Unable to set TCP_NODELAY with setsockopt on CentOS

Question

The call to setsockopt seems to succeed, but checking with getsockopt shows that TCP_NODELAY has not been set properly.

Below is the relevant section of my strace.

Am I missing something?

[00007fd101b10327] socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 4
[00007fd102c6d520] connect(4, {sa_family=AF_INET, sin_port=htons(31695), sin_addr=inet_addr("[removed]")}, 16) = 0
[00007fd102c6d1e0] write(3, "110816.344860 [I.COMM_NET] TCP c"..., 155) = 155
[00007fd101a4e880] rt_sigprocmask(SIG_BLOCK, [CHLD], [PIPE], 8) = 0
[00007fd101a4e75d] rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
[00007fd101a4e880] rt_sigprocmask(SIG_SETMASK, [PIPE], NULL, 8) = 0
[00007fd101ad6470] nanosleep({1, 0}, 0x7fff66f9df40) = 0
[00007fd101b102ca] setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
[00007fd101a4e880] rt_sigprocmask(SIG_BLOCK, [CHLD], [PIPE], 8) = 0
[00007fd101a4e75d] rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
[00007fd101a4e880] rt_sigprocmask(SIG_SETMASK, [PIPE], NULL, 8) = 0
[00007fd101ad6470] nanosleep({1, 0}, 0x7fff66f9df40) = 0
[00007fd101b102ca] setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
[00007fd101a4e880] rt_sigprocmask(SIG_BLOCK, [CHLD], [PIPE], 8) = 0
[00007fd101a4e75d] rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
[00007fd101a4e880] rt_sigprocmask(SIG_SETMASK, [PIPE], NULL, 8) = 0
[00007fd101ad6470] nanosleep({1, 0}, 0x7fff66f9df40) = 0
[00007fd101b0ff8a] getsockopt(4, SOL_TCP, TCP_NODELAY, "", [0]) = 0

Answer 1

The strace output isn't telling you anything about the value of the TCP_NODELAY option. What it is telling you is that you are calling getsockopt with a zero-length buffer for the response. The kernel will not be able to return any useful information when the length of the buffer is zero.

In the output [0] is an indication of the size of the buffer you gave it. You need to specify a buffer size which is at least the size of an integer in order to get a useful response back.