why it cause a Segmentation fault?

Hello,all.
Following are codes.It 's function is to set RTC.

Code:

---

#include <time.h>
#include <stdio.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/stat.h>

/* Copied from linux/rtc.h to eliminate the kernel dependency */
struct linux_rtc_time {
        int tm_sec;
        int tm_min;
        int tm_hour;
        int tm_mday;
        int tm_mon;
        int tm_year;
        int tm_wday;
        int tm_yday;
        int tm_isdst;
};

#define RTC_SET_TIME  _IOW('p', 0x0a, struct linux_rtc_time) /* Set RTC time    */

static void write_rtc(time_t t)
{
        int rtc;
        struct tm tm;

        if (( rtc = open ( "/dev/rtc", O_WRONLY )) < 0 ) {
                if (( rtc = open ( "/dev/misc/rtc", O_WRONLY )) < 0 )
                        fprintf(stdout,"Could not access RTC\n" );
        }

        tm = *(localtime ( &t ));
        tm. tm_isdst = 0;

        if ( ioctl ( rtc, RTC_SET_TIME, &tm ) < 0 )
                fprintf(stdout,"Could not set the RTC time.\n" );

        close ( rtc );
}

int sync_time(const unsigned char *cmd)
{       
        time_t tma;
        struct tm * tmb;
        char tmp[4];
       
//        time(&result);
//        my_time = gmtime(&result);
//        my_time->tm_year += 1900;
//        my_time->tm_mon += 1;

        tmp[0] = cmd[2] >> 4;                                // get the year
        tmp[1] = cmd[2] & 0x0f;
        tmp[2] = cmd[3] >> 4;
        tmp[3] = cmd[3] &0x0f;
        tmb->tm_year = (tmp[0] * 1000 + tmp[1] * 100 + tmp[2] * 10 + tmp[3]) - 1900;

        tmp[0] = cmd[4] >>4;                                //get month
        tmp[1] = cmd[4] & 0x0f;
        tmb->tm_mon = (tmp[0] * 10 + tmp[1]) - 1;

        tmp[0] = cmd[5] >> 4;                                //day
        tmp[1] = cmd[5] & 0x0f;
        tmb->tm_mday = tmp[0] * 10 + tmp[1];

        tmp[0] = cmd[6] >> 4;                                //hour
        tmp[1] = cmd[6] & 0x0f;
        tmb->tm_hour = tmp[0] * 10 + tmp[1];

        tmp[0] = cmd[7] >> 4;                                //minute
        tmp[1] = cmd[7] & 0x0f;
        tmb->tm_min =  tmp[0] * 10 + tmp[1];

        tmp[0] = cmd[8] >> 4;                                //second
        tmp[1] = cmd[8] & 0x0f;
        tmb->tm_sec =  tmp[0] * 10 + tmp[1];
        printf("%02d:%02d:%02d",tmb->tm_hour,tmb->tm_min,tmb->tm_sec);
        tma = mktime(tmb);                //conver tm to time_t
                /* write_rtc(tma); */                // set RTC .

        return 0;
       
}

int main(void)
{
        unsigned char cmd[9]={0x00,0x00,0x20,0x06,0x09,0x26,0x11,0x23,0x11};
        int re;
        re=sync_time(cmd);
        return 0;
}

---

>why it cause a Segmentation fault?
> struct tm * tmb;
Because you haven't allocated any memory for tmb.
You could change this to a non-pointer:

Code:

---

        struct tm tmb;

---

Then you would change all references to tmb pointer access:
> tmb->tm_year = (tmp[0] * 1000 + tmp[1] * 100 + tmp[2] * 10 + tmp[3]) - 1900;
to structure access:

Code:

---

        tmb.tm_year = (tmp[0] * 1000 + tmp[1] * 100 + tmp[2] * 10 + tmp[3]) - 1900;

---

> tma = mktime(tmb); //conver tm to time_t
And this would become:

Code:

---

        tma = mktime(&tmb);                //conver tm to time_t

---