4.7 Register Variables

A register declaration advises the compiler that the variable in question will be heavily used. The idea is that register variables are to be placed in machine registers, which may result in smaller and faster programs. But compilers are free to ignore the advice.

The register declaration looks like

   register int  x;
   register char c;

and so on. The register declaration can only be applied to automatic variables and to the formal parameters of a function. In this later case, it looks like

   f(register unsigned m, register long n)
   {
       register int i;
       ...
   }

In practice, there are restrictions on register variables, reflecting the realities of underlying hardware. Only a few variables in each function may be kept in registers, and only certain types are allowed. Excess register declarations are harmless, however, since the word register is ignored for excess or disallowed declarations. And it is not possible to take the address of a register variable (a topic covered in Chapter 5), regardless of whether the variable is actually placed in a register. The specific restrictions on number and types of register variables vary from machine to machine.

编者或作者: 我有闲    收录日期:
参考或来源:

上一页: Static Variables 返回上级目录: 第四章 函数和程序结构 下一页: Block Structure