Multiple clients conflict - c

Question

I have a problem with multiple clients sending a notification, every interval of time, with some kind of information to a server. The problem occurs when more than one client sends the notification at the same moment, only one will be sent.
Specifically, they both print "Sending nofication", but after that only one prints "Sent ...", the server receives only one aswell.
This is my client thread function
void* Notification_Handler (void* args) {
int nc = *((int*)args); // id of the client
struct sockaddr_un addr;
memset(&addr, '0', sizeof(addr));
addr.sun_family= AF_UNIX;
strncpy(addr.sun_path, SOCKNAME_MANAGER, strlen(SOCKNAME_MANAGER)+1);
int fd_socket;
SYSCALL(fd_socket, socket(AF_UNIX, SOCK_STREAM, 0), "socket");
int notused;
SYSCALL(notused, connect(fd_socket, (struct sockaddr*)&addr, sizeof(addr)), "connect");
int res;
struct timespec *ts_notification = (struct timespec *)malloc(sizeof(struct timespec));
ts_notification->tv_sec = (4000)/1000;
ts_notification->tv_nsec = ((4000)%1000)*1000000;
while (c_status[nc-1]==1) { // while this client is open (1: open, 0:closed)
do {
res = nanosleep(ts_notification, ts_notification); // notification every interval
} while (res && errno==EINTR);
if (c_status[nc-1]==0) break;
sendToManager(nc, fd_socket);
}
free(ts_notification);
close(fd_socket);
pthread_exit((void*)EXIT_SUCCESS);
}
and this is the SendToManager function for sending the data
void sendToManager (int nc, int fd_socket) {
printf("Handler[%d]: sending notification\n", nc);
char message[64];
snprintf(message, sizeof(message), "Cashier:%d", nc);
int len= strlen(message);
int n;
if ((n=write_h(fd_socket, &len, sizeof(int)))==-1) {
fprintf(stderr, "write len failed\n");
pthread_exit((void*)EXIT_FAILURE);
}
if ((n=write_h(fd_socket, message, len))==-1) {
fprintf(stderr, "write info failed\n");
pthread_exit((void*)EXIT_FAILURE);
}
printf("Sent: %s\n", message);
}
SYSCALL and write_h are just some macro/function from an header file
#define SYSCALL(r, sc, str) \
if ((r=sc) == -1) { \
perror(str); \
exit(errno); \
}
static inline int write_h (long fd, void* buffer, size_t size) {
int res;
size_t left = size;
char *bufptr = (char*)buffer;
while(left>0) {
if ((res=write((int)fd ,bufptr,left))==-1) {
if (errno == EINTR) continue;
return -1;
}
if (res==0) return 0;
left -= res;
bufptr += res;
}
return 1;
}
The server spawns a thread for every connect request he receives. Is my function not thread-safe or some info/variables are shared?