I have a tcp server running on my localhost. My program connects to this tcp server, which responds with data on connection. I'm using signals elsewhere in the program, so I need this to be non-blocking.
Unfortunately, I also need to use clock_gettime
. When I do this, the tcp connection hangs.
The only alternatives that I can think of are to use pipes, switch over to udp, but those two are not desired results.
Relevant code below:
....
std::string tcpsens::getstr(void){
int outsock;
struct sockaddr_in servaddr;
outsock = socket(AF_INET,SOCK_STREAM,0);
bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = ip_addr;
servaddr.sin_port=htons(port);
if (fcntl(outsock, F_SETFL, O_NDELAY | O_NONBLOCK) < 0) {
perror("Can't set socket to non-blocking");
exit(0);
}
connect(outsock, (struct sockaddr *)&servaddr, sizeof(servaddr));
while(1) {
buflen=recv(outsock,buf,255,0);
if (buflen>0){
buflen=-1;}
else if (buflen==0){
break;}
//sleep(1);
//printf("still looping.\n");
}
close(outsock);
return buf;
}
float tcpsens::gettruespeed(){
float speed,val;
buffer = getstr();
sscanf(buffer.c_str(),"%f,%f\n",&speed,&val);
return speed;}
...
#include <stdio.h>
#include "tcpsens.h"
#include <string>
#include <iostream>
#include <sys/time.h>
#include <time.h>
int main(void){
//timing functions
//struct timespec timer;
// initialize the tcpsensor
tcpsens t;
// connect to tcp and get sensor and speed reading
std::cout << "TEST WITHOUT INITIALIZING TIMESPEC:\n\n";
std::cout << "expecting value of ~0.00, got " << t.gettruespeed() << "\n";
std::cout << "expecting value ~0.00, got " << t.getsensval() << "\n";
return 0;
}
compiled with $ g++-4.7 -std=c++11 testtcp1.cpp -lrt
, gives result
$ ./a.out
TEST WITHOUT INITIALIZING TIMESPEC:
expecting value of ~0.00, got 0
expecting value ~0.00, got 0
same as testtcp1.cpp except uncommented the line struct timespec timer;
, and changing the message. gives result
$ ./a.out
TEST WITH INITIALIZING TIMESPEC:
and hangs forever.
It isn't hanging, it's spinning, because you're in non-blocking mode, you're ignoring errors, and you're incurring buflen == -1
and errno == EAGAIN/EWOULDBLOCK
whenever there is no data. Use blocking mode, or at least handle errors properly.