C51: Naming Conventions for Function Symbols

Information in this article applies to:

• C51 Version 5.50a and later

QUESTION

I have problems passing parameters from a C file to an assembly function. I think the problem is with the declarations.

My assembly function receives a pointer as an argument. The C prototype I'm using is:

extern unsigned char Function (unsigned char xdata *);

The assembler code is:

public  Function
Function:
  MOV   A,R7
  .
  .
  .
  RET

When I build my project, the linker outputs the warning Unresolved External Symbol for Function in the C file. If I change the function prototype to have a void argument list and if I pass no parameters to Function, the program compiles and links without warnings.

What could be the problem?

ANSWER

The C51 compiler adds an underline character ('_') to the beginning of function names to indicate that all arguments are passed in registers. This is documented in the Cx51 User's Guide under Program Objects. If you add an underline character ('_') before the function name in the assembler source file your project will build as you expect.

MORE INFORMATION

• Refer to Program Objects in the Cx51 User's Guide.

SEE ALSO

• C51: Calling Assembly Routines from C
• C51: Mixing C and Assembly

Last Reviewed: Thursday, February 25, 2021