c - 内存分配代码错误

Question

我正在开发一个客户端-服务器程序，我何时尝试使用此结构实现用户链表：

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

我正在尝试找出代码有什么问题，因为编译器没有给我任何错误，但是当我尝试让用户打印用户列表时，它根本不显示任何内容。

添加用户功能：

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（短版）

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

将其他人在评论中已经指出的内容放在一起：

1. 改变main。代替

   int Main()
   

   利用

   int main()
   

2. , 的值在您更改时head不会更改。这是一个解决方案。从 AddUser返回。mainAddUserhead

   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. AddUser捕获in的返回值main。而不仅仅是

   AddUser(head, req);
   

   利用

   head = AddUser(head, req);