C51 error written in keil *** WARNING L1

C51 error written in keil *** WARNING L1: UNRESOLVED EXTERNAL SYMBOL SYMBOL: ?C_START 

Solution:
1. The file is not added to the project.
2. It may be because there are defined functions that have not been called.
3. I don’t know if you have deleted A51.C under the Source group. If not, right-click A51.c and select remove file " ".
4. Create a new c file, write an empty function in it, and add the file to the project. Note that the generate assembler SRC file and assemble SRC file options cannot be selected for this file. Recompile the project. If it warns that the function has not been called, call it in the main file.
5. Create a new c file, move several functions in the main file to this file, and add the file to the project. Note that the generate assembler SRC file and assemble SRC file options cannot be selected for this file. Recompile the project.

Others: UNRESOLVED EXTERNAL SYMBOL indicates that the external variable extern xdata unsigned char RxPnt; was not found during the connection.
It only indicates a reference to an external variable, not an actual declaration.
There should be xdata unsigned char RxPnt in a .c file.

The variable is declared in extern, saying that the variable will be used in this file, but it is not defined,

which means that the variable does not really exist.

It needs to be defined somewhere,

or it may be that you have defined it, but it is removed by conditional compilation. Check all the source files in the project.