UDP multicast receives 2^24 bytes of data which should be impossible - c

Question

have a stand alone PC running VS6 on WinXP - yes ancient technology.
I am porting a C code app from Linux.
Stuck on multicast problem
#include "stdafx.h"
#include <stdlib.h>
#include <stdio.h> /* for printf(), fprintf() */
#include <conio.h>
#include <winsock.h> /* for socket(),... */
#include <stdlib.h> /* for exit() */
#define MAXRECVSTRING 255 /* Longest string to receive */
int main(int argc, char* argv[])
{
char msg[100];
char loopchar = 0;
int iOptVal = 0;
char iOptVal2 = 1;
int iLenOptVal = sizeof(int);
int result = -1;
int retval = -1;
int set_option_on = 1;
int sock; /* Socket */
struct sockaddr_in multicastAddr; /* Multicast Address */
char *multicastIP; /* IP Multicast Address */
unsigned short multicastPort; /* Port */
char recvString[MAXRECVSTRING+1]; /* Buffer for received string */
unsigned int recvStringLen; /* Length of received string */
struct ip_mreq multicastRequest; /* Multicast address join structure */
WSADATA wsaData; /* Structure for WinSock setup communication */
if (argc != 3) /* Test for correct number of arguments */
{
fprintf(stderr,"Usage: %s <Multicast IP> <Multicast Port>\n", argv[0]);
exit(1);
}
multicastIP = argv[1]; /* First arg: Multicast IP address (dotted quad) */
//WW Silversilsied //WW SilversilsiedmulticastIP = inet_addr("224.000.010.101"); /* First arg: Multicast IP address (dotted quad) */
//multicastIP = inet_addr("224.000.010.101"); /* First arg: Multicast IP address (dotted quad) */
multicastPort = atoi(argv[2]);/* Second arg: Multicast port */
//multicastPort = 6600);/* Second arg: Multicast port */
if (WSAStartup(MAKEWORD(2, 0), &wsaData) != 0) /* Load Winsock 2.0 DLL */
{
fprintf(stderr, "WSAStartup() failed");
exit(1);
}
/* Create a best-effort datagram socket using UDP */
if ((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
{
printf("\nsocket() failed - error = %d\n", sock);
}
/* Construct bind structure */
memset(&multicastAddr, 0, sizeof(multicastAddr)); /* Zero out structure */
multicastAddr.sin_family = AF_INET; /* Internet address family */
multicastAddr.sin_addr.s_addr = htonl(INADDR_ANY); /* Any incoming interface */
multicastAddr.sin_port = htons(multicastPort); /* Multicast port */
/* Bind to the multicast port */
retval = bind(sock, (struct sockaddr *) &multicastAddr, sizeof(multicastAddr));
if (retval < 0)
{
printf("\nbind() failed - error = %d\n", retval);
}
#if 0
/* Specify the multicast group */
multicastRequest.imr_multiaddr.s_addr = inet_addr(multicastIP);
/* Accept multicast from any interface */
multicastRequest.imr_interface.s_addr = htonl(INADDR_ANY);
/* Join the multicast address */
//if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&multicastRequest, sizeof(multicastRequest)) < 0)
result = setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (char*)&set_option_on, sizeof(set_option_on));
if (result < 0)
{
printf("\n setsockopt() failed");
perror(" setsockopt ");
}
/* Receive a single datagram from the server */
while(1)
{
if ((recvStringLen = recvfrom(sock, recvString, MAXRECVSTRING, 0, NULL, 0)) < 0)
{
printf("\nrecvfrom() failed");
}
Sleep(1000);
}
#else
strcpy(msg,"default test message");
struct sockaddr_in address1;
int len;
int bytes_sent = -1;
memset(&address1, 0, sizeof(address1));
address1.sin_family = AF_INET;
address1.sin_port = multicastPort;
address1.sin_addr.s_addr = inet_addr(multicastIP);
//msg = "abcdefghijklmnopqrstuvwxyz";
//socklen_t len;
//size = strlen(msg);
if ((recvStringLen = recvfrom(sock, recvString, 1024, 0, (struct sockaddr*)&address1, &len)) < 0)
{
printf("\nrecvfrom() failed ");
perror(" recvfrom ");
}else{
recvString[recvStringLen] = '\0';
printf("Received: %d bytes %s\n", recvStringLen,recvString); /* Print the received string */
perror(" received from ");
}
bytes_sent = sendto(sock,
msg,
sizeof(msg),
0,
(struct sockaddr*)&address1,
sizeof(address1));
printf("bytes sent = %d \n", bytes_sent);
printf("size of msg = %d \n ", sizeof(msg));
perror( "sendto ");
#endif
getch();
closesocket(sock);
WSACleanup(); /* Cleanup Winsock */
exit(0);
return 0;
}
When I single step through, I get a successful socket creation and valid socket descriptor.
I get a successful bind, at which point the port shows up as udp when I do a cmd line netstat -p UDP -a
setsockopt also completes without error.
When I step through the recvfrom it receives 2^24 bytes all of which are the same -52
The machine is stand alone, not on a network.

Answer 1

recvString is 256 bytes long, but your recvFrom() is asking for up to 1024 bytes.