c - 在出列之前返回链表中的第一个元素时出现问题

Question

我正在学习 C 中链表的实现。我的以下实现有问题:

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

typedef struct msg * Message;
struct msg{
    int SN;
    Message next;
};

Message dequeue(Message *head);
void new_msg(Message *head);

int main(){

Message M, head = NULL;

new_msg(&head);
M= dequeue(&head);
if(M != NULL) printf("Message %d has been deleted:", M->SN);

}


Message dequeue(Message *head)
{

    Message m, temp;

    if(*head == NULL){
        return NULL;
    }
    else
    {
        temp = *head;
        m = temp;  // To return the message before removing it from the queue
        head = head->next; // ERROR IS HERE
        free(temp);
    }

    return (m);
}

void new_msg(Message *head){

    Message m, last;
    m = malloc(sizeof(struct msg));

    last = *head;
    srand(0);
    m->SN = rand();

    if (*head == NULL)
    {
        *head = m;
    }
    else
    {
        while (last->next != NULL)
        {
            last = last->next;
        }
        last->next = m;
    }


}

我将扩展我的程序以充当一个队列，它需要在完全出队之前返回队列中的第一个节点(即头)，但我不断收到此错误:

/bin/sh -c 'make -j 8 -e -f  Makefile'
----------Building project:[ hello - Debug ]----------
gcc -c  "/Users/CFC/Library/Application Support/codelite/test/hello/main.c" -g -O0 -Wall  -o ./Debug/main.c.o -I. -I.
/Users/CFC/Library/Application Support/codelite/test/hello/main.c:38:20: error: member reference base type 'Message' (aka 'struct msg *') is not a structure or union
        head = head->next;
               ~~~~^ ~~~~
1 error generated.
make[1]: *** [Debug/main.c.o] Error 1
make: *** [All] Error 2
====1 errors, 0 warnings====

在 head = head->next; 这行 Message dequeue(Message *head) 中。

请问有人能解释一下原因吗？

谢谢。

Answer 1

Message dequeue(Message *head)
{

    Message  temp=NULL;

    if(*head )
    {
        temp = *head;

        *head = temp->next;  // ERROR HERE
    //  ^____________________ you have to dereference the pointer head
        //free(temp); // do not free it 
    }

    return temp;
}



int main()
{
    Message M, head = NULL;

    new_msg(&head);
    M= dequeue(&head);
    if(M) // check if not NULL
    {
        printf("Message %d has been deleted:", M->SN);
       free(M); // now its time to free it
    }

}