c - strncpy 只复制部分字符串

Question

假设我有这样的结构

typedef struct _student {
    int studentID;
    char name[30];
    char class[10];
    char department[10];
} Student;

下面的函数创建了 Student 类型的新变量:

Student *new_student(int id, char *name, char *class, char *dept) {
    Student *s = (Student *)malloc(sizeof(Student *));

    s->studentID = id;
    strncpy(s->name, name, sizeof(s->name) - 1);
    s->name[sizeof(s->name) - 1] = '\0';
    strncpy(s->class, class, sizeof(s->class) - 1);
    s->class[sizeof(s->class) - 1] = '\0';
    strncpy(s->department, dept, sizeof(s->department) - 1);
    s->department[sizeof(s->department) - 1] = '\0';
    return s;
}

void display_student(Student *s) {
    printf("Student: %d | %s | %s | %s\n", s->studentID, s->name, s->class, s->department);
}

为了测试我的代码，我只是在 main() 中写了一些简单的东西

int main() {

    Student *s1 = new_student(20111201, "Lurther King Anders Something", "ICT-56", "SoICT");
    Student *s2 = new_student(20111202, "Harry Potter", "ICT-56", "SoICT");
    Student *s3 = new_student(20111203, "Hermione Granger", "ICT-56", "SoICT");
    Student *s4 = new_student(20111204, "Ron Weasley", "ICT-56", "SoICT");
    display_student(s1);
    display_student(s2);
    display_student(s3);
    display_student(s4);

    return 0;
}

然而，结果对我来说是出乎意料和奇怪的:

有人可以为我解释为什么奇怪的结果吗!我认为我做事的方式是正确的，我安全地使用了 strncpy，但我不理解输出。

Answer 1

这个

 ... malloc(sizeof(Student *));

分配

sizeof(Student *)

字节。通常为 4 或 8，因为 Student * 是指针类型。

你可能想要

     ... malloc(sizeof(Student));

甚至更好:

Student * s = malloc(sizeof(*s));

甚至没有无用的括号:

Student * s = malloc(sizeof *s); /* sizeof is an operator, not a function. */

阅读 malloc(sizeof *s) 为:“分配与 s 所指向的一样多的字节。”