Good Programming Practices for Macro Definitions (#define) in C

Question

For example, never define a macro like this:

#define DANGER 60 + 2

This can potentially be dangerous when we do an operation like this:

int wrong_value = DANGER * 2; // Expecting 124

Instead, define like this because you don't know how the user of the macro may use it:

#define HARMLESS (60 + 2)

The example is trivial, but that pretty much explains my question. Are there any set of guidelines or best practices that you would recommend when writing a macro?

Thanks for your time!

Answer 1

When doing a macro that is to run its argument and behave like an expression, this is idiomatic:

 #define DOIT(x) do { x } while(0)

This form has the following advantages:

1. It needs a terminating semicolon
2. It works with nesting and braces, e.g. with if/else

Answer 2

Not only should you put parens around the arguments, you should put parens around the expression returned.

#define MIN(a,b)  a < b ? a : b     // WRONG  

int i = MIN(1,2); // works
int i = MIN(1,1+1); // breaks

#define MIN(a,b)  (a) < (b) ? (a) : (b)   // STILL WRONG

int i = MIN(1,2); // works
int i = MIN(1,1+1); // now works
int i = MIN(1,2) + 1; // breaks

#define MIN(a,b)  ((a) < (b) ? (a) : (b))   // GOOD

int i = MIN(1,2); // works
int i = MIN(1,1+1); // now works
int i = MIN(1,2) + 1; // works

However, MIN(3,i++) is still broken...

The best rule is only to use #defines only when NO OTHER APPROACH WILL WORK! I know you're asking about C instead of C++, but still bear his in mind.

Answer 3

Use parenthesis around the entire macro and around each argument referred to in the expansion list:

#define MAX(x, y) ((x) > (y) ? (x) : (y))

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Avoid writing macros that evaluate their arguments multiple times. Such macros will not behave as expected when arguments have side effects:

MAX(a++, b);

Will evaluate a++ twice if a is greater than b.

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Use UPPERCASE names for macros to make it clear it is a macro and not a function so that the differences can be considered accordingly (another general good practice is not passing arguments that have side effects to functions either).

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Don't use macros to rename types like this:

#define pint int *

because it won't behave as expected when someone types

pint a, b;

Use typedefs instead.

Answer 4

use static const values instead of macros for constant values, integral or other. The compiler can often optimize them away, and they remain a 1-st class citizen in the language's type system.

static const int DANGER = 60 + 2;

Answer 5

In the expansion, put parenthesis around the arguments, so that if they pass in a expression you will get the intended behavior.

#define LESS_THAN(X,Y) (((X) < (Y) ? (X) : (Y))

Answer 6

Response to the MAX/MIN macros, taken from GCC hacks in the Linux kernel:

#define min(x, y) ({                       \
        typeof(x) _min1 = (x);             \
        typeof(y) _min2 = (y);             \
        (void) (&_min1 == &_min2);         \
        _min1 < _min2 ? _min1 : _min2; })

Answer 7

Use fairly unique names for your macros, since they have global scope and can clash with anything, so:

#define MAX 10

could easily clash with other code, so:

#define MYPROJECT_MAX 10

or something even more unique, would be better.

I've seen cases where a clash of this kind didn't produce a compile error, but generated slightly wrong code, so it can be quite insidious.

Answer 8

Undefine your macros.

Your #defines should matched with an #undef. This prevents the preprocessor from getting clogged up and affecting unintended pieces of code.

Answer 9

For multiple line macros, use a do { } while (0):

#define foo(x) do {  \
    (x)++;           \
    printf("%d", x); \
} while(0)

Had you done

#define foo(x) {     \
    (x)++;           \
    printf("%d", x); \
}

instead,

if (xyz)
    foo(y);
else
    foo(z);

would've failed.

Also, be careful when introducing temporary variables in macros:

#define foo(t) do {    \
    int x = (t);       \
    printf("%d\n", x); \
} while(0)

int x = 42;
foo(x);

will print 0 and not 42.

If you have a complicated expression which needs to return a value, you can use the comma operator:

#define allocate(foo, len) (foo->tmp = foo->head, foo->head += len, foo->tmp)

Answer 10

If you're careful and expert, you may be able to accomplish DRY (Don't-Repeat-Yourself) code, by using macros as simple code generators. You do have to explain to other programmers what you're doing, but it can save a lot of code. For example, the list-macro technique:

// define a list of variables, error messages, opcodes
// or anything that you have to write multiple things about
#define VARLIST \
    DEFVAR(int, A, 1) \
    DEFVAR(double, B, 2) \
    DEFVAR(int, C, 3) \

// declare the variables
#define DEFVAR(typ, name, val) typ name = (val);
    VARLIST
#undef  DEFVAR

// write a routine to set a variable by name
void SetVar(string varname, double value){
    if (0);
    #define DEFVAR(typ, name, val) else if (varname == #name) name = value;
        VARLIST
    #undef  DEFVAR
    else printf("unrecognized variable %s\n", varname);
}

// write a routine to get a variable's value, given its name
// .. you do it ..

Now, if you want to add a new variable, delete one, or rename one, it's a 1-line edit.

Answer 11

See how I hate this:

void bar(void) {
    if(some_cond) {
        #define BAZ ...
        /* some code */
        #undef BAZ
    }
}

Always put them like this:

void bar(void) {
    if(some_cond) {
#define BAZ ...
        /* some code */
#undef BAZ
    }
}