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