Problem creating a new project with existing source code.

Please refer to Nios II Developer's Handbook, page 6-37 and following,

namely the section named "Boot Sequence and Entry Point".

alt_main() is the standard entry point for a raw Nios application, while main() is the entry point for a HAL based application. In the latter, an alt_main function is automatically generated which provides system initialization and calls your application main().

If you don't use any template, you must define the entry point function:

void alt_main(void)