C socket connection timeout

Question

I have a simple program to check if a port is open  but I want to shorten the timeout length on the socket connection because the default is far too long  I m not sure how to do this though  Here s the code    include  lt sys socket h gt   include  lt sys time h gt   include  lt sys types h gt   include  lt arpa inet h gt   include  lt netinet in h gt   include  lt errno h gt   include  lt fcntl h gt   include  lt stdio h gt   include  lt netdb h gt   include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   int main int argc  char   argv        u short port                    user specified port number        char addr 1023                  will be a copy of the address entered by u        struct sockaddr in address      the libc network address data structure        short int sock   -1             file descriptor for the network socket         if  argc    3            fprintf stderr   Usage  s  lt port num gt   lt address gt    argv 0            return EXIT FAILURE             address sin addr s addr   inet addr argv 2       assign the address        address sin port   htons atoi argv 2                   translate int2port num         sock   socket AF INET  SOCK STREAM  0       if  connect sock  struct sockaddr    amp address sizeof address      0            printf   i is open n   port               close sock       return 0

User · Answer

The answers about using select   poll   are right and code should be written this way to be portable   However  since you re on Linux  you can do this   int synRetries   2     Send a total of 3 SYN packets   gt  Timeout  7s setsockopt fd  IPPROTO TCP  TCP SYNCNT   amp synRetries  sizeof synRetries      See man 7 tcp and man setsockopt   I used this to speed up the connect-timeout in a program I needed to patch quickly  Hacking it to timeout via select   poll   was not an option

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Here is a modern connect with timeout implementation  using poll  with proper error and signal handling    include  lt sys socket h gt   include  lt fcntl h gt   include  lt poll h gt   include  lt time h gt   int connect with timeout int sockfd  const struct sockaddr  addr  socklen t addrlen  unsigned int timeout ms        int rc   0         Set O NONBLOCK     int sockfd flags before      if  sockfd flags before fcntl sockfd F GETFL 0  lt 0   return -1      if fcntl sockfd F SETFL sockfd flags before   O NONBLOCK  lt 0  return -1         Start connecting  asynchronously      do           if  connect sockfd  addr  addrlen  lt 0                   Did connect return an error  If so  we ll fail              if   errno    EWOULDBLOCK   amp  amp   errno    EINPROGRESS                     rc   -1                               Otherwise  we ll wait for it to complete              else                      Set a deadline timestamp  timeout  ms from now  needed b c poll can be interrupted                  struct timespec now                  if clock gettime CLOCK MONOTONIC   amp now  lt 0    rc -1  break                    struct timespec deadline      tv sec   now tv sec                                                tv nsec   now tv nsec   timeout ms 1000000l                      Wait for the connection to complete                  do                          Calculate how long until the deadline                     if clock gettime CLOCK MONOTONIC   amp now  lt 0    rc -1  break                        int ms until deadline    int     deadline tv sec  - now tv sec  1000l                                                      deadline tv nsec - now tv nsec  1000000l                       if ms until deadline lt 0    rc 0  break                           Wait for connect to complete  or for the timeout deadline                      struct pollfd pfds          fd   sockfd   events   POLLOUT                          rc   poll pfds  1  ms until deadline                          If poll  succeeded   make sure it  really  succeeded                     if rc gt 0                            int error   0  socklen t len   sizeof error                           int retval   getsockopt sockfd  SOL SOCKET  SO ERROR   amp error   amp len                           if retval  0  errno   error                          if error  0  rc -1                                                             If poll was interrupted  try again                  while rc  -1  amp  amp  errno  EINTR                      Did poll timeout  If so  fail                  if rc  0                        errno   ETIMEDOUT                      rc -1                                                  while 0          Restore original O NONBLOCK state     if fcntl sockfd F SETFL sockfd flags before  lt 0  return -1         Success     return rc

User · Answer

On Linux you can also use   struct timeval timeout  timeout tv sec    7      after 7 seconds connect   will timeout timeout tv usec   0  setsockopt fd  SOL SOCKET  SO SNDTIMEO   amp timeout  sizeof timeout    connect        Don t forget to clear SO SNDTIMEO after connect   if you don t need it

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Set the socket non-blocking  and use select    which takes a timeout parameter    If a non-blocking socket is trying to connect  then select   will indicate that the socket is writeable when the connect   finishes  either successfully or unsuccessfully    You then use getsockopt   to determine the outcome of the connect     int main int argc  char   argv        u short port                    user specified port number        char  addr                      will be a pointer to the address        struct sockaddr in address      the libc network address data structure        short int sock   -1             file descriptor for the network socket        fd set fdset      struct timeval tv       if  argc    3            fprintf stderr   Usage  s  lt port num gt   lt address gt  n   argv 0            return EXIT FAILURE             port   atoi argv 1        addr   argv 2        address sin family   AF INET      address sin addr s addr   inet addr addr      assign the address        address sin port   htons port                 translate int2port num         sock   socket AF INET  SOCK STREAM  0       fcntl sock  F SETFL  O NONBLOCK        connect sock   struct sockaddr    amp address  sizeof address         FD ZERO  amp fdset       FD SET sock   amp fdset       tv tv sec   10                 10 second timeout        tv tv usec   0       if  select sock   1  NULL   amp fdset  NULL   amp tv     1                int so error          socklen t len   sizeof so error           getsockopt sock  SOL SOCKET  SO ERROR   amp so error   amp len            if  so error    0                printf   s  d is open n   addr  port                        close sock       return 0

User · Answer

The two socket options SO RCVTIMEO and SO SNDTIMEO have no effect on connect   Below is a link to the screenshot which includes this explanation  here I am just briefing it  The apt way of implementing timeouts with connect are using signal  or select or poll   Signals  connect can be interrupted by a self generated signal SIGALRM by using syscall  wrapper  alarm  But  a signal disposition should be installed for the same signal otherwise the program would be terminated  The code goes like this      include lt stdio h gt   include lt stdlib h gt   include lt unistd h gt   include lt sys socket h gt   include lt netinet in h gt   include lt signal h gt   include lt errno h gt   static void signal handler int signo        return     Do nothing just interrupt     int main            Register signal handler         struct sigaction act  oact       act sa handler   signal handler      sigemptyset  amp act sa mask       act sa flags   0    ifdef SA INTERRUPT     act sa flags    SA INTERRUPT   endif      if sigaction SIGALRM   amp act   amp oact   lt  0      Error registering signal handler                fprintf stderr   Error registering signal disposition n            exit 1                 Prepare your socket and sockaddr structures         int sockfd      struct sockaddr  servaddr          Implementing timeout connect         int sec   30       if alarm sec     0          fprintf stderr   Already timer was set n         if connect sockfd  servaddr  sizeof struct sockaddr    lt  0                if errno    EINTR              fprintf stderr   Connect timeout n            else             fprintf stderr   Connect failed n             close sockfd            exit 1              alarm 0       turn off the alarm         sigaction SIGALRM   amp oact  NULL       Restore the default actions of SIGALRM            Use socket             End program         close sockfd       return 0      Select or Poll  As already some users provided nice explanation on how to use select to achieve connect timeout  it would not be necessary for me to reiterate the same  poll can be used in the same way  However  there are few mistakes that are common in all of the answers  which I would like to address    Even though socket is non-blocking  if the server to which we are connecting is on the same local machine  connect may return with success  So it is advised to check the return value of connect before calling select  Berkeley-derived implementations  and POSIX  have the following rules for non-blocking sockets and connect   1  When the connection completes successfully  the descriptor becomes writable  p  531 of TCPv2     2  When the connection establishment encounters an error  the descriptor becomes both readable and writable  p  530 of TCPv2     So the code should handle these cases  here I just code the necessary modifications       All the code stays        Modifications at connect     int conn ret   connect sockfd  servaddr  sizeof struct sockdaddr     if conn ret    0      goto done      Modifications at select     int sec   30  for              struct timeval timeo      timeo tv sec   sec      timeo tv usec   0       fd set wr set  rd set      FDZERO  amp wr set       FD SET sockfd   amp wr set       rd set   wr set      int sl ret   select sockfd   1   amp rd set   amp wr set  NULL   amp timeo           All the code stays        done          Use your socket      Text from Unix Network Programming Volume 1

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This one has parametrized ip  port  timeout in seconds  handle connection errors and give you connection time in milliseconds    include  lt sys socket h gt   include  lt sys time h gt   include  lt sys types h gt   include  lt arpa inet h gt   include  lt netinet in h gt   include  lt errno h gt   include  lt fcntl h gt   include  lt stdio h gt   include  lt netdb h gt   include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt time h gt   int main int argc  char   argv        struct sockaddr in addr s      char  addr      short int fd -1      int port      fd set fdset      struct timeval tv      int rc      int so error      socklen t len      struct timespec tstart  0 0   tend  0 0       int seconds       if  argc    4            fprintf stderr   Usage   s  lt ip gt   lt port gt   lt timeout seconds gt  n   argv 0            return 1             addr   argv 1       port   atoi argv 2        seconds   atoi argv 3         addr s sin family   AF INET     utilizzo IPv4     addr s sin addr s addr   inet addr addr       addr s sin port   htons port        clock gettime CLOCK MONOTONIC   amp tstart        fd   socket AF INET  SOCK STREAM  0       fcntl fd  F SETFL  O NONBLOCK      setup non blocking socket         make the connection     rc   connect fd   struct sockaddr    amp addr s  sizeof addr s        if   rc    -1   amp  amp   errno    EINPROGRESS             fprintf stderr   Error   s n   strerror errno            close fd           return 1            if  rc    0               connection has succeeded immediately         clock gettime CLOCK MONOTONIC   amp tend           printf  socket  s  d connected  It took   5f seconds n               addr  port     double tend tv sec   1 0e-9 tend tv nsec  -   double tstart tv sec   1 0e-9 tstart tv nsec              close fd           return 0          else              connection attempt is in progress               FD ZERO  amp fdset       FD SET fd   amp fdset       tv tv sec   seconds      tv tv usec   0       rc   select fd   1  NULL   amp fdset  NULL   amp tv       switch rc        case 1     data to read         len   sizeof so error            getsockopt fd  SOL SOCKET  SO ERROR   amp so error   amp len            if  so error    0                clock gettime CLOCK MONOTONIC   amp tend               printf  socket  s  d connected  It took   5f seconds n                   addr  port     double tend tv sec   1 0e-9 tend tv nsec  -   double tstart tv sec   1 0e-9 tstart tv nsec                 close fd               return 0            else      error             printf  socket  s  d NOT connected   s n   addr  port  strerror so error                      break      case 0    timeout         fprintf stderr   connection timeout trying to connect to  s  d n   addr  port           break             close fd       return 0

User · Answer

Is there anything wrong with Nahuel Greco s solution aside from the compilation error   If I change one line     Compilation error setsockopt fd  SO SNDTIMEO   amp timeout  sizeof timeout      to     Fixed  setsockopt fd  SOL SOCKET  SO SNDTIMEO   amp timeout  sizeof timeout      then it seems to work as advertised - socket   returns a timeout error   Resulting code   struct timeval timeout  timeout tv sec    7      after 7 seconds connect   will timeout timeout tv usec   0  setsockopt fd  SOL SOCKET  SO SNDTIMEO   amp timeout  sizeof timeout    connect        I m not versed enough to know the tradeoffs are between a send timeout and a non-blocking socket  but I m curious to learn

User · Answer

This article might help   Connect with timeout  or another use for select      Looks like you put the socket into non-blocking mode until you ve connected  and then put it back into blocking mode once the connection s established   void connect w to void       int res     struct sockaddr in addr     long arg     fd set myset     struct timeval tv     int valopt     socklen t lon         Create socket    soc   socket AF INET  SOCK STREAM  0      if  soc  lt  0          fprintf stderr   Error creating socket   d  s  n   errno  strerror errno          exit 0            addr sin family   AF INET     addr sin port   htons 2000      addr sin addr s addr   inet addr  192 168 0 1           Set non-blocking    if   arg   fcntl soc  F GETFL  NULL    lt  0          fprintf stderr   Error fcntl      F GETFL    s  n   strerror errno          exit 0           arg    O NONBLOCK     if  fcntl soc  F SETFL  arg   lt  0          fprintf stderr   Error fcntl      F SETFL    s  n   strerror errno          exit 0              Trying to connect with timeout    res   connect soc   struct sockaddr    amp addr  sizeof addr       if  res  lt  0          if  errno    EINPROGRESS             fprintf stderr   EINPROGRESS in connect   - selecting n             do               tv tv sec   15              tv tv usec   0              FD ZERO  amp myset               FD SET soc   amp myset               res   select soc 1  NULL   amp myset  NULL   amp tv               if  res  lt  0  amp  amp  errno    EINTR                   fprintf stderr   Error connecting  d -  s n   errno  strerror errno                   exit 0                             else if  res  gt  0                      Socket selected for write                lon   sizeof int                  if  getsockopt soc  SOL SOCKET  SO ERROR   void    amp valopt    amp lon   lt  0                      fprintf stderr   Error in getsockopt    d -  s n   errno  strerror errno                      exit 0                                      Check the value returned                   if  valopt                      fprintf stderr   Error in delayed connection    d -  s n   valopt  strerror valopt                        exit 0                                   break                            else                  fprintf stderr   Timeout in select   - Cancelling  n                   exit 0                            while  1                 else            fprintf stderr   Error connecting  d -  s n   errno  strerror errno             exit 0                      Set to blocking mode again       if   arg   fcntl soc  F GETFL  NULL    lt  0          fprintf stderr   Error fcntl      F GETFL    s  n   strerror errno          exit 0           arg  amp     O NONBLOCK      if  fcntl soc  F SETFL  arg   lt  0          fprintf stderr   Error fcntl      F SETFL    s  n   strerror errno          exit 0              I hope that is all

[c] C: socket connection timeout

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