Pthread semaphore TCP - pthreads

Question

I wanna create 2 threads in client, each thread will send a message to server and get rebounce or download a file from server, using TCP protocole. This program run very well when I didn't add pthread in it. After I created 2 threads in client, It doesn't communicate with server. Normally, thread will tell server which operation it wants, then server respond, but when one thread send the message, there is no respond from server, and this thread exit immediately, next thread occupy the semaphore but exit without chose the operation.
Code
void semaphore()
{
int nThread = 2;
int nSemaphore = 1;
int nRet = -1;
pthread_t threadIDs[nThread];
nRet = sem_init(&sem, 0, nSemaphore);
if(nRet == -1)
{
perror("Semaphore intialization failed!!!\n");
exit(EXIT_FAILURE);
}
int i;
for (i = 0; i < nThread; ++i)
{
nRet = pthread_create(&threadIDs[i], NULL, thread, NULL);
if(nRet != 0)
{
perror("pthreas_create failed!!!\n");
exit(EXIT_FAILURE);
}
}
for(i = 0; i < nThread; ++i)
{
nRet = pthread_join(threadIDs[i], NULL);
if(nRet != 0)
{
printf("Threan %d join failed!!!\n", i);
exit(EXIT_FAILURE);
}
}
sem_destroy(&sem);
}
enter code here
void *thread(void* p)
{
pthread_t id = pthread_self();
sem_wait(&sem);
pthread_mutex_lock(&mutex);
int operation ;
//向服务器（特定的IP和端口）发起请求
struct sockaddr_in serv_addr;
memset(&serv_addr, 0, sizeof(serv_addr)); //每个字节都用0填充
serv_addr.sin_family = AF_INET; //使用IPv4地址
serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1"); //具体的IP地址
serv_addr.sin_port = htons(1234); //端口
printf("\nThis is client %ld", id);
printf("\nWhich operation do you want:\n");
printf("1:Sending message to server and get rebound\n");
printf("2:Downlown a file from server\n");
scanf("Your chose:%d", &operation);
switch(operation)
{
case 1:
{
sendMessage(serv_addr);
break;
}
case 2:
{
download(serv_addr);
break;
}
default:
break;
}
sem_post(&sem);
pthread_mutex_unlock(&mutex);
}
From terminal
This is client 139671599236864
Which operation do you want:
1:Sending message to server and get rebound
2:Downlown a file from server
Your choise:1
This is client 139671590844160
Which operation do you want:
1:Sending message to server and get rebound
2:Downlown a file from server
Can someone tell me where is the problem?