c - 理解指向结构的指针是指访问它时c中的指针成员

Question

我有一个下面的程序并且编译成功但是在运行时，程序在 eclipse 中崩溃了

struct Student
{
   unsigned int  *ptr;  //Stores address of integer Variable
}*s1;

int main()
{
    unsigned int roll = 20;
    s1->ptr   = &roll;

    printf("\nRoll Number of Student : %d",*(s1->ptr));

    return(0);
}

如何使用指向结构的指针打印 roll 的值

Answer 1

创建一个Student结构，分配并使用

typedef struct Student
{
   unsigned int  *ptr;  //Stores address of integer Variable
} Student;

int main()
{
    Student *s1;
    unsigned int roll = 20;
    s1 = malloc(sizeof(Student));
    if (s1 == NULL) {
        return -1;
    }
    s1->ptr   = &roll;

    printf("\nRoll Number of Student : %d",*(s1->ptr));
    free(s1);

    return(0);
}