What are strong and weak symbols in embedded systems?

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source ： Online material

__attribute__   It's a compiler instruction , It's actually GNU C A mechanism of , The essence is a compiler instruction , You can provide some attributes when declaring , Works at compile time , To do diversified error checking and advanced optimization .

Used in C、C++、Objective-C Modify variables in 、 function 、 Parameters 、 Method 、 Class etc. .

The rational use of  __attribute__  What are the benefits ？

Give the compiler context , Help compiler optimize , Reasonable use can achieve significant optimization effect . The compiler will respond by  __attribute__  Generate some compilation warnings , Make the code more standard . Provide necessary comments to code readers , Help them understand the intent of the code .

All in all , __attribute__  It serves as a context for the compiler , If the wrong use  __attribute__  Instructions , Because the compiler is given the wrong context , The resulting errors are often difficult to detect .

Strong sign and weak sign

You cannot define the same variable or function under the same scope , quite a lot C Language learners take it for granted that .

This is actually biased ,GNU C To the standard C The language has been extended , stay GCC in , For symbols ( At compile time , Variables and functions are abstracted into symbols ) for , There are strong signs and weak signs . Yes , Whether this feature is supported depends on C Language standards .

about C/C++ for , Compiler default functions and initialized global variables are strong symbols , Uninitialized global variables are weakly signed , When the programmer does not display the specified , The compiler has some default behavior for the definition of strong and weak symbols , At the same time, developers can also specify symbols , Use "attribute((weak))" To declare a symbol as weak .

Define the same variable , When both are not strong symbols ,gcc No error will be reported during compilation , But follow certain rules to choose ：

• When both are strong signs , Report errors ：redefinition of 'xxx'

• When the two are strong and weak , Pick the value of the strong sign

• When both are weak , Select the symbol that takes up more space , This is actually a good understanding , The compiler doesn't know what the programmer intended , Choosing symbols that take up a lot of space will at least not cause problems such as overflow 、 Cross the border and other serious consequences .

• In the case of the default symbol type , Strong and weak symbols can coexist , Something like this ：

int x;
int x = 1;

Compile without error , At compile time x The value of will be 1.

But use  __attribute__((weak))  Convert strong symbols to weak symbols , But cannot coexist with a strong symbol , Something like this ：

int __attribute__((weak)) x = 0;
int x = 1;

The compiler will report a duplicate definition error .

Strong quotes and weak quotes

Apart from the difference between strong and weak symbols ,GNUC Another feature is strong and weak references , What we know is , The compiler is only responsible for compiling the source file into the target file in the compilation phase ( Binary file ), Then the linker links all binary files .

In separate compilation , When the compiler checks that the currently used function or variable is only declared but not defined in this module , The compiler uses this symbol directly , Transfer work to linker , The linker is responsible for finding the entity address of these functions or variables based on this information .

Because during the execution of the program , The program must know exactly the address of each function and global variable . If the entity of the symbol is not found , Will report undefined reference error , Such references between symbols are called strong references .

The compiler defaults to strong references for all variables and functions , At the same time, programmers can use  __attribute__((weakref))  To declare a function , Note that this is a declaration, not a definition , Since it's a citation , Then it is to use the entities defined in other modules , For functions , We can use this expression ：

__attribute__((weakref)) void func(void);

And then call it in the function. func(), If func() Not defined , be func The value of is 0, If func Defined , Then call the corresponding func, stay 《 Self cultivation of programmers 》 This book introduces , It's written like this ：

__attribute__((weakref)) void func(void);
void main(void)
{
    if(func) {func();}
}

But in modern compilation systems , This is wrong , Although the compilation passed ( There are warning messages ), But it's not true :

warning: ‘weakref’ attribute should be accompanied with an ‘alias’ attribute [-Wattributes]

The warning shows ：weakref It needs to be accompanied by an alias to work properly

strong / Weak sign and strong / The role of weak references

This weak sign 、 The extension mechanism of weak references is very useful in the implementation of libraries . We can use weak notation and weak reference mechanism in the library , So for a weakly signed function , Users can customize the function that extends the function to cover the weak symbolic function .

At the same time, we can define some extended function functions as weak references , When users need to use extended functions , Define it , Link to the program , If the user does not define , Then the link will not report an error , This makes it easy to cut and combine the functions of the library .

Be careful ：C There is no mention of strong in the standard 、 Weak sign . This is just GCC This implementation defines the features , stay MS C There is no such concept in the compiler .

Reference blog ：

https://www.cnblogs.com/downey-blog/p/10470674.html https://blog.csdn.net/astrotycoon/article/details/8008629

link ：https://xiaoyege.blog.csdn.net/article/details/93674095

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