問題返回第一個元素時

Question

我學習鏈表中C.執行我有一個問題，我下面的實現：

#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; 
    } 


} 

我會延長我的計劃爲行動隊列需要完全出隊之前返回隊列（即頭）的第一個節點，但我不斷收到此錯誤：

/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==== 

在這條線 head = head->next;

在此功能Message dequeue(Message *head)。

請問，有人可以解釋爲什麼嗎？

謝謝。

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 
    } 

}