setjmp 和 longjmp函数的使用

setjmp和longjmp函数可以实现函数之间的跳跃（goto），下面是一个实例

#include <stdio.h>#include <setjmp.h>static void f1(int, int, int, int);static void f2(void);static jmp_buf jmpbuffer;static intglobval;int main(void){intautoval;register intregival;volatile intvolaval;static intstatval;globval = 1;autoval = 2;regival = 3;volaval = 4;statval = 5;if(setjmp(jmpbuffer) != 0) {printf("after longjmp:\n");printf("globval = %d, autoval = %d, regival = %d,""volaval = %d, statval = %d\n",globval, autoval, regival, volaval, statval);exit(0);}globval = 95;autoval = 96;regival = 97;volaval = 98;statval = 99;f1(autoval, regival, volaval, statval);exit(0);}static void f1(int i, int j, int k, int l){printf("in f1():\n");printf("globval = %d, autoval = %d, regival = %d,""volaval = %d, statval = %d\n",globval, i, j, k, l);f2();}static void f2(){longjmp(jmpbuffer, 1);}

jay@jay-vibox:~/workspace/UNIX/7-6$ cc main.c
main.c: In function ‘main’:
main.c:28:3: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
main.c:38:2: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
jay@jay-vibox:~/workspace/UNIX/7-6$ ./a.out
in f1():
globval = 95, autoval = 96, regival = 97,volaval = 98, statval = 99
after longjmp:
globval = 95, autoval = 96, regival = 97,volaval = 98, statval = 99
jay@jay-vibox:~/workspace/UNIX/7-6$

然后使用优化编译后执行

jay@jay-vibox:~/workspace/UNIX/7-6$ cc -O main.c
main.c: In function ‘main’:
main.c:28:3: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
main.c:38:2: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
jay@jay-vibox:~/workspace/UNIX/7-6$ ./a.out
in f1():
globval = 95, autoval = 96, regival = 97,volaval = 98, statval = 99
after longjmp:
globval = 95, autoval = 2, regival = 3,volaval = 98, statval = 99
jay@jay-vibox:~/workspace/UNIX/7-6$

可以发现2次输出不一样，不进行优化时，所有5个变量都存放在存储器中。而进行了优化后，autoval和regival都存放在寄存器中，volatile变量则仍存放在存储器中。

如果要编写一个使用非局部跳转的可移植程序，则必须使用volatile属性。