Thread: Why does this call to accept hang this way?

I sure thought I was familiar with the Sockets API but I dont understand why the server code doesn't print "got a connection from [ ip addr && port]" string until something is written on the socket from the client. I've used the library for a few things in the past but perhaps there is something I'm missing here.

server.c

Code:

#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <errno.h>


void error(const char *err_str){
	printf("%s: %s\n",err_str, strerror(errno));
	exit(-1);
}

int main(int argc, char **argv){

	if(argc != 2){
		printf("Usage: %s <port>\n", argv[0]);
		exit(-1);
	}

	int listen_fd, accept_fd, nread, port;
	socklen_t length;
	char buffer[32], ipv4[INET_ADDRSTRLEN];
	struct sockaddr_in server, client;

	if((listen_fd = socket(AF_INET, SOCK_STREAM, 0)) <= 0)
		error("socket()");

	bzero(&server, sizeof(server));
	bzero(&client, sizeof(client));


	server.sin_family = AF_INET;
	server.sin_port = htons(atoi(argv[1]));
	server.sin_addr.s_addr = htonl(INADDR_ANY);

	if(bind(listen_fd, (struct sockaddr *)&server, sizeof(server)) == -1)
		error("bind()");

	listen(listen_fd, 10);

	for(;;){
		length = sizeof(client);

		accept_fd = accept(listen_fd,(struct sockaddr *)&client, &length);

		inet_ntop(AF_INET, &client.sin_addr, ipv4, INET_ADDRSTRLEN);
		port = ntohs(client.sin_port);

		printf("*************** Got a connection from %s %u **********************",ipv4,port);

		while(nread = read(accept_fd, buffer, sizeof(buffer))){
			buffer[nread] = 0;
			fputs(buffer,stdout);
		}
		close(accept_fd);
		fputs("**************** Connection closed *********************", stdout);
	}

	return 0;
}

client.c

Code:

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <errno.h>

void error(const char *err_str){
	printf("%s: %s\n", err_str, strerror(errno));
	exit(-1);
}

int main(int argc, char **argv){

	if(argc != 3){
		printf("Usage: %s <ip address> <port number>\n",argv[0]);
		exit(-1);
	}

	int sock_fd, nread, nleft, nwritten;
	char buffer[32];
	struct sockaddr_in server;

	if((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) <= 0)
		error("socket()");

	bzero(&server, sizeof(server));
	server.sin_family = AF_INET;
	server.sin_port = htons(atoi(argv[2]));

	inet_pton(AF_INET, argv[1], &server.sin_addr);

	if(connect(sock_fd, (struct sockaddr *)&server, sizeof(server)) < 0)
		error("connect()");

	fputs("************ Got a connection from remote host *************\n", stdout);


	fgets(buffer, sizeof(buffer), stdin);
	while(nwritten = write(sock_fd, buffer, strlen(buffer))){
		bzero(&buffer, sizeof(buffer));
		fgets(buffer,sizeof(buffer), stdin);
	}

	close(sock_fd);

	return 0;

}

So I added these few printing lines as sort of a debugging measure and when I do, now it seems like my calls to printf and fputs hang. What the heck? how can that be?

Code:

#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <errno.h>


void error(const char *err_str){
	printf("%s: %s\n",err_str, strerror(errno));
	exit(-1);
}

int main(int argc, char **argv){

	if(argc != 2){
		printf("Usage: %s <port>\n", argv[0]);
		exit(-1);
	}

	int listen_fd, accept_fd, nread, port;
	socklen_t length;
	char buffer[32], ipv4[INET_ADDRSTRLEN];
	struct sockaddr_in server, client;

	if((listen_fd = socket(AF_INET, SOCK_STREAM, 0)) <= 0)
		error("socket()");
fputs("socket()\n",stdout);
	bzero(&server, sizeof(server));
	bzero(&client, sizeof(client));


	server.sin_family = AF_INET;
	server.sin_port = htons(atoi(argv[1]));
	server.sin_addr.s_addr = htonl(INADDR_ANY);

	if(bind(listen_fd, (struct sockaddr *)&server, sizeof(server)) == -1)
		error("bind()");
fputs("bind()\n",stdout);
	listen(listen_fd, 10);
fputs("listen()\n",stdout);	
	length = sizeof(client);
	for(;;){


		accept_fd = accept(listen_fd,(struct sockaddr *)&client, &length);
fputs("accept()\n",stdout);

		inet_ntop(AF_INET, &client.sin_addr, ipv4, sizeof(ipv4));
fputs("inet_ntop()\n",stdout);
		port = ntohs(client.sin_port);
fputs("ntohs()\n",stdout);
		printf("*************** Got a connection from %s %u **********************",ipv4,port);
fputs("printf()\n",stdout);
		while(nread = read(accept_fd, buffer, sizeof(buffer))){
			buffer[nread] = 0;
			fputs(buffer,stdout);
		}
		close(accept_fd);
fputs("close()\n",stdout);
		fputs("**************** Connection closed *********************", stdout);
	}

	return 0;
}

You might be encountering output stream buffering. Either call fflush(stdout) after each printf/fputs or switch to using stderr which is unbuffered by default.

I found that re-writing the code solved my problem. Not sure what went wrong here. But those few printf/fputs statements on the server would only occur after my calls to write in the client code. Never had that happen.

Originally Posted by Syscal

I found that re-writing the code solved my problem. Not sure what went wrong here. But those few printf/fputs statements on the server would only occur after my calls to write in the client code. Never had that happen.

If you are just writing diagnostics to a console screen, use puts() ... primative but very fast.