关于__attribute__中section部分的一些了解

转自：https://blog.csdn.net/sadshen/article/details/9419267

诸如我辈菜鸟，对编译器了解比较少，面对代码中出现的陌生字眼真是茫然不知所措。今天查阅了一些资料，总算是有了一点了解，现在将些许理解记录在案。

__attribute__这个关键词是GNU编译器中的编译属性，ARM编译器也支持这个用法。__attribute__主要用于改变所声明或定义的函数或 数据的特性，它有很多子项，用于改变作用对象的特性。比如对函数，noline将禁止进行内联扩展、noreturn表示没有返回值、pure表明函数除 返回值外，不会通过其它（如全局变量、指针）对函数外部产生任何影响。当然，__attribute__肯定有很多的用法，今天就用到了section部分，所以就只针对这个做一些记录。

提到section，就得说RO RI ZI了，在ARM编译器编译之后，代码被划分为不同的段，RO Section(ReadOnly)中存放代码段和常量，RW Section(ReadWrite)中存放可读写静态变量和全局变量，ZI Section(ZeroInit)是存放在RW段中初始化为0的变量。


于是本文的大体意思就清晰了，__attribute__((section("section_name")))，其作用是将作用的函数或数据放入指定名为"section_name"对应的段中。


MDK给出的帮助文档如下，他将__attribute__的用法归类到编译器特性里，以变量和函数的两种用法做区分。

1.编译时为变量指定段

__attribute__((section("name")))
RealView Compilation Tools for µVision Compiler Reference Guide Version 4.0

Home > Compiler-specific Features > Variable attributes > __attribute__((section("name")))

4.5.6. __attribute__((section("name")))
Normally, the ARM compiler places the objects it generates in sections like data and bss. However, you might require additional data sections or you might want a variable to appear in a special section, for example, to map to special hardware. The section attribute specifies that a variable must be placed in a particular data section. If you use the section attribute, read-only variables are placed in RO data sections, read-write variables are placed in RW data sections unless you use the zero_init attribute. In this case, the variable is placed in a ZI section.

Note
This variable attribute is a GNU compiler extension supported by the ARM compiler.

Example
/* in RO section */
const int descriptor[3] __attribute__ ((section ("descr"))) = { 1,2,3 };
/* in RW section */
long long rw[10] __attribute__ ((section ("RW")));
/* in ZI section *
long long altstack[10] __attribute__ ((section ("STACK"), zero_init));/

2.编译时为函数指定段

__attribute__((section("name")))
RealView Compilation Tools for µVision Compiler Reference Guide Version 4.0

Home > Compiler-specific Features > Function attributes > __attribute__((section("name")))

4.3.13. __attribute__((section("name")))
The section function attribute enables you to place code in different sections of the image.

Note
This function attribute is a GNU compiler extension that is supported by the ARM compiler.

Example
In the following example, Function_Attributes_section_0 is placed into the RO section new_section rather than .text.

void Function_Attributes_section_0 (void)
    __attribute__ ((section ("new_section")));
void Function_Attributes_section_0 (void)
{
    static int aStatic =0;
    aStatic++;
}
In the following example, section function attribute overrides the #pragma arm section setting.

#pragma arm section code="foo"
  int f2()
  {
      return 1;
  }                                  // into the 'foo' area
  __attribute__ ((section ("bar"))) int f3()
  {
      return 1;
  }                                  // into the 'bar' area
  int f4()
  {
      return 1;
  }                                  // into the 'foo' area
#pragma arm section

记录完毕，希望能够给遇到同样问题的朋友些许帮助。