How to set socket timeout in C when making multiple connections

Question

I m writing a simple program that makes multiple connections to different servers for status check  All these connections are constructed on-demand  up to 10 connections can be created simultaneously  I don t like the idea of one-thread-per-socket  so I made all these client sockets Non-Blocking  and throw them into a select   pool   It worked great  until my client complained that the waiting time is too long before they can get the error report when target servers stopped responding   I ve checked several topics in the forum  Some had suggested that one can use alarm   signal or set a timeout in the select   function call  But I m dealing with multiple connections  instead of one  When a process wide timeout signal happens  I ve no way to distinguish the timeout connection among all the other connections   Is there anyway to change the system-default timeout duration

User · Answer

am not sure if I fully understand the issue, but guess it's related to the one I had, am using Qt with TCP socket communication, all non-blocking, both Windows and Linux..

wanted to get a quick notification when an already connected client failed or completely disappeared, and not waiting the default 900+ seconds until the disconnect signal got raised. The trick to get this working was to set the TCP_USER_TIMEOUT socket option of the SOL_TCP layer to the required value, given in milliseconds.

this is a comparably new option, pls see http://tools.ietf.org/html/rfc5482, but apparently it's working fine, tried it with WinXP, Win7/x64 and Kubuntu 12.04/x64, my choice of 10 s turned out to be a bit longer, but much better than anything else I've tried before ;-)

the only issue I came across was to find the proper includes, as apparently this isn't added to the standard socket includes (yet..), so finally I defined them myself as follows:

#ifdef WIN32
    #include <winsock2.h>
#else
    #include <sys/socket.h>
#endif

#ifndef SOL_TCP
    #define SOL_TCP 6  // socket options TCP level
#endif
#ifndef TCP_USER_TIMEOUT
    #define TCP_USER_TIMEOUT 18  // how long for loss retry before timeout [ms]
#endif

setting this socket option only works when the client is already connected, the lines of code look like:

int timeout = 10000;  // user timeout in milliseconds [ms]
setsockopt (fd, SOL_TCP, TCP_USER_TIMEOUT, (char*) &timeout, sizeof (timeout));

and the failure of an initial connect is caught by a timer started when calling connect(), as there will be no signal of Qt for this, the connect signal will no be raised, as there will be no connection, and the disconnect signal will also not be raised, as there hasn't been a connection yet..

User · Answer

Can t you implement your own timeout system   Keep a sorted list  or better yet a priority heap as Heath suggests  of timeout events  In your select or poll calls use the timeout value from the top of the timeout list  When that timeout arrives  do that action attached to that timeout    That action could be closing a socket that hasn t connected yet

User · Answer

connect timeout has to be handled with a non-blocking socket  GNU LibC documentation on connect   You get connect to return immediately and then use select to wait with a timeout for the connection to complete   This is also explained here   Operation now in progress error on connect  function  error   int wait on sock int sock  long timeout  int r  int w        struct timeval tv    0 0       fd set fdset      fd set  rfds   wfds      int n  so error      unsigned so len       FD ZERO   amp fdset       FD SET   sock   amp fdset       tv tv sec   timeout      tv tv usec   0       TRACES   wait in progress tv   ld  ld      n               tv tv sec  tv tv usec        if  r  rfds    amp fdset  else rfds   NULL      if  w  wfds    amp fdset  else wfds   NULL       TEMP FAILURE RETRY  n   select  sock 1  rfds  wfds  NULL   amp tv        switch  n        case 0          ERROR   wait timed out n            return -errno      case -1          ERROR SYS   error during wait n            return -errno      default             select tell us that sock is ready  test it         so len   sizeof so error           so error   0          getsockopt  sock  SOL SOCKET  SO ERROR   amp so error   amp so len           if  so error    0              return 0          errno   so error          ERROR SYS   wait failed n            return -errno

User · Answer

You can use the SO RCVTIMEO and SO SNDTIMEO socket options to set timeouts for any socket operations  like so       struct timeval timeout            timeout tv sec   10      timeout tv usec   0       if  setsockopt  sockfd  SOL SOCKET  SO RCVTIMEO   char    amp timeout                  sizeof timeout    lt  0          error  setsockopt failed n         if  setsockopt  sockfd  SOL SOCKET  SO SNDTIMEO   char    amp timeout                  sizeof timeout    lt  0          error  setsockopt failed n      Edit  from the setsockopt man page   SO SNDTIMEO is an option to set a timeout value for output operations  It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for output operations to complete   If a send operation has blocked for this much time  it returns with a partial count or with the error EWOULDBLOCK if no data were sent   In the current implementation  this timer is restarted each time additional data are delivered to the protocol  implying that the limit applies to output portions ranging in size from the low-water mark to the high-water mark for output   SO RCVTIMEO is an option to set a timeout value for input operations  It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for input operations to complete  In the current implementation  this timer is restarted each time additional data are received by the protocol  and thus the limit is in effect an inactivity timer  If a receive operation has been blocked for this much time without receiving additional data  it returns with a short count or with the error EWOULDBLOCK if no data were received  The struct timeval parameter must represent a positive time interval  otherwise  setsockopt   returns with the error EDOM

[c] How to set socket timeout in C when making multiple connections?

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