Malloc在adt列表程序中返回null

Question

I was working on a program with ADT list, queue, etc... Made by various .c and .h files.

I'm showing you a little extract of the program, just to let you understand better where is the problem; this little program, however, is compilable and runnable.

The problem is that a simple malloc in a function (something I've always did!), returns NULL, or causes segmentation fault (Visual Studio behavior is different than CodeBlocks for example). The malloc I'm talking about is the one in the InsertTop function. Here is the short code, thanks for your help !

#include <stdio.h>
#include <stdlib.h>

#define M 35+1

typedef struct student* Student;
struct student
{
    char name[M];
    char surname[M];
    char number[M];
};


typedef struct node* Node;
struct node
{
    Node prev;
    Student student;
    Node next;
};

typedef struct list* List;
struct list
{
    Node first;
    Node last;
};

/* PROTOTYPES */
List Initialize(List list);
List InsertTop(List list, Student student);
Student PromptStudent(Student student);

/* MAIN */
int main()
{
    /* DECLARATIONS */
    List list = NULL;
    Student student = NULL;

    /* LIST OPERATIONS */
    list=Initialize(list);
    while (0==0)
    {
        student=PromptStudent(student);
        list=InsertTop(list, student);
    }

    return EXIT_SUCCESS;
}

/* FUNCTIONS */
/* 1 */
List Initialize(List list)
{
    list = malloc(sizeof(List));

    list->first = list->last = NULL;

    return list;
}

/* 2 */
List InsertTop(List list, Student student)
{
    Node p;

    /* THIS LINE CREATES THE ERROR! */
    p = malloc(sizeof(Node));

    /* New node */
    p->prev = NULL;
    p->next = list->first;
    p->student = student;

    /* Update list with new node */
    if (list->first == NULL)
        list->first = list->last = p;
    else
    {
        list->first->prev = p;
        list->first = p;
    }

    printf(" -> Done!\n");

    return list;
}

/* 3 */
Student PromptStudent(Student student)
{
    student = malloc(sizeof(Student));

    printf("Name: "); scanf("%s", student->name);
    printf("Surname: "); scanf("%s", student->surname);
    printf("Number: "); scanf("%s", student->number);

    return student;
}

Thanks again!

Answer 1

Node is a pointer type. You want to allocate as much memory to a pointer as of the type it is pointing to. This however isn't Node , but struct node or simply what p is pointing to, that is *p .

This

 p = malloc(sizeof(Node));

shall be

 p = malloc(sizeof (struct node));

or even better

 p = malloc(sizeof *p);

---

As a general rule: Do not typedef pointers, this very easily leads to confusion, as here.

---

Some other issues with your code:

• This #define M 35+1 shall be #define M (35+1) .
• Instead of while (0==0) you can do while (1) .
• scanf("%s" ... shall be scanf("%35s" ... to avoid buffer overflows, if the user enters more then 35 char s.

Answer 2

Make below suggested changes: Memory should be allocated for sizeof structure as shown below and not for size of pointer to a structure.This needs to be updated in all the malloc() in your program.

   list = malloc(sizeof(struct list));

   p = malloc(sizeof(struct node));

   student = malloc(sizeof(struct student));