TCP_CORK (aka TCP_NOPUSH) does not work

Brian Inglis Brian.Inglis@SystematicSw.ab.ca
Wed Jul 31 03:50:00 GMT 2019


On 2019-07-30 15:30, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via cygwin wrote:
> Consider the following code:
> $ cat cork.c
> #include <arpa/inet.h>
> #include <netinet/in.h>
> #include <netinet/tcp.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/socket.h>
> #include <sys/types.h>
> #if defined(TCP_NOPUSH)  &&  !defined(TCP_CORK)
> #  define TCP_CORK  TCP_NOPUSH
> #endif

For POSIX only non-Nagle TCP_NODELAY is required:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/netinet_tcp.h.html
and even then says:
"The implementation need not allow the value of the option to be set via
setsockopt() or retrieved via getsockopt()."

TCP_CORK is Linux only; TCP_NOPUSH is BSD only; Windows does its own thing:
https://baus.net/on-tcp_cork/
https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-tcp-socket-options

Regular SO options on Windows:
https://docs.microsoft.com/en-us/windows/win32/winsock/sol-socket-socket-options

You can abuse Nagle to get similar behaviour cross-platform:
https://stackoverflow.com/a/22118709

> int main(int argc, const char* argv[])
> {
>     union {
>         struct sockaddr_in in;
>         struct sockaddr    sa;
>     } addr;
>     int sock, cork = 1;
> 
>     memset(&addr, 0, sizeof(addr));
>     if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
>         perror("socket");
>     addr.in.sin_family = AF_INET;
>     addr.in.sin_addr.s_addr = inet_addr(argv[1]);
>     addr.in.sin_port = htons((unsigned short) atoi(argv[2]));
>     if (connect(sock, &addr.sa, sizeof(addr.in)) < 0)
>         perror("connect");
>     if (setsockopt(sock, IPPROTO_TCP, TCP_CORK, (char*) &cork, sizeof(cork)) != 0)
>         perror("cork");
>     return 0;
> }
> When compiled and run under Cygwin, the last syscall, setsockopt(), returns
> an error, Protocol not available:
> gcc cork.c
> ./a.exe 8.8.8.8 443
> cork: Protocol not available

This is error ENOPROTOOPT, where the message is misleading, but as suggested by
the name, means that the socket option is not supported for that protocol;
getsockopt(3p/3posix) says: "The option is not supported by the protocol."
getsockopt(2) for Linux and FreeBSD say: "The option is unknown at the level
indicated." and given the POSIX statement above, that error should be treated as
a warning to do something different.

> The same code works under Linux just fine.  I straced both.
> gcc cork.c
> ./a.out 8.8.8.8 443
> Any ideas?  Is TCP_NOPUSH (which is a BSDism, BTW) not actually usable on
Cygwin?  If not, why is it in the header file <netinet/tcp.h>?

If a socket option is defined, perhaps for compatibility, it should either be
used or ignored, rather than giving an error.
If you are not going to support a socket option, and generate an error, it would
be better to not define the option and generate the error at compile time,
instead of failing at run time.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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