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 int	globval;

int main(void)
{
	int		autoval;
	register int	regival;
	volatile int	volaval;
	static int	statval;

	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);
}

先使用不带优化来编译程序

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。