goto语句可以用于同一个函数内异常处理，不幸的是，goto是本地的，它只能跳到所在函数内部的标号上。为了解决这个限制，C函数库提供了setjmp()和longjmp()函数，它们分别承担非局部标号和goto作用。头文件<setjmp.h>申明了这些函数及同时所需的jmp_buf数据类型。
1.setjmp(jbuf)设置“jump”点，用正确的程序上下文填充jmp_buf对象jbuf。这个上下文包括程序存放位置、栈和框架指针，其它重要的寄存器和内存数据。当初始化完jump的上下文，setjmp()返回0值。
2. 以后调用longjmp(jbuf,r)的效果就是一个非局部的goto或“长跳转”到由jbuf描述的上下文处（也就是到那原来设置jbuf的setjmp()处）。当作为长跳转的目标而被调用时，setjmp()返回r （或1，如果r设为0的话）。（setjmp()不能在这种情况时返回0，否则就和设置jump点的返回值冲突了）
- #include "apue.h"
- #include <setjmp.h>
- #define TOK_ADD 5
- jmp_buf jmpbuffer;
- int
- main(void)
- {
- char line[MAXLINE];
- if (setjmp(jmpbuffer) != 0)
- printf("error");
- while (fgets(line, MAXLINE, stdin) != NULL)
- do_line(line);
- exit(0);
- }
- ...
- void
- cmd_add(void)
- {
- int token;
- token = get_token();
- if (token < 0) /* an error has occurred */
- longjmp(jmpbuffer, 1);
- /* rest of processing for this command */
- }
setjump和longjump返回时对auto变量、register变量、volatile变量和静态变量的恢复情况示例。cc -O选项表示编译优化。
longjump返回时一般情况下不会恢复到setjump时的值，此种情况属于未定义。
如果你不想让anto变量的值回滚，需要加上volatile属性。
- #include "apue.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);
- }
- /*
- * Change variables after setjmp, but before longjmp.
- */
- globval = 95; autoval = 96; regival = 97; volaval = 98;
- statval = 99;
- f1(autoval, regival, volaval, statval); /* never returns */
- 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(void)
- {
- longjmp(jmpbuffer, 1);
- }
-
- $ cc testjmp.c compile without any optimization
- $ ./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
- $ cc -O testjmp.c compile with full optimization
- $ ./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
