undefined reference to `main'

This error message usually occurs when the compiler cannot find the main function in your code. The main function is the entry point of your program, and it must be defined in your code for the program to run.

To fix this error, make sure that you have defined the main function in your code and that it is spelled correctly. Also, check that the file containing the main function is linked correctly during the compilation process.

If you are using an IDE, make sure that you have created a new project and added the source files correctly. If you are compiling from the command line, make sure that you have specified all the necessary flags and options.

If you are still having trouble, try to simplify your code and remove any unnecessary dependencies. Sometimes, this error can occur due to a missing library or header file.