Why does a broken network connection cause an EOF on stdin? - c

Question

I have a simple server that in a separate thread waits for a network connection and then sends information periodically to the client. The main thread accepts commands via stdin. What I don't understand is why stdin receives EOF when the client is terminated.
For the sample code below, the client can be as simple as 'nc 127.0.0.1 1234' in the command line. When the client is interrupted with 'kill' or Ctl-C the server exits due to EOF on stdin. I'd certainly appreciate an explanation for this behavior and a workaround that keeps the server running.
static void *WaitForConnections(void *p) {
int sockfd, newsockfd;
struct sockaddr_in server = { sizeof(server), AF_INET, htons(1234), INADDR_ANY};
if ( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket call failed");
exit(1);
}
if ( bind(sockfd, (struct sockaddr *)&server, sizeof(struct sockaddr_in)) == -1){
perror("bind call failed");
exit(1);
}
if ( listen(sockfd, 0) == -1 ) {
perror("listen call failed");
exit(1);
}
for (;;) {
if ( (newsockfd = accept(sockfd, NULL, NULL)) != -1) { // new connection
for ( ;;) {
char c = 'd';
if (send(newsockfd, &c, sizeof(c), 0) != sizeof(c)) {
break;
}
sleep(1);
}
close(newsockfd);
}
else {
break;
}
}
close(sockfd);
return NULL;
}
int main(int argc, const char * argv[]) {
pthread_attr_t attr;
pthread_t p;
void * status;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
if (0 != pthread_create( &p, &attr, WaitForConnections, NULL )) {
fprintf(stderr, "thread creation failed\n");
exit(1);
}
while (getchar() != EOF) {}
pthread_join(p, &status);
return 0;
}
It shouldn't matter, but this is under MacOS X 10.10.1, Xcode 6.1.1 and Apple LLVM 6.0.

Answer 1

Your server doesn't exit because of EOF on stdin, it exits because it tries to send data on a broken TCP connection, which causes the SIGPIPE signal to be delivered - and the default action for SIGPIPE is to terminate the process.
You should ignore SIGPIPE with
signal(SIGPIPE,SIG_IGN);
And that will cause send()/write() calls to instead return -1 and set errno to EPIPE, which your code can handle.