EXTRN CODE ( FUNCTION ) gives L16 if FUNCTION is BIT

Have you got the naming convention correct?

C51 puts various prefixes on the generated name to indicate things like return type & register usage - check the Manual

Or, try writing a dummy call in 'C', and see what assembler it generates.

The lst gives:

IS_OK_INC_DTARGET_SPD. PUBLIC CODE PROC 0000H -----

and the link map shows

CODE 35A8H 0096H INBLOCK ?PR?IS_OK_INC_DTARGET_SPD?SPDLIM

for the PRogram part and

BIT 0020H.3 0000H.1 UNIT ?BI?IS_OK_INC_DTARGET_SPD?SPDLIM

which, I assume, is for the return value.

By the way, I read the C manual (Bit Types, p. 98 and Func. Return Values, p. 120) before my original post. I did not see any special note or warning for interfacing C to assy where the return value from a C function is BIT.

Am I missing something?

Created main.c, cf.c, af.a51 for project. The following paste does not show main's call to af(). This simple example still shows the linker warning.

// cf.c
bit cf( void );
bit cf( void )
{
bit retval;
  retval = 1;
  return ( retval );
}

;; af.a51
CS SEGMENT CODE
   RSEG    CS
extrn code ( cf )
public af
af:
  call    cf
  ret
END

(In my opinion) The ONLY way to make assembler functions that can be seamlessly called from C is to make a skeleton function in C and use the generated assembler as a template. Make sure all variables are used in the skeleton.

Erik

IMHO, the problem is calling a C function from assembly, not calling assembly from C.

IMHO, the problem is calling a C function from assembly, not calling assembly from C.

To which the same strategy applies: do it (correctly, i.e. with a prototype declaration of the called function in scope) in
a C program, and have C51 make you A51 soruce code demonstrates what exactly has to be done.

That's the single most important rule about mixed assembly + C programming: the compiler knows how to do it, so don't ever hesitate to ask it.