problem in select() in a Multiprotocol client

Hi,

I am new to socket programming and I am trying to write code for a client that sends and receives messages on both TCP and UDP. I am using select() system call for it. My problem is, select() works fine for TCP but it doesn't go inside the if loop for UDP and hence no messages are being received on UDP. The sendto() seems to be working fine as it returns a positive integer value.
I am not able to figure out the reason. Here is some of my code. Any help is appreciated.

fd_set master; // master file descriptor list

FD_ZERO(&master);

while(1)
{

FD_SET(child_fd,&master); //child_fd is socket descriptor for TCP
FD_SET(sockudpfd,&master); //sockudpfd is socket descriptor for UDP
struct timeval t;
t.tv_sec=15;

if(select(255, &master, (fd_set *)0, (fd_set *)0,(struct timeval *)0) < 0)
perror("select error \n");

if(FD_ISSET(child_fd, &master))
{

recv_msg=recv(child_fd, message,500,MSG_WAITALL);

// some computation goes here...where "sendmsg" is calculated

cliaddr.sin_port=htons(destination_port);

udp_send=sendto(sockudpfd, sendmsg, strlen(sendmsg), 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));
}

else if(FD_ISSET(sockudpfd, &master)) // this loop doesnt work.
{

int m=recvfrom(sockudpfd, udpmesg, 500, 0, (struct sockaddr *) &cliaddr, &cli_len);
}
}

Are you sure that sockudpfd is actually getting set by your FD_SET()? Trying running FD_ISSET immediately after to check. You may want to see what exact status select is at when it exits instead of just checking for '< 0'.

Your code is written so the sockudpfd socket only gets serviced if the child_fd socket doesn't have anything. Don't you want to change that loop to:

Code:

if(FD_ISSET(child_fd, &master)) { // ... } if(FD_ISSET(sockudpfd, &master)) { // ... }