unknown type name callback

The error message "unknown type name callback" typically occurs in programming languages like C or C++ when the compiler cannot recognize the data type "callback". This error can occur for several reasons, such as:

1. The header file containing the definition of "callback" is not included in the program.
2. The definition of "callback" is misspelled or not declared in the correct syntax.
3. The program is using a library or API that defines "callback" differently than expected.

To resolve this error, check if the header file defining "callback" is included in the program and ensure that the syntax of the definition is correct. If the error persists, consult the documentation of the library or API being used to verify the correct usage of "callback".