Can you bind() and connect() both ends of a UDP connection

by

Hello from the distant future which is the year 2018, to the year 2012.

There’s, in fact, a reason behind connect()ing an UDP socket in practice (though blessed POSIX and its implementations don’t in theory require you to).

An ordinary UDP socket doesn’t know anything about its future destinations, so it performs a route lookup each time sendmsg() is called.

However, if connect() is called beforehand with a particular remote receiver’s IP and port, the operating system kernel will be able to write down the reference to the route and assign it to the socket, making it significantly faster to send a message if subsequent sendmsg() calls do not specify a receiver (otherwise the previous setting would be ignored), choosing the default one instead.

Look at the lines 1070 through 1171:

if (connected)
    rt = (struct rtable *)sk_dst_check(sk, 0);

if (!rt) {
    [..skip..]

    rt = ip_route_output_flow(net, fl4, sk);

    [..skip..]
}

Until Linux kernel 4.18, this feature had been mostly limited to the IPv4 address family only. However, since 4.18-rc4 (and hopefully Linux kernel release 4.18 as well), it’s fully functional with IPv6 sockets as well.

It may be a source of a serious performance benefit, though it will heavily depend on the OS you’re using. At least, if you’re using Linux and don’t use the socket for multiple remote handlers, you should give it a try.

More Related Contents:

  1. Problems with SO_BINDTODEVICE Linux socket option
  2. C – list all files in current directory then move in directory above, list files, and so on until root directory is reached [closed]
  3. How to open, read, and write from serial port in C?
  4. Hide password input on terminal
  5. Pass arguments into C program from command line
  6. How to list files in a directory in a C program?
  7. Segmentation fault handling
  8. What are the use cases of SO_REUSEADDR?
  9. Why does the C preprocessor interpret the word “linux” as the constant “1”?
  10. Why does ENOENT mean “No such file or directory”?
  11. Is there an alternative sleep function in C to milliseconds?
  12. How to compile .c file with OpenSSL includes?
  13. Clearing output of a terminal program Linux C/C++
  14. How to use printf to display off_t, nlink_t, size_t and other special types?
  15. `bash: ./a.out: No such file or directory` on running executable produced by `ld`
  16. Linking against older symbol version in a .so file
  17. What is the difference between using _exit() & exit() in a conventional Linux fork-exec?
  18. How can you flush a write using a file descriptor?
  19. Why getch() returns before press any key?
  20. Does linux allow any system call to be made from signal handlers?
  21. How can I get the keyboard state in Linux?
  22. Problem usage memory in C
  23. How does a pipe work in Linux?
  24. How to flush the CPU cache for a region of address space in Linux?
  25. How to dynamically load often re-generated c code quickly?
  26. get command output in pipe, C for Linux
  27. Checking if errno != EINTR: what does it mean?
  28. mmap, msync and linux process termination
  29. How does GCC behave if passed conflicting compiler flags?
  30. Where can I find system call source code?

Leave a Comment