Special attribute Mechanism

The _ attribute _ mechanism is a major feature of gnu c. It can be used to set attributes of functions, variables, and data types. Next, we will give a brief introduction to the two attributes that are useful in our work.

This attribute specifies that the function is executed before the main function is executed.

  RunBeforeMain( RunBeforeMain(% main(% 

The output sequence is as follows:

RunbeforeMain

Main

 

2. weak, alias

Weak: weak symbol. if two identical global symbols exist, a redefinition error is thrown. if weak attribute is used, when both weak symbol and non-weak symbol exist, linker uses non-weak symbol. if only weak symbol exists, only weak symbol is used.

Alias: sets the alias of a function, as shown in figure

 example() __attribute__((alias()));

Set the example function as the alias of _ example.

These two attributes can be used together to compile and run successfully even if the non-weak symbol is not declared.

// Strong. c

#include <stdio.h>

// Weak. c

#include <stdio.h> StrongFun()__attribute__((weak, alias( main( argc,  ** 

For separate links,

Gcc-o weak. o

./Weak

Output result:

Weak. c: WeakFun

At the same time,

Gcc-o strong weak. o strong. o

./Strong

Output result

Strong. c: StrongFun