gettimeofday()为什么会出现段异常

gettimeofday()为什么会出现段错误
我两个程序，同样的程序流程，只不过一个是结构体变量，一个是结构体指针，但是为什么结构体指针运行完之后报错，说段错误。

正确的程序是这个：
#gettimeofday_r.c

C/C++ code

#include <sys/time.h>#include <stdio.h>#include <unistd.h>void foo(){        int i = 0;        for (; i <= 10000; i++); }int main(int argc, char *argv[]){        struct timeval start;        struct timeval end;        float timeuse;        gettimeofday(&start, NULL);        printf("start time is %d:%d\n", start.tv_sec, start.tv_usec);        foo();        gettimeofday(&end, NULL);        printf("end time is %d:%d\n", end.tv_sec, end.tv_usec);        timeuse = 1000000 * (end.tv_sec - start.tv_sec) + ( end.tv_usec - start.tv_usec );        printf("timeuse1 is %f \n", timeuse);        timeuse /= 1000000;        printf("timeuse2 is %f \n", timeuse);        printf("use time is %f\n", timeuse);        return 0;}

运行结果是：

C/C++ code

start time is 1331190741:726493end time is 1331190741:726691timeuse1 is 198.000000 timeuse2 is 0.000198 use time is 0.000198

错误的代码是：
#gettimeofday_w.c

C/C++ code

#include <sys/time.h>#include <stdio.h>#include <unistd.h>void foo(){        int i = 0;        for (; i <= 10000; i++); }int main(int argc, char *argv[]){        struct timeval *start;        struct timeval *end;        float timeuse;        gettimeofday(start, NULL);        printf("start time is %d:%d\n", start->tv_sec, start->tv_usec);        foo();        gettimeofday(end, NULL);        printf("end time is %d:%d\n", end->tv_sec, end->tv_usec);        timeuse = 1000000 * (end->tv_sec - start->tv_sec) + ( end->tv_usec - start->tv_usec );        printf("timeuse1 is %f \n", timeuse);        timeuse /= 1000000;        printf("timeuse2 is %f \n", timeuse);        printf("use time is %f\n", timeuse);        return 0;}

运行结果是：

C/C++ code

start time is 1331190767:111354end time is 1331190767:111558timeuse1 is 204.000000 timeuse2 is 0.000204 use time is 0.000204Segmentation fault

错误的代码我用gdb调试，最后的程序到最后一个括号结束了，然后提示：
Cannot find bounds of current function
Program received signal SIGSEGV，segmentation fault

[解决办法]
看一下函数声明就知道了，第一个参数必须是指针。
int gettimeofday(struct timeval *tv, struct timezone *tz);
[解决办法]
你没有给指针分配空间啊

start=(struct timeval *)malloc（struct timeval);
gettimeofday(start, NULL);
[解决办法]
函数需要一个指针，而你给了它一个未初始化的指针，程序没崩溃就算运气好了。
第一种才是标准用法，第二种需要这样：

struct timeval start;
struct timeval *pstart ＝ &start;
gettimeofday(...);

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