recv function in C can't get value - c

Question

I am new to C language, and when I used c to write a socket program, I encountered a problem.
I want to implement two-way communication between two computers (Linux and Simulink). In Linux, I need to implement a connection-based UDP client and server. And for other needs, I divided the process of creating UDP communication into four parts: creating the sender socket, server binding process, sending data and accepting data.
The code is compiled successfully. During the run, the value can be successfully sent to Simulink, but it is blocked on the 'recv()' function. I used Wireshark to view the data packets of the computer where Simulink is located. The data packets of this computer are normal. In other words, the socket in Linux received the datagram, but my program could not read it.
The code in Linux is as below:
void check_comm_error(int ret);// just for check error
void sendsock_creat(); //creat socket and server ip address
void sendsock_send(void* send_buf); //call send function and send data
void recvsock_creat(); called by sendsock_creat function
void recvsock_recv(); just for receieve data
int client_fd; //定义套接字描述符
double senddata[1] = {10};
double recvdata[1];
struct sockaddr_in local_addr;//定义本地地址 <netinet/in.h>
void sendsock_creat()
{
int check_ret = 0;//定义操作返回值，以传给check_comm_error函数
struct sockaddr_in server_addr;//定义服务器地址 <netinet/in.h>
int ser_addr_len;//定义服务端地址长度
client_fd = socket(AF_INET, SOCK_DGRAM, 0); //创建客户端套接字
check_ret = client_fd;
check_comm_error(check_ret);//判断是否出错
recvsock_creat();//Need to run the bind function before connect function
memset(&server_addr, 0, sizeof(server_addr)); //清空服务器地址结构体
server_addr.sin_family = AF_INET; //定义地址中的地址类型
server_addr.sin_addr.s_addr = inet_addr(SERVER_ADDR);//给出服务端的ip地址
server_addr.sin_port = htons(SERVER_PORT); //
check_ret=connect(client_fd, (struct sockaddr*)&server_addr,
sizeof(server_addr)); //连接服务端
check_comm_error(check_ret);//判断是否出错
}
void sendsock_send(void* send_buf)
{
//void* send_buf 为待发送的数据缓冲区
int check_ret = 0;
check_ret = send(client_fd, send_buf, sizeof(send_buf), 0);
check_comm_error(check_ret);
}
void check_comm_error(int ret)
{
if(ret < 0)
{
perror("create socket fail!\n");
exit(1);
}
}
void recvsock_creat()
{
int check_ret = 0;//定义操作返回值，以传给check_comm_error函数
int recv_flag; //接受标志
int ser_addr_len;//定义本地地址长度
memset(&local_addr, 0, sizeof(struct sockaddr_in));
local_addr.sin_family=AF_INET;//bind to a local port
local_addr.sin_addr.s_addr=htonl(INADDR_ANY);
local_addr.sin_port=htons(LOCAL_PORT);
check_ret = bind(client_fd,(struct sockaddr *)&local_addr, sizeof(struct
sockaddr_in));
check_comm_error(check_ret);
recv(client_fd, recvdata, sizeof(recvdata), 0); //This function can get
the right value
printf("recv from simulink %f \n", recvdata[0]);
}
void recvsock_recv()
{
int recv_flag;
int addr_len;
recv_flag = recv(client_fd, recvdata, sizeof(recvdata),0);
}
int main()
{
sendsock_creat();
sendsock_send((double *)senddata);
recvsock_recv();
printf("recv from matlab %f .\n",recvdata[0]);
return 0;
}
Because the program has been compiled, the head file part is definitely no problem, So I ignore the include that part.