一、獲取任意一個線程的Call Stack

如果要獲取當前線程的調用棧，可以直接使用現有API：[NSThread callStackSymbols]。

但是並沒有相關API支持獲取任意線程的調用棧，所以只能自己編碼實現。

1. 基礎結構

一個線程的調用棧是什麼樣的呢？

我的理解是應該包含當前線程的執行地址，並且從這個地址可以一級一級回溯到線程的入口地址，這樣就反向構成了一條鏈：線程入口執行某個方法，然後逐級嵌套調用到當前現場。

（圖片來源於維基百科）

如圖所示，每一級的方法調用，都對應了一張活動記錄，也稱為活動幀。也就是說，調用棧是由一張張幀結構組成的，可以稱之為棧幀。

我們可以看到，一張棧幀結構中包含著Return Address，也就是當前活動記錄執行結束後要返回的地址（展開）。<喎?/kf/ware/vc/" target="_blank" class="keylink">vcD4NCjxwPsTHw7SjrNTaztLDx7vxyKG1vdW71qG686Osvs2/ydLUzai5/be1u9i12Na3wLS9+NDQu9jL3cHLoaM8L3A+DQo8aDIgaWQ9"2-指令指針和基址指針">2. 指令指針和基址指針

我們明確了兩個目標：(1)當前執行的指令，(2)當前棧幀結構。

以x86為例，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，我們可以通過指令指針來獲取當前指令地址，以及通過棧基址指針獲取當前棧幀地址。

那麼問題來了，我們怎麼獲取到相關寄存器呢？

3. 線程執行狀態

考慮到一個線程被掛起時，後續繼續執行需要恢復現場，所以在掛起時相關現場需要被保存起來，比如當前執行到哪條指令了。

那麼就要有相關的結構體來為線程保存運行時的狀態，經過一番查閱，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

所以我們可以通過這個API搭配相關參數來獲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);
}

這裡引入了一個結構體叫_STRUCT_MCONTEXT。

4. 不同平台的寄存器

_STRUCT_MCONTEXT在不同平台上的結構不同：

x86_64，如iPhone 6模擬器：

_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模擬器：

_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 */
};

可以對照《iOS ABI Function Call Guide》，其中在ARM64相關章節中描述到：

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，通過了解以上不同平台的寄存器結構，我們可以編寫出比較通用的回溯功能。

5. 算法實現

/**
 * 關於棧幀的布局可以參考：
 * 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) {
        /* 由於lr保存的也是返回地址，所以在lr有效時，應該會產生重復的地址項 */
        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、編碼實現對一個地址進行符號化解析

後續iOS中線程Call Stack的捕獲和解析（二）。