C preprocessor directive problem

[handsomecat]

In the main body of my code i have something like this:

Cmin_vec[1][1][1] = rptr->cmin[3];

I want to replace this with a macro

#define COPY_CMIN( cmin, rptr)  (cmin)[1][1][1] = (rptr)->cmin[3]

and call it in from my code

COPY_CMIN( Cmin_vec, rptr)

But i get the error message saying "structure has no member named `Cmin_vec'.

how should I go about doing it?

 

[D H]

I want to replace this with a macro

#define COPY_CMIN( cmin, rptr)  (cmin)[1][1][1] = (rptr)->cmin[3]

and call it in from my code

COPY_CMIN( Cmin_vec, rptr)

This expands to

(Cmin_vec)[1][1][1] = (rptr)->Cmin_vec[3]

If you wan't to preserve that cmin[3] on the right-hand side then you must not use cmin as one of the macro argument names. The macro processor replaces all occurrences of a macro parameter in the macro definition text with the corresponding argument value. The expansion is not limited to parameters in parentheses. The preprocessor just sees those parentheses as part of a string.

 

[jim mcnamara]

If you want to see macro expansions, the compiler often has an option to let you see input from the "cpp" phase of pre-compilation.

[g]cc -E myfile.c

will do that. Check your compiler docset or a man page.
You can also invoke cpp on some UNIX systems directly.
Make sure the command "which cpp" is able to find the executable image, then try:

cpp myfile.c

Note that you cannot debug code in a debugger when the code is inside a macro.

 

[chroot]

Don't use a macro in the first place. They're evil and generally should never be used.

- Warren

 

[CRGreathouse]

Don't use a macro in the first place. They're evil and generally should never be used.

There are times that C macros are useful, though.

 

[jim mcnamara]

Warren -

Macros are evil? Is this like void main()? I don't think so. Try reading the the Linux kernel source or the macros that the C standard defines. Macros cause problems when used in a problematic way; the OP's example falls into that category.

 

[chroot]

The vast majority of programmers are not skilled enough to properly use macros, or to easily solve the often mystifying bugs they cause. They're just not a good idea. There's almost always a better way.

- Warren