內存分配碼錯誤

Question

我正在開發一個客戶端 - 服務器程序，當我試圖用這個結構來實現用戶鏈接列表：

typedef struct user { 
    char username[50]; 
    int user_pid; 
    struct user *next; 
} user_list; 

我試圖找出什麼是錯的代碼，因爲編譯器不會給我任何錯誤，但是當我嘗試用戶打印用戶列表時，它根本不顯示任何內容。

ADDUSER功能：

AddUser(user_list *head, req req) 
{ 
    if(head == NULL) 
    { 
     head = malloc(sizeof(user_list)); 

     if (head == NULL) 
      fprintf(stdout,"[SERVER] Error memory allocation "); 

     strcpy(head->username, req.str); 
     head->user_pid = req.client_pid; 
     head->next = NULL; 
    } 
    else 
    { 
     user_list *current = head; 

     while (current->next != NULL) 
      current = current->next; 

     current->next = malloc(sizeof(user_list)); 
     strcpy(current->next->username, req.str); 
     current->next->user_pid = req.client_pid; 
     current->next->next = NULL; 
    } 
    num_users++; 
} 

主要功能（精簡版）

int Main() 
{ 
struct request req; 
struct answer ans; 
user_list *head = NULL; 

do{ 

    read(fifo_1, &req, sizeof(req)); // Read client request 

    if(strcasecmp(req.str, "adduser") == 0) 
    {    
     AddUser(head, req); 
     strcpy(ans.str, "User added with success! You're logged!"); 
    } 

    if(strcasecmp(req.str, "users") == 0) // Print on the screen the users list 
    { 
     user_list *current = head; 

     while (current != NULL) 
     { 
      fprintf(stdout, "%s\n", current->username); 
      fprintf(stdout, "%d\n", current->user_pid); 
      current = current->next; 
     } 

    } 
}while(strcmp(req.str,"exit") != 0); 
} 

Answer 1

放在一起別人已經在評論中指出：

1. 變化main。取而代之的

   int Main() 
   

   使用

   int main() 
   

2. 的head的價值，不main當你在AddUser改變而改變。這是一個解決方案。從AddUser返回head。

   user_list* AddUser(user_list *head, req req) 
   { 
       if(head == NULL) 
       { 
        head = malloc(sizeof(user_list)); 
   
        if (head == NULL) 
         fprintf(stdout,"[SERVER] Error memory allocation "); 
   
        strcpy(head->username, req.str); 
        head->user_pid = req.client_pid; 
        head->next = NULL; 
       } 
       else 
       { 
        user_list *current = head; 
   
        while (current->next != NULL) 
         current = current->next; 
   
        current->next = malloc(sizeof(user_list)); 
        strcpy(current->next->username, req.str); 
        current->next->user_pid = req.client_pid; 
        current->next->next = NULL; 
       } 
       num_users++; 
       return head; 
   } 
   

3. 捕獲的AddUser在main返回值。而不是僅僅

   AddUser(head, req); 
   

   使用

   head = AddUser(head, req);