How do I change a TCP socket to be non-blocking

Question

How do you make a socket non-blocking   I am aware of the fcntl   function  but I ve heard it s not always reliable

User · Answer

Generally you can achieve the same effect by using normal blocking IO and multiplexing several IO operations using select(2), poll(2) or some other system calls available on your system.

See The C10K problem for the comparison of approaches to scalable IO multiplexing.

User · Answer

If you want to change socket to non blocking   precisely accept   to NON-Blocking state then       int flags fcntl master socket  F GETFL           fcntl master socket  F SETFL flags  O NONBLOCK      Change the socket into non-blocking state  F SETFL is a command saying set flag and flag is 0 NONBLOCK                                                   while 1       if  newSocket   accept master socket   struct sockaddr     amp address   amp addr size   lt 0                   if errno  EWOULDBLOCK                          puts   n No clients currently available              n                           continue                        else            puts   nClient approched              n

User · Answer

fcntl   or ioctl   are used to set the properties for file streams  When you use this function to make a socket non-blocking  function like accept    recv   and etc  which are blocking in nature will return error and errno would be set to EWOULDBLOCK  You can poll file descriptor sets to poll on sockets

User · Answer

What do you mean by  not always reliable   If the system succeeds in setting your socket non non-blocking  it will be non-blocking  Socket operations will return EWOULDBLOCK if they would block need to block  e g  if the output buffer is full and you re calling send write too often    This forum thread has a few good points when working with non-blocking calls

User · Answer

The best method for setting a socket as non-blocking in C is to use ioctl   An example where an accepted socket is set to non-blocking is following   long on   1L  unsigned int len  struct sockaddr storage remoteAddress  len   sizeof remoteAddress   int socket   accept listenSocket   struct sockaddr    amp remoteAddress   amp len  if  ioctl socket   int FIONBIO   char    amp on         printf  ioctl FIONBIO call failed n

User · Answer

fcntl   has always worked reliably for me   In any case  here is the function I use to enable disable blocking on a socket    include  lt fcntl h gt       Returns true on success  or false if there was an error    bool SetSocketBlockingEnabled int fd  bool blocking       if  fd  lt  0  return false    ifdef  WIN32    unsigned long mode   blocking   0   1     return  ioctlsocket fd  FIONBIO   amp mode     0    true   false   else    int flags   fcntl fd  F GETFL  0      if  flags    -1  return false     flags   blocking    flags  amp   O NONBLOCK     flags   O NONBLOCK      return  fcntl fd  F SETFL  flags     0    true   false   endif

User · Answer

On Linux and BSD you can directly create the socket in non-blocking mode  https   www man7 org linux man-pages man7 socket 7 html   int fd   socket res- gt ai family  res- gt ai socktype   SOCK NONBLOCK   SOCK CLOEXEC  res- gt ai protocol   if  fd    -1        perror  quot socket quot        return -1     When accepting connections you can use the accept4 function to directly accept new connections in non-blocking mode  https   man7 org linux man-pages man2 accept 2 html   int fd   accept4 lfd  NULL  0  SOCK NONBLOCK   SOCK CLOEXEC   if  fd    -1        perror  quot accept4 quot        return -1     I don t know why the accepted answer doesn t mention this

User · Answer

It is sometimes convenient to employ the  send recv  family of system calls  If the flags parameter contains the MSG DONTWAIT flag  each call will behave similar to a socket having the O NONBLOCK flag set   ssize t send int sockfd  const void  buf  size t len  int flags    ssize t recv int sockfd  void  buf  size t len  int flags

User · Answer

You re misinformed about fcntl   not always being reliable   It s untrue   To mark a socket as non-blocking the code is as simple as      where socketfd is the socket you want to make non-blocking int status   fcntl socketfd  F SETFL  fcntl socketfd  F GETFL  0    O NONBLOCK    if  status    -1     perror  calling fcntl         handle the error   By the way  I ve never seen fcntl fail in this way     Under Linux  on kernels   2 6 27 you can also create sockets non-blocking from the outset using socket   and accept4     e g          client side    int socketfd   socket AF INET  SOCK STREAM   SOCK NONBLOCK  0          server side - see man page for accept4 under linux     int socketfd   accept4        SOCK NONBLOCK     It saves a little bit of work  but is less portable so I tend to set it with fcntl

User · Answer

I know it s an old question  but for everyone on google ending up here looking for information on how to deal with blocking and non-blocking sockets here is an in depth explanation of the different ways how to deal with the I O modes of sockets - http   dwise1 net pgm sockets blocking html   Quick summary    So Why do Sockets Block  What are the Basic Programming Techniques for Dealing with Blocking Sockets    Have a design that doesn t care about blocking Using select Using non-blocking sockets  Using multithreading or multitasking

[c] How do I change a TCP socket to be non-blocking?

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