全局构造函数调用不在.init_array部分 [英] Global constructor call not in .init_array section
问题描述
我想在嵌入式目标（ARM Cortex-M3）上添加全局构造函数支持。
假设我有以下代码：
I'm trying to add global constructor support on an embedded target (ARM Cortex-M3). Lets say I've the following code:
class foobar
{
int i;
public:
foobar()
{
i = 100;
}
void inc()
{
i++;
}
};
foobar foo;
int main()
{
foo.inc();
for (;;);
}
我这样编译：
arm-none-eabi-g++ -O0 -gdwarf-2 -mcpu=cortex-m3 -mthumb -c foo.cpp -o foo.o
当我使用objdump查看.init_array节时，它显示.init_section的大小为零。
When I look at the .init_array section with objdump it shows the .init_section has a zero size.
我得到一个名为 _Z41__static_initialization_and_destruction_0ii 的符号。
当我反汇编对象文件时，我看到全局构造是在static_initialization_and_destruction符号中完成的。
I do get an symbol named _Z41__static_initialization_and_destruction_0ii.
When I disassemble the object file I see that the global construction is done in the static_initialization_and_destruction symbol.
为什么在.init_section？
Why isn't a pointer added to this symbol in the .init_section?
推荐答案
我知道这个问题几乎已经两年了， GCC自己的裸机C ++初始化的机制，所以我想我会在这里分享细节。结果是在网络上有很多过时的或混乱的信息。例如，由于其任意段支持支持下面描述的方法，所以 collect2 包装器似乎不用于ARM ELF目标。
I know it has been almost two years since this question was asked, but I just had to figure out the mechanics of bare-metal C++ initialization with GCC myself, so I thought I'd share the details here. There turns out to be a lot of out-of-date or confusing information on the web. For example, the oft-mentioned collect2 wrapper does not appear to be used for ARM ELF targets, since its arbitrary section support enables the approach described below.
首先，当我使用Sourcery CodeBench Lite 2012.09-63编译上面的代码时，我会看到正确的 .init_array 区段大小4：
First, when I compile the code above with the given command line using Sourcery CodeBench Lite 2012.09-63, I do see the correct .init_array section size of 4:
$ arm-none-eabi-objdump -h foo.o
foo.o: file format elf32-littlearm
Sections:
Idx Name Size VMA LMA File off Algn
...
13 .init_array 00000004 00000000 00000000 0000010c 2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
...
当我看看节内容，它只包含0：
When I look at the section contents, it just contains 0:
$ arm-none-eabi-objdump -j .init_array -s foo.o
Contents of section .init_array:
0000 00000000 ....
但是，还有一个重定位部分可以正确设置为 _GLOBAL__sub_I_foo ：
However, there is also a relocation section that sets it correctly to _GLOBAL__sub_I_foo:
$ arm-none-eabi-objdump -x foo.o
...
RELOCATION RECORDS FOR [.init_array]:
OFFSET TYPE VALUE
00000000 R_ARM_TARGET1 _GLOBAL__sub_I_foo
一般来说， .init_array 指向所有 _GLOBAL__sub_I_XXX 初始化器存根，每个都调用自己的 _Z41__static_initialization_and_destruction_0ii 副本（是的，它是乘法
In general, .init_array points to all of your _GLOBAL__sub_I_XXX initializer stubs, each of which calls its own copy of _Z41__static_initialization_and_destruction_0ii (yes, it is multiply-defined), which calls the constructor with the appropriate arguments.
因为我在我使用 -nostdlib build，我不能使用CodeSourcery的 __ libc_init_array 为我执行 .init_array ，所以我需要调用static initializers myself：
Because I'm using -nostdlib in my build, I can't use CodeSourcery's __libc_init_array to execute the .init_array for me, so I need to call the static initializers myself:
extern "C"
{
extern void (**__init_array_start)();
extern void (**__init_array_end)();
inline void static_init()
{
for (void (**p)() = __init_array_start; p < __init_array_end; ++p)
(*p)();
}
}
__ init_array_start 和 __ init_array_end 由链接描述文件定义：
__init_array_start and __init_array_end are defined by the linker script:
. = ALIGN(4);
.init_array :
{
__init_array_start = .;
KEEP (*(.init_array*))
__init_array_end = .;
}
这种方法似乎适用于CodeSourcery交叉编译器和本机ARM GCC ，例如在Ubuntu 12.10 for ARM。支持两个编译器是使用 -nostdlib 而不依赖于CodeSourcery CS3裸机支持的一个原因。
This approach seems to work with both the CodeSourcery cross-compiler and native ARM GCC, e.g. in Ubuntu 12.10 for ARM. Supporting both compilers is one reason for using -nostdlib and not relying on the CodeSourcery CS3 bare-metal support.
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