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.

Answer 1

fcntl() has always worked reliably for me. In any case, here is the function I use to enable/disable blocking on a socket:

#include <fcntl.h>

/** Returns true on success, or false if there was an error */
bool SetSocketBlockingEnabled(int fd, bool blocking)
{
   if (fd < 0) return false;

#ifdef _WIN32
   unsigned long mode = blocking ? 0 : 1;
   return (ioctlsocket(fd, FIONBIO, &mode) == 0) ? true : false;
#else
   int flags = fcntl(fd, F_GETFL, 0);
   if (flags == -1) return false;
   flags = blocking ? (flags & ~O_NONBLOCK) : (flags | O_NONBLOCK);
   return (fcntl(fd, F_SETFL, flags) == 0) ? true : false;
#endif
}

Answer 2

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() .

eg

   // 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() .

Answer 3

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 (eg 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.

Answer 4

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.

Answer 5

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

Answer 6

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 *) &address, &addr_size))<0){
                if(errno==EWOULDBLOCK){
                       puts("\n No clients currently available............ \n");
                       continue;
               }
    }else{
          puts("\nClient approched............ \n");
    }

}

Answer 7

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->ai_family, res->ai_socktype | SOCK_NONBLOCK | SOCK_CLOEXEC, res->ai_protocol);
if (fd == -1) {
    perror("socket");
    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("accept4");
    return -1;
}

I don't know why the accepted answer doesn't mention this.

Answer 8

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.

Answer 9

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 *)&remoteAddress, &len)
if (ioctl(socket, (int)FIONBIO, (char *)&on))
{
    printf("ioctl FIONBIO call failed\n");
}

Answer 10

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);