Reading socket: EAGAIN: Resource temporarily unavailable - c++

Question

I have created a socket in C++ and I needed it to have certain connection timeout. So that's what is happening:
Create socket
Make it NON_BLOCKING
Call connect
It returns -1 and errno EINPROGRESS as expected
Call select
Returns >0, so connection has been made
Make the socket BLOCKING again
Code for this part is the following:
bool mastControl::prepareSocket(char * address, int port, int * sockfd) {
struct sockaddr_in serv_addr;
struct timeval timeout = {0,100000};
struct timeval connTimeout;
struct hostent * server = NULL;
fd_set socketSet;
socklen_t lon;
int sockOpt = 0;
long socketFlags = 0;
int buffersize = 8;
int res = 0;
int connectReturn = 0;
const int WAIT_TO_RECONN = 15;
server = gethostbyname(address);
*sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (*sockfd < 0) {
qDebug()<<"Impossible to open socket: "<<strerror(errno);
return false;
}
if (server == NULL) {
qDebug()<<"No such host: "<<strerror(h_errno);
return false;
}
// Initializating server direction struct:
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr,
(char *)&serv_addr.sin_addr.s_addr,
server->h_length);
serv_addr.sin_port = htons(port);
// Making socked non-blocking in order to set a timeout value for connection:
if((socketFlags = fcntl(*sockfd, F_GETFL, NULL)) < 0){
qDebug()<<"Impossible to retrieve sockets descriptor flags "<<strerror(errno);
return false;
}
socketFlags |= O_NONBLOCK;
if(fcntl(*sockfd, F_SETFL, socketFlags) <0){
qDebug()<<"Impossible to update sockets descriptor flags: "<<strerror(errno);
return false;
}
connectReturn = connect(*sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr));
if(connectReturn < 0){
if(errno == EINPROGRESS){
do{
// Establishing a 15 seconds timeout:
connTimeout.tv_sec = 15;
connTimeout.tv_usec = 0;
FD_ZERO(&socketSet); // Initialising set of sockets as an empty set
FD_SET(*sockfd, &socketSet); // Adding socket to set
connectReturn = select(*sockfd+1, NULL, &socketSet, NULL, &connTimeout);
if(connectReturn<0 && errno!=EINTR){ // Error in select
qDebug()<<"Connection error in select function: "<<strerror(errno);
return false;
}
else if(connectReturn>0){ // Socket selected for writing
lon = sizeof(int);
if(getsockopt(*sockfd, SOL_SOCKET, SO_ERROR, (void*)(&sockOpt), &lon) <0){
qDebug()<<"Unnable to get socket options: "<<strerror(errno);
return false;
}
// Checking the value returned:
if(sockOpt){
qDebug()<<"Error in delayed connection: "<<strerror(errno);
return false;
}
break;
}
else{ // Timeout
qDebug()<<"Connection timeout exceeded: "<<strerror(errno);
return false;
}
} while (1);
}
else{
qDebug()<<"Connection error: "<<strerror(errno);
sleep(WAIT_TO_RECONN); // Wait 15 seconds
return false;
}
}
//Connected
// Must set the socket as blocking again:
if((socketFlags = fcntl(*sockfd, F_GETFL, NULL)) < 0){
qDebug()<<"Impossible to retrieve sockets descriptor flags "<<strerror(errno);
return false;
}
socketFlags &= (~O_NONBLOCK);
if(fcntl(*sockfd, F_SETFL, socketFlags) <0){
qDebug()<<"Impossible to update sockets descriptor flags "<<strerror(errno);
return false;
}
if (setsockopt (*sockfd, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout,
sizeof(timeout)) < 0) {
qDebug()<< "ERR - setsockopt failed";
return false;
}
if (setsockopt (*sockfd, SOL_SOCKET, SO_SNDTIMEO, (char *)&timeout,
sizeof(timeout)) < 0) {
qDebug()<< "ERR - setsockopt failed";
return false;
}
if ((res = setsockopt (*sockfd, SOL_SOCKET, SO_SNDBUF, &buffersize,
sizeof(buffersize))) == -1) {
qDebug()<< "ERR - setsockopt failed (SO_SNDBUF) = " << res;
return false;
}
//Socket Ready
return true;
}
That works ok. But then I have a loop where I'm calling a function which checks if there is a new packet received to read:
bool mastControl::findPacket(int sockfd, st_messageMastToPc * messageReceived, bool * connected) {
int n = 0;
bool messageFound = false;
char * buffer = (char *) messageReceived;
unsigned int pos = 0;
while ( ((n = read(sockfd, &(buffer[pos]), 1)) > 0) and not messageFound) {
//qDebug() << "read output " << n;
if (n == 1) {
pos++;
if ( (pos == 1) && (buffer[0] == 2)) {
// Some stuff...
} else if ( (pos == 2) && (buffer[1] == 2) ) {
// Some stuff...
} else if (pos >= uiMessageMastToPcSize) {
messageFound = true;
//Complete message received
} else if (pos < 2) {
// Reseting pos
pos = 0;
}
}
}
if (n < 0){
qDebug()<< "Disconnected. Reason #" << errno << ": " << strerror(errno);
*connected = false;
}
return messageFound;
}
Read function is giving EAGAIN as errno, which means "Resource temporarily unavailable". Then I'm supposing I am disconnected, and since boolean connected is now false, in the loop I will be trying to reconnect creating the socket again, ans so on.
So, in the first place I don't know why I'm receiving this error.
In the second place, I don't know if I am disconnected at all, or it's me the one who is disconnecting when creating the new socket after this error.
Any help?

Answer 1

EAGAIN does not mean you're disconnected, it just means "there's nothing to read now; try again later".
You could either unset O_NONBLOCK with fcntl(2) (making read wait until there's something available), or just wait on the socket with something like select(2) before calling read.
EDIT: Now that you've added more code, I can see that you're setting SO_RCVTIMEO for the socket. This can cause a blocking read to return EAGAIN (so if you don't want that to happen, simply leave SO_RCVTIMEO alone).