Choosing Between Functions and Macros

Article
09/15/2006

Most Microsoft run-time library routines are compiled or assembled functions, but some routines are implemented as macros. When a header file declares both a function and a macro version of a routine, the macro definition takes precedence, because it always appears after the function declaration. When you invoke a routine that is implemented as both a function and a macro, you can force the compiler to use the function version in two ways:

Enclose the routine name in parentheses.

#include <ctype.h>
a = toupper(a);    //use macro version of toupper
a = (toupper)(a);  //force compiler to use function version of toupper

“Undefine” the macro definition with the #undef directive:
```
#include <ctype.h>
#undef toupper
```

If you need to choose between a function and a macro implementation of a library routine, consider the following trade-offs:

Speed versus size. The main benefit of using macros is faster execution time. During preprocessing, a macro is expanded (replaced by its definition) inline each time it is used. A function definition occurs only once regardless of how many times it is called. Macros may increase code size but do not have the overhead associated with function calls.
Function evaluation. A function evaluates to an address; a macro does not. Thus you cannot use a macro name in contexts requiring a pointer. For instance, you can declare a pointer to a function, but not a pointer to a macro.
Macro side effects. A macro may treat arguments incorrectly when the macro evaluates its arguments more than once. For instance, the toupper macro is defined as:
```
#define toupper(c) ( (islower(c)) ? _toupper(c) :  (c) )
```
In the following example, the toupper macro produces a side effect:
```
#include <ctype.h>

int a = 'm';
a = toupper(a++);
```
The example code increments a when passing it to toupper. The macro evaluates the argument a++ twice, once to check case and again for the result, therefore increasing a by 2 instead of 1. As a result, the value operated on by islower differs from the value operated on by toupper.
Type-checking. When you declare a function, the compiler can check the argument types. Because you cannot declare a macro, the compiler cannot check macro argument types, although it can check the number of arguments you pass to a macro.

Choosing Between Functions and Macros

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