Issues with select () function in Multicast communication - c

Question

I will elaborate as much as possible without being too lengthy about the issue I would like help with, if possible:
I'm writing a program that communicates with two sockets, I listen on the multicast socket, while I delegate to the other socket "Unicast" important information stemming from the communicated data coming from the first socket "Multicast".
There are two issues that I think they are related:
1- I run the program in one comuputer "Linux" communicating to another "Linux", and the program performs as expected. But when i take it to another computer "running both my program and the other programs, all in one host" with similar Multicast configuration, I get the following error:
Select : Interrupted system call
This is a perror message, but i am not sure if it is due to error in select() or my multicast configuration.
2- As a result of the first issue, I am unable to delegate to the "Unicast" client socket, but the Unicast works because there is some periodic checking between "Unicast client" and my program running all the time.
My code is as a follow:
struct ConfigStruct
{
struct sockaddr_in Hinfo1, Hinfo2;
struct sockaddr_in Rinfo;
int sock1, sock2;
};
int main()
{
ConfigStruct StructArg;
int fd1, fd2;
int POS(1);
/****************** Network parameters declaration *************************/
// Declaration for socket addresses
struct sockaddr_in Host_info1, Host_info2;
struct sockaddr_in Remote_info;
struct in_addr localInterface;
struct ip_mreq Group;
memset((char *)&Host_info1,0,sizeof(Host_info1));
memset((char *)&Host_info2,0,sizeof(Host_info2));
memset((char *)&Remote_info,0,sizeof(Remote_info));
memset((char *)&Group,0,sizeof(Group));
//**** Reads configuration file****************
cout<<"Reading configuration file..........."<<endl;
std::string input1 ="192.***.**.**";
std::string input2 = "8888";
std::string input3 ="192.***.**.**";
std::string input4 = "8889";
const char* addr_input = input1.data();
const char* port_input = input2.data();
const char* addr_input2 = input3.data();
const char* port_input2 = input4.data();
Remote_info.sin_addr.s_addr=inet_addr(addr_input);
Remote_info.sin_port = htons((uint16_t)stoi(port_input,nullptr,0));
Remote_info.sin_family=AF_INET;
Host_info1.sin_addr.s_addr=inet_addr(addr_input2);//htonl(INADDR_ANY);
Host_info1.sin_port = htons((uint16_t)stoi(port_input2,nullptr,0));
Host_info1.sin_family=AF_INET;
//***** First socket *******
fd1= socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
if (fd1 == -1)
{
std::cout<<"A problem occured"<<endl;
cease("socket", wd) ;
}
if (setsockopt(fd1,SOL_SOCKET,SO_REUSEADDR, &POS, sizeof(POS)) == -1)
{
perror(" Error in setsockopt");
exit(1);
}
// **** I'M NOT SURE IF THIS NECESSARY **************
int opts;
opts = fcntl(fd1,F_GETFL);
if (opts < 0)
{
perror("fcntl(F_GETFL)");
exit(EXIT_FAILURE);
}
opts = (opts | O_NONBLOCK);
if (fcntl(fd1,F_SETFL,opts) < 0)
{
perror("fcntl(F_SETFL)");
exit(EXIT_FAILURE);
}
//*****************************************************
if (bind(fd1,(struct sockaddr *)&Host_info1,sizeof(Host_info1)) < 0)
{
cease("Bind",wd);
}
else
{
cout<<" Socket ID number "<<fd1<<endl;
cout<<" Bound socket..."<<endl;
}
//********** The multicast network setup ***********************
std::string input5 ="230.*.**.**";
std::string input6 = "192.***.***"; // The same host IP address as above
std::string input7 = "1500" ; // The port number to listen to for Multicast message
const char* Group_Multi_Addr = input5.data();
const char* Group_Interface_Addr = input6.data();
const char* Host_port_input = input7.data();
Group.imr_multiaddr.s_addr = inet_addr(Group_Multi_Addr);
Group.imr_interface.s_addr = inet_addr(Group_Interface_Addr);
Host_info2.sin_family = AF_INET;
Host_info2.sin_addr.s_addr = INADDR_ANY;
Host_info2.sin_port = htons((uint16_t)stoi(Host_port_input,nullptr,0));
//***** The second socket *******
fd2 = socket(AF_INET, SOCK_DGRAM, 0);
if(fd2 < 0)
{
perror("Opening datagram socket error");
exit(1);
}
else
printf("Opening the datagram socket...OK.\n");
int reuse = 1;
if(setsockopt(fd2, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof(reuse)) < 0)
{
close(fd2);
cease("Setting SO_REUSEADDR error", wd);
}
else
printf("Setting SO_REUSEADDR...OK.\n");
if(bind(fd2, (struct sockaddr*)&Host_info2, sizeof(Host_info2)))
{
close(fd2);
cease("Binding datagram socket error",wd);
}
else
printf("Binding datagram socket...OK.\n");
if(setsockopt(fd2, IPPROTO_IP, IP_ADD_MEMBERSHIP,(char *)&Group,sizeof(Group)) < 0)
{
perror("Adding multicast group error");
close(fd2);
exit(1);
}
else
printf("Adding multicast group...OK.\n");
StructArg.Hinfo1= Host_info1;
StructArg.Hinfo2= Host_info2 ;
StructArg.Rinfo= Remote_info ;
StructArg.sock1=fd1;
StructArg.sock2=fd2;
fd_set readfds ,rd_fds;
struct timeval tv;
// clear the set ahead of time
FD_ZERO(&readfds);
// add our descriptors to the set
FD_SET(StructArg.sock1, &readfds);
FD_SET(StructArg.sock2, &readfds);
nls = StructArg.sock2 + 1;
tv.tv_sec = 0;
tv.tv_usec = 50;
char Recv_buffer[125];
char TX_buffer[125];
memset((char *)&Recv_buffer,'0',sizeof(Recv_buffer));
memset((char *)&TX_buffer,'0',sizeof(TX_buffer));
int lenremote(sizeof(StructArg.Rinfo));
ssize_t rs, rs2;
uint8_t MsgSize;
uint8_t MsgID;
do
{
rd_fds=readfds;
if (select(nls, &rd_fds, NULL, NULL, &tv) < 0)
{
perror("select"); // error occurred in select()
}
else
{
// one or both of the descriptors have data
if (FD_ISSET(StructArg.sock1, &rd_fds))
{
rs = recvfrom(StructArg.sock1,....,...,0,...,...) ;
if ( rs > 0 )
{
Do bunch of routines
}
}
if (FD_ISSET(StructArg.sock2, &rd_fds))
{
rs2 = recv(StructArg.sock2,&Recv_buffer,sizeof(Recv_buffer),0);
if ( rs2 > 0 )
{
send some data to StructArg.sock1
}
}
// I do some work here , i send somethings to Sock 1 (Is this appropriate ??)
}
while(1);
return 0;
}
So most importantly, why do I get Select : System interrupt call in one computer but not another?

Answer 1

I am not sure if it is due to error in Select() or my multicast configuration.
Neither. It is due to a signal being caught during the system call. Just loop around this condition, eg:
do
{
// ...
rc = select(nls, &rd_fds, NULL, NULL, &tv);
} while (rc == -1 && errno == EINTR);
if (rc == -1)
{
perror("select");
}
else
{
// ...
}
most importantly, why do i get , Select : System interrupt call in one computer but not another
Because you got a signal on one computer and not the other. And it isn't important.

Answer 2

how about the timeouts, because the man page, says that timeout
becomes undefined when an error arise, should i put the tv.sec and
tv.usec inside the loop.
man 2 select:
On Linux, select() modifies timeout to reflect the amount of time not
slept; most other implementations do not do this. (POSIX.1-2001
permits either behavior.)
On Linux thus, the code shown in the question probably doesn't act as intended, because the timeout is set before the loop only once and zeroed if it expires - from then on, it remains zero. Here, it would be appropriate to set the desired timeout inside the main loop, but outside the proposed loop … while (rc == -1 && errno == EINTR).
On systems that don't update timeout accordingly, there is in the presence of interruptions no such easy way to maintain the correct timeout.