IO map

[Damon lbj]

Submit my codes,compile is ok,when i run it ,after input name, segment fault!
I found the mistake cause by this statement -scanf("%s",s[count].name).but i can't fix it,so i need some help.
any suggestions i will appericate it.
output:
input your name:hello
Segmentation fault (core dumped)

#include<stdio.h>
#include<fcntl.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/mman.h>
#include<string.h>
#include<sys/stat.h>
#include<sys/types.h>
struct stu{
    char name[20];
    int age;
    float score;
};
main()
{
    int fd; 
    struct stu *s; 
    struct stat st; 
    int size;
    int count;
    fd =open("newstu.dat",O_RDWR|O_CREAT|O_EXCL,0666);
        if(fd==-1)
        {   
            fd=("newstu.dat",O_RDWR);
            if(fd==-1)
                printf("error:%m\n"),exit(-1);
        }   
    fstat(fd,&st);
    size=(int)st.st_size;
    count=size/sizeof(struct stu);
    s=mmap(0,size+sizeof(struct stu),
            PROT_READ|PROT_WRITE,
            MAP_SHARED,fd,0);

    printf("input your name:");
    scanf("%s",s[count].name);
    printf("input your age:");
    scanf("%d",&(s[count].age));
    printf("input your scores:");
    scanf("%f",&(s[count].score));
    munmap(s,sizeof(struct stu)+size);
    close(fd);
}

[kooth]

The first thing I would do is to check the value of s on line 31. It could be returning NULL.