C language implementation of raii

Although there are many disputes , But I still think C++ Medium RAII Idioms are a good thing , Also write C Code is the only thing I miss C++ characteristic . Here are some C Language implementation RAII Methods ：

gcc

GCC Top available cleanup Extended implementation

#define RAII_VARIABLE(vartype,varname,initval,dtor) \
    void _dtor_ ## varname (vartype * v) { dtor(*v); } \
    vartype varname __attribute__((cleanup(_dtor_ ## varname))) = (initval)
 
void example_usage() {
  RAII_VARIABLE(FILE*, logfile, fopen("logfile.txt", "w+"), fclose);
  fputs("hello logfile!", logfile);
}

Windows

Windows You can use SEH Of __try/__finally, Words Windows In fact, there are many things that are quite convenient .

standard C

For better performance and portability , You can also use setjmp/longjmp Mechanism , A simple package is as follows , Use TRY/FINALLY that will do

#define TRY do{ jmp_buf ex_buf__; switch( setjmp(ex_buf__) ){ case 0: while(1){
#define CATCH(x) break; case x:
#define FINALLY break; } default:
#define ETRY } }while(0)
#define THROW(x) longjmp(ex_buf__, x)

The code above is switch Used in while, It uses Duff Device technology （ As shown below ）. It's all because C Not allowed in language goto To switch Any of the statements case It's about .

Duff Device

C-FAQ The form given in is ：

int n = (count + 7) / 8;   /* count > 0 assumed */
switch (count % 8)
{
case 0:    do { *to = *from++;
case 7:     *to = *from++;
case 6:     *to = *from++;
case 5:     *to = *from++;
case 4:     *to = *from++;
case 3:     *to = *from++;
case 2:     *to = *from++;
case 1:     *to = *from++;
          } while (--n > 0);
}

This is in C It's legal in language ,switch The only function of a statement is ” Fall ” Start at the right place to start . In the above example, such a comparison can be performed 8 Secondary copy operations , At the same time, don't care count Can not be 8 Division and other problems . This operation is a special loop expansion mechanism (loop-unrolling mechanism).(switch Statement case It's actually a label , This is easy to understand ).

This technology is used in particularly low-level code （ Such as driving ） Or specific needs （ Such as C Language implementation RAII） You can use .

 

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author ：Virbox Technology blogs
source ：Virbox Technology blogs
original text ：http://blog.virbox.com/2019/06/25/c-%E8%AF%AD%E8%A8%80%E5%AE%9E%E7%8E%B0-raii/
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