Receiving garbage from GSocket - c

Question

I am using the below functions to create and open a GSocket and then listen for UDP messages to display in a GTK_ENTRY field. The open_listen_socket function is called, and then after that I have a function on a timeout calling the get_incoming_messages function every second or so.
The problem is that when I launch the program, I only get a bunch of garbage characters showing up in the GTK_ENTRY field, and my console is repeating an error message "Pango-WARNING: Invalid UTF-8 string passed to pango_layout_set_text()."
I've been able to send UDP messages no problem, but am going crazy trying to figure out receiving them, even though it feels like it shouldn't be so difficult!
Thank you for any help, here are my functions:
static void open_listen_socket()
{
GInetAddress *localAddress;
GSocketAddress *localSocketAddress;
localAddress = g_inet_address_new_from_string("127.0.0.1");
guint16 listenPort = atoi(gtk_entry_get_text (GTK_ENTRY (listenPortField)));
localSocketAddress = g_inet_socket_address_new(localAddress, listenPort);
listenSocket = g_socket_new(G_SOCKET_FAMILY_IPV4, G_SOCKET_TYPE_DATAGRAM, 17, NULL);
g_socket_bind (listenSocket, localSocketAddress, FALSE, NULL);
g_socket_set_blocking(listenSocket, FALSE);
g_socket_listen (listenSocket, NULL);
}
static int get_incoming_message()
{
gchar buffer[1024];
int input_length;
input_length = g_socket_receive(listenSocket, (gchar *)buffer, 1024, NULL, NULL);
gtk_entry_set_text (GTK_ENTRY (current_status_message_box), (const gchar *)buffer);
return 1;
}
Also, if it helps at all, I had to add the "return 1;" at the end of get_incoming_message, because without it that process seemed to get stuck inside that function even though the rest of the application continued working ok.
/EDIT AND UPDATE BELOW*/
Ok, using the advice below I have things working, but am hitting a new problem. It appears that my call to g_socket_receive returns true every single iteration, printing an empty line if no message has been sent. That means that I do see my sent messages come through, but they disappear from the GTK_ENTRY a split second later as they are replaced by an empty line on the next iteration.
static gboolean get_incoming_message()
{
gchar buffer[10] = {0};
GError *err = NULL;
if (g_socket_receive(listenSocket, (gchar *)buffer, 10, NULL, &err) > 0 );
{
printf("\n%s", buffer);
gtk_entry_set_text (GTK_ENTRY (current_status_message_box), (const gchar *)buffer);
return TRUE;
}
return FALSE;
}
I don't know what checks to use to tell the difference between an actual message and these empty lines! It's almost as if this socket is receiving an endless stream of empty data anytime a legitimate message is not being sent. Does that make sense?
/Figured it out!/
I had to add an ELSE statement to give it something else to do when there was no data, to prevent it from trying to write the empty line. Not sure I understand that, so I'd love an explanation if anyone has one, but I'm back in business! Thanks again to everyone!

Answer 1

There are few pointers for your consideration:
Whenever function provides facility to use GError please use it. It is very helpful to find out error message
Check return value of the functions to make sure they have done what they were intended to (Already pointed out by Joachim)
Use enum when available. In this case GSocketProtocol. Don't use 17 use G_SOCKET_PROTOCOL_UDP instead, as your application will break if the enum value changes
UDP sockets don't really "listen" so g_socket_listen call is redundant
Please find below sample code based on what you have posted at your disposal:
/* gcc -Wall -Wextra `pkg-config --cflags --libs glib-2.0 gio-2.0` gsock.c -o gsock */
#include <glib.h>
#include <gio/gio.h>
#include <stdio.h>
#define MAX_RECV 10
#define TIMEOUT_INTERVAL 1
static GSocket *listenSocket;
static GMainLoop *loop;
static gboolean
recv_msg(gpointer data)
{
(void)data;
static unsigned int count = 0;
gchar buffer[1024] ={0};
GError *err = NULL;
if( g_socket_receive(listenSocket, (gchar *)buffer, 1024, NULL, &err) > 0 )
{
printf("buff = %s\n", buffer);
}
else
{
printf(" Nothing posted in last %d sec (Error: %s)\n", TIMEOUT_INTERVAL, err->message);
g_error_free(err);
}
if(count++ < MAX_RECV)
return TRUE;
/* This is fugly!! :\ */
g_main_loop_quit(loop);
return FALSE;
}
static int
open_listen_socket(void)
{
GInetAddress *localAddress;
GSocketAddress *localSocketAddress;
GError *err = NULL; /* This is mandatory */
localAddress = g_inet_address_new_from_string("127.0.0.1");
guint16 listenPort = 31337; /* Can you recongnize this port? xD*/
localSocketAddress = g_inet_socket_address_new(localAddress, listenPort);
listenSocket = g_socket_new(G_SOCKET_FAMILY_IPV4, G_SOCKET_TYPE_DATAGRAM, G_SOCKET_PROTOCOL_UDP, &err);
if( NULL == listenSocket)
{
printf("\n Failed to create socket! Error: %s\n", err->message);
g_error_free(err);
return -1;
}
if( FALSE == g_socket_bind (listenSocket, localSocketAddress, FALSE, &err))
{
printf("\n Failed to bind! Error: %s\n", err->message);
g_error_free(err);
return -1;
}
g_socket_set_blocking(listenSocket, FALSE);
/* UDP socket don't "listen". Uncomment below to see the error message*/
/*
if(FALSE == g_socket_listen (listenSocket, &err))
{
printf("\n Failed to listen! Error: %s\n", err->message);
g_error_free(err);
}
*/
return 0;
}
int main(void)
{
g_type_init();
if(open_listen_socket() < 0)
{
printf("\n Socket creation went wrong!!\n");
}
else
{
loop = g_main_loop_new (NULL, TRUE);
g_timeout_add_seconds(TIMEOUT_INTERVAL, recv_msg, NULL);
g_main_loop_run(loop);
}
return 0;
}
Sample run:
Terminal #1:
$ ./gsock
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
buff = hello
buff = world
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Nothing posted in last 1 sec (Error: Error receiving data: Resource temporarily unavailable)
Terminal #2:
$ nc 127.0.0.1 31337 -u
hello
world
^C
Hope this helps!

Answer 2

Check the line where you receive data again... Which socket do you read from? The listening socket! That socket is only used to accept new connections (via the g_socket_accept) function.
g_socket_accept returns a new socket (that you also have to make non-blocking), that can be used for reading and sending.
Also, you should really check the return value from g_socket_receive! In this case I can almost bet it's returning -1 for an error, but you "print" the buffer anyway which can contain anything.