Problems with calling an assembly function from C-Collection of common programming errors

… the claim that the external function is an ‘undefined reference’

LOL! Linkers do not “claim” falsehoods. You will not convince it to change its mind by insisting that you are correct or it is wrong. Accept what the tools tell you to be the truth without delay. This is key to rapidly identifying the problem.

Almost every C compiler, including those you are using, generates global symbols with an underscore prefix to minimize name collisions with assembly language symbols. For example, change your code to

extern _printf
...
call  _printf

and error messages about printf being undefined will go away. If you do get an undefined reference to _printf, it is because the linker is not accessing the C runtime library. The link command can be challenging to get correct. Usually doing so is not very educational, so crib from a working project, or look for an example. This is way that IDEs are very helpful.

As for the C code calling the assembly function, it is usually easiest to write the assembly function using C’s conventions:

    global  _do_str

_do_str:

Alternatively, you could declare the function to use the Pascal calling convention:

extern int pascal do_str ( whatever parameters are needed);
...

retval = do_str ("hello world");

The Pascal calling convention is substantially different from C’s: it does not prepend a leading underscore to the symbol, the caller is responsible for removing the parameters after return, and the parameters are in a different order, possibly with some parameter data types being passed in registers rather than on the stack. See the compiler references for all the details.

Originally posted 2013-11-09 22:55:00.