What is RONG that I am doing with sprintf() function?

[J-M-L]

found this:

Additional compiler options

pert: You might be interested to know about this nice feature: Arduino IDE 1.5 3rd party Hardware specification · arduino/Arduino Wiki · GitHub rather than modifying platform.txt directly, you can instead define the compiler.cpp.extra_flags property in a file named platform.local.txt that you place in the same folder as platform.txt. The property definitions in platform.local.txt override those in platform.txt. This is convenient because every time you update to a new version of the hardware…