posix connect() blocks even when O_NONBLOCK is set.

[JonathanHenson]

This bug-tracker is monitored by developers and other technical types. We like detail! So please use this form and tell us, concisely but precisely, what's up. Please fill out ALL THE FIELDS!

If you have a feature request, please post to the UserVoice. If you're reporting a BSOD, don't post here! Instead, e-mail "secure@microsoft.com", and if possible attach the minidump from "C:\Windows\minidump".

• Your Windows build number: (Type ver at a Windows Command Prompt)
  Microsoft Windows [Version 10.0.15063]

• What you're doing and what's happening: (Copy&paste specific commands and their output, or include screen shots)
  I'm working on Async-IO network programming. While testing, I noticed that connect() always blocks, even when O_NONBLOCK is set. So I decided to put in a bad endpoint, to see if it would block even when the operation should take a long time. Instead, on WSL it hangs the application for 30 seconds or so.

int flags = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, flags | O_NONBLOCK);
//this line will always block on WSL
int error_code = connect(socket->socket_handle, (struct sockaddr *)&addr_in, sizeof(addr_in));

• What's wrong / what should be happening instead:

• Strace of the failing command, if applicable: (If <cmd> is failing, then run `strace -o strace.txt -ff

On both my Ubuntu box and my Centos box, this happens:

socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 10
fcntl(10, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(10, F_SETFL, O_RDWR|O_NONBLOCK) = 0
write(1, "made connect call\n", 18made connect call
) = 18
connect(10, {sa_family=AF_INET, sin_port=htons(801), sin_addr=inet_addr("216.58.216.174")}, 16) = -1 EINPROGRESS (Operation now in progress)
write(1, "connect call finished with error"..., 41connect call finished with error code -1
) = 41

Returning EINPROGRESS immediately

On WSL, I get:

socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 10
fcntl(10, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(10, F_SETFL, O_RDWR|O_NONBLOCK) = 0
write(1, "made connect call\n", 18made connect call
) = 18
connect(10, {sa_family=AF_INET, sin_port=htons(801), sin_addr=inet_addr("216.58.216.174")}, 16) = -1 EAGAIN (Resource temporarily unavailable)
write(1, "connect call finished with error"..., 41connect call finished with error code -1
) = 41

After 30 seconds or so.

What should happen is it should return EINPROGRESS as the linux man says it should.

See our contributing instructions for assistance.

[benhillis]

Support for non-blocking connect was added recently will be available in an Insider build soon.

[JonathanHenson]

awesome, glad it's on your radar! And that it wasn't just a bug in my code.

[benhillis]

Adding links to some duplicates: #903, #1391, #1584, #1585, #1829.