C++中全局对象的初始化
全局对象初始化
查看如下C++代码：
#include <cstdio>
class A {
public:
int val;
A() {
printf("construct ok \n");
}
};
A g_al;
int main()
{
return 0;
}
这段代码的运行结果是对象A的构造函数被调用，串口输出"construct ok"。根据全局对象的初始化规则，这个调用是发生在main函数之前的，而且可以是多个函数依次调用。那么，这种调用时如何实现的？
通过gcc生成arm64汇编，汇编代码如下:
.arch armv8-a
.file "main.cpp"
.text
.section .rodata
.align 3
.LC0:
.string "construct ok "
.section .text._ZN1AC2Ev,"axG",@progbits,_ZN1AC5Ev,comdat
.align 2
.weak _ZN1AC2Ev
.type _ZN1AC2Ev, %function
_ZN1AC2Ev:
.LFB1:
.cfi_startproc
stp x29, x30, [sp, -32]!
.cfi_def_cfa_offset 32
.cfi_offset 29, -32
.cfi_offset 30, -24
add x29, sp, 0
.cfi_def_cfa_register 29
str x0, [x29, 24]
adrp x0, .LC0
add x0, x0, :lo12:.LC0
bl puts
nop
ldp x29, x30, [sp], 32
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa 31, 0
ret
.cfi_endproc
.LFE1:
.size _ZN1AC2Ev, .-_ZN1AC2Ev
.weak _ZN1AC1Ev
.set _ZN1AC1Ev,_ZN1AC2Ev
.global g_al
.bss
.align 3
.type g_al, %object
.size g_al, 4
g_al:
.zero 4
.text
.align 2
.global main
.type main, %function
main:
.LFB3:
.cfi_startproc
mov w0, 0
ret
.cfi_endproc
.LFE3:
.size main, .-main
.align 2
.type _Z41__static_initialization_and_destruction_0ii, %function
_Z41__static_initialization_and_destruction_0ii:
.LFB4:
.cfi_startproc
stp x29, x30, [sp, -32]!
.cfi_def_cfa_offset 32
.cfi_offset 29, -32
.cfi_offset 30, -24
add x29, sp, 0
.cfi_def_cfa_register 29
str w0, [x29, 28]
str w1, [x29, 24]
ldr w0, [x29, 28]
cmp w0, 1
bne .L6
ldr w1, [x29, 24]
mov w0, 65535
cmp w1, w0
bne .L6
adrp x0, g_al
add x0, x0, :lo12:g_al
bl _ZN1AC1Ev
.L6:
nop
ldp x29, x30, [sp], 32
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa 31, 0
ret
.cfi_endproc
.LFE4:
.size _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
.align 2
.type _GLOBAL__sub_I_g_al, %function
_GLOBAL__sub_I_g_al:
.LFB5:
.cfi_startproc
stp x29, x30, [sp, -16]!
.cfi_def_cfa_offset 16
.cfi_offset 29, -16
.cfi_offset 30, -8
add x29, sp, 0
.cfi_def_cfa_register 29
mov w1, 65535
mov w0, 1
bl _Z41__static_initialization_and_destruction_0ii
ldp x29, x30, [sp], 16
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa 31, 0
ret
.cfi_endproc
.LFE5:
.size _GLOBAL__sub_I_g_al, .-_GLOBAL__sub_I_g_al
.section .init_array,"aw",%init_array
.align 3
.xword _GLOBAL__sub_I_g_al
.ident "GCC: (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04) 7.5.0"
.section .note.GNU-stack,"",@progbits
关键的汇编代码是以下几部分；
构造函数的具体实现如下：
.LC0:
.string "construct ok "
.section .text._ZN1AC2Ev,"axG",@progbits,_ZN1AC5Ev,comdat
.align 2
.weak .
.type _ZN1AC2Ev, %function
_ZN1AC2Ev:
.LFB1:
.cfi_startproc
stp x29, x30, [sp, -32]!
.cfi_def_cfa_offset 32
.cfi_offset 29, -32
.cfi_offset 30, -24
add x29, sp, 0
.cfi_def_cfa_register 29
str x0, [x29, 24]
adrp x0, .LC0
add x0, x0, :lo12:.LC0
bl puts
nop
ldp x29, x30, [sp], 32
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa 31, 0
ret
.cfi_endproc
梳理汇编中的bl指令，可以得出以下的调用过程：
_GLOBAL__sub_I_g_al -> _Z41__static_initialization_and_destruction_0ii -> _ZN1AC1Ev(_ZN1AC2Ev)
其中_ZN1AC1Ev是一个弱符号，最终解析成_ZN1AC2Ev，即class A的构造函数
.weak _ZN1AC1Ev
.set _ZN1AC1Ev,_ZN1AC2Ev
那现在问题就变成_GLOBAL__sub_I_g_al是谁调用的，查看汇编里的相关信息，如下：
.LFE5:
.size _GLOBAL__sub_I_g_al, .-_GLOBAL__sub_I_g_al
.section .init_array,"aw",%init_array
.align 3
.xword _GLOBAL__sub_I_g_al
.ident "GCC: (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04) 7.5.0"
.section .note.GNU-stack,"",@progbits
可以发现，_GLOBAL__sub_I_g_al存在一个单独的，名为"init_array"的section中。
关于这个section，ARM的官方文档中，描述如下:
The C++ Standard places certain requirements on the construction and destruction of objects with static storage duration.
The ARM C++ compiler uses the .init_array area to achieve this. This is a const data array of self-relative pointers to functions.
The linker collects each .init_array from the various translation units together. It is important that the .init_array is accumulated in the same order.
The library routine __cpp_initialize__aeabi_ is called from the C library startup code, __rt_lib_init, before main. __cpp_initialize__aeabi_ walks through the .init_array calling each function in turn. On exit, __rt_lib_shutdown calls __cxa_finalize.
Usually, there is at most one function for T::T(), mangled name _ZN1TC1Ev, one function for T::~T(), mangled name _ZN1TD1Ev, one sti function, and four bytes of .init_array for each translation unit. The mangled name for the function f() is _Z1fv. There is no way to determine the initialization order between translation units.
Function-local static objects with destructors are also handled using __aeabi_atexit.
.init_array sections must be placed contiguously within the same region for their base and limit symbols to be accessible. If they are not, the linker generates an error.
查看init_array里的信息
通过以下命令，可以解析出具体调用的函数信息：
objdump -D -j .init_array
在我的环境中，输出如下:
a.out: file format elf64-littleaarch64
Disassembly of section .init_array:
0000000000010d78 <__frame_dummy_init_array_entry>:
10d78: 00000770 .word 0x00000770
10d7c: 00000000 .word 0x00000000
10d80: 000007c0 .word 0x000007c0
10d84: 00000000 .word 0x00000000
通过以下命令，将具体的地址，解析成相应的函数符号:
addr2line 0xc1000a68 -e
在我的环境中，输出如下:
/tmp/tmp.KS3wdL1X4n/main.cpp:16
需要注意的是，这部分信息属于调试信息，编译的时候，加上"-g"，编译器才会生成这些额外信息。