Using the function of a specific library

Question

I have a problem: I would like to use the function `abs` of the library `complex`. However, I undergo an error warning me the function `abs` used is `#define abs(x) (x > 0) ? x : -(x)`. Thus, I think the problem comes from my imports. Because of I also include the stdio and stdlib libraries, the compiler may use the function `abs` defined in one of these libraries. So my question is: how can I use the function `abs` of the library `complex` without removing any import ? Thanks a lot in advance for your response.

Accepted Answer

Wrap parens around it.

```
(abs)(whatever);
```

This will force the compiler to use the function version because the macro no longer matches.

Function-like macros work by matching an identifier followed by a left paren `(`. Since we've wrapped the function name itself in parens, we have instead an identifier followed by a right paren `)`, which fails to match the macro. The parens are semantically transparent, but they inhibit the macro syntax.

IIRC, it was `splint` the C checker which taught this to me. While writing a postscript interpreter, I created nice short macros to access the stack.

```
#define push(o) (*tos++ = (o))
#define pop() (*--tos)
```

Which were great until the tricky parts where they were part of an expression involving `tos`. To avoid undefined behavior, I had to create function versions and use those for those tricky spots. For the new design, I skipped the macros altogether.

Edit: I've got a nagging feeling that it was actually the Coelocanthe book (Peter Van Der Linden's Deep C Secrets) where I learned this, the above situation being where I first needed it. IIRC his example involved `putchar` or `getchar` which are often implemented as both functions and macros in conforming C implementations.

Using the function of a specific library

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