.SYNOPSISkern_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
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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);}
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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;};
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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;};
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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 */};
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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 */};
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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;}
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如上。