output from spdlog is truncated when stdin be non blocked

[chenchuanyin]

Hi spdlog maintainer, When stdin set to nonblock, some logs are truncated in spdlog. How can I avoid the case? Thanks

[tt4g]

Since you have not provided the code, I am not sure what exactly you are doing.
I can only answer with a guess that the OS is not writing the STDOU buffer.
Please call logger->flush().

[chenchuanyin]

thanks for the quick update. this code logic is I use ansicolor_stdout_sink_mt in the code, and spdlog always flush() when doing log(), and in somewhere I need set stdin to nonblock in other thread for some case. And. I also test removing stdin nonblock, the spdlog output is not truncated and expected. and I found most of truncated log is multiple line case. I am not sure why stdout is impacted when setting stdin to nonblock?

[tt4g]

I am not sure why stdout is impacted when setting stdin to nonblock?

If you are running the application on a console (tty), this is expected behavior.
For a very simple reason, when the shell executes a command, STDOUT, STDIN, and STDERR all default to the same console as input source and output destination.

Assuming that there is a virtual input/output file called console, when STDIN, STDOUT, and STDERR are not explicitly specified, the shell implicitly uses console for input and output, as in the following command line:

$ your_app <console 1>console 2>console

The shell opens console for input/output redirection. The file descriptors will also be the same since it is the same file.
STDOUT and STDERR will also be in non-blocking mode if STDIN is set to non-blocking mode since they are the same file descriptors.

If you want to avoid this behavior, open another output destination (/dev/tty on UNIX) and prepare a new file descriptor and set it to STDOUT or STERR.

See also:

• c - When non-blocking I/O is turned on for stdout, is it correct for the OS to turn it on for stdin too? - Stack Overflow
• c - Are stdin and stdout actually the same file? - Stack Overflow

[tt4g]

I do not know why multiple lines of logs are truncated when STDOUT is in non-blocking mode.
It may be that the console implementation that is reading the STDOUT output does not support non-blocking mode.

Perhaps disabling buffering with setbuf(stdout, NULL); may solve the problem. However, I don't know what the impact would be.

[chenchuanyin]

Thank you for the professional advice.