Unbuffered stdout

[novakboskov]

I invoke iperf3 from a C++ via popen:

FILE *iperf_pipe = popen("iperf3 -c <SOME_IP> -t 1 | awk 'NR==4 {printf \"%s %s\\n\", $(NF - 4), $(NF - 3); system(\"\")}'", "r");
char *buffer[128];
if (fgets(buffer, 128, iperf_pipe) == NULL)
   exit 1;
court << buffer;

Note that -t 1 option.

And the output differs depending on the network speed. When <SOME_IP>=localhost then a number is printed (e.g., 12Gbits/s, because localhost). When iperf3 needs to go over a slow connection (some Kbps), then fgets does not read anything from iperf_pipe, and the program exits.

To make the issue even more interesting, when I run from a tty:

$ iperf3 -c <SOME_IP> -t 1 | awk 'NR==4 {printf "%s %s\n", $(NF - 4), $(NF - 3); system("")}'

It always prints the network speed, regardless of whether the connection is fast or slow.

This makes me think that iperf3 buffers its stdout depending on whether it goes to a tty. How do I force iperf3 to flush its stdout as soon as possible, even when I'm not on a tty?

I use the following iperf3 version:

iperf 3.1.3
Linux sg 5.4.0-110-generic #124~18.04.1-Ubuntu SMP Fri Apr 22 12:01:21 UTC 2022 x86_64
Optional features available: CPU affinity setting, IPv6 flow label, TCP congestion algorithm setting, sendfile / zerocopy, socket pacing

Unfortunately, I'd have to stick to it unless there is a self-contained binary of a newer iperf3 version.

[davidBar-On]

Some flash commands were added in later iperf3 versions. You can with newer iperf3 version for Windows from here (not official site - maintained by BudMan).

[novakboskov]

Thanks!

[novakboskov]

What I ended up doing is actually two things:

• installed the latest version of iperf3, it's 3.10 (compiling it from source from the official website, Linux build), and
• got rid of awk. Now I read iperf3 output directly via fgets and parse the text on my own.

Apparently, the buffering problem is not there anymore.