c - C中Struct的恼人问题

Question

struct Users{
        int id;
        char msg[];
};

int nUsers;

struct Users users[10];

void connectUser(struct Users user){
     if(nUsers<10){
        for(int i=0;i<10;i++){
                if(users[i]==NULL){
                        users[i]=user;
                        printf("user %d connected!\n", user.id);
                        nUsers++;
                }
        }
    }else
        printf("number of users reached!\n");
}

那是我的代码，当我尝试编译时，出现错误:

[s3450124@csitprdap01 ~]$ gcc -std=c99 socketserver.c -o socketserver
socketserver.c: In function ‘connectUser’:
socketserver.c:24: error: invalid operands to binary == (have ‘struct Users’ and ‘void *’)
socketserver.c:21: note: The ABI of passing struct with a flexible array member has changed in GCC 4.4
socketserver.c: In function ‘disconnectUser’:
socketserver.c:37: error: incompatible types when assigning to type ‘struct Users’ from type ‘void *’

每次我尝试编译时，都会出现这些错误。你们能帮帮我吗？

Answer 1

当 i = 0 时，users[i] 的类型是 Struct Users。它不是指针，因此不能具有 NULL 值。这就是您的编译器所提示的。您正在检查该值是否为 NULL 而不是。您只在比较指针时检查 NULL。

// you already check here that you do not exceed bounds of array - 10
for(int i=0;i<10;i++){
    users[i]=user;
    etc