what is meanging of gcc option "-fmessage-length"?

man page for gcc says

-fmessage-length=n
       Try to format error messages so that they fit on lines of about n characters.  The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC.  If n is
       zero, then no line-wrapping will be done; each error message will appear on a single line.

Just a formatting option for error messages.

I didnt find it in GCC Command-Line Options.

That's because you're looking at "a modified version of the Command-Line Options section of the GCC Manual."

This is the official list of all possible GCC command-line options, which leads to this section: "3.7 Options to Control Diagnostic Messages Formatting". This is what the section has to say:

3.7 Options to Control Diagnostic Messages Formatting

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.

-fmessage-length=n

Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping is done; each error message appears on a single line.

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