If you use the "Small C library", the printf() family of functions cannot handle floating point data.
(%f, %g options are not supported)

For details, please refer to the information at the following URL.
　https://www.altera.co.jp/support/support-resources/knowledge-base/solutions/rd03232005_29.html
　https://www.altera.com/en_US/pdfs/literature/hb/nios2/n2sw_nii52004.pdf

If you enable the "Small C library" option in the Nios II IDE's system library properties, the system library will now use the reduced Newlib ANSI C standard library.
In that case the printf() family of functions (printf(), fprintf(), sprintf(), etc.) will no longer support floating point data.
This minified library is optimized for smaller memory size at the expense of speed.