What is #undef Directive in C Programs?

Answer: #undef directive undefines a previously defined #define. For example,

#define LIMIT 100
 
#undef LIMIT

Preprocessor undefines LIMIT by removing its definition. Even if we haven’t defined LIMIT, for ex.,

#define LIMIT

It’s possible to undefine it.

#undef LIMIT

Notice further that even if we are not sure a given symbol is defined or not, we can undefine it to be on the safe side before using it.

A few predefined macros like __DATE__, __FILE__, __TIME__ etc. can’t be undefined.

Let’s Consider the fragment of code below,

#define MAX(a, b)        ((a) > (b)) ? (a) : (b)

Now let’s use this in a program,

int main(void)
{
    printf("Larger of %d, %d is %d\n", a, b, MAX(a, b));
    return 0;
}

If we want to redefine the MAX(a, b), we have to remove its prior definition. This we can do with #undef as follows,

/* #undef un-#define's MAX(a, b) by removing its definition */
#undef MAX(a, b)

/* then redefine MAX(a, b) */
#define MAX(a, b) \
if ((a) > (b)) \
printf(“%d\n”, (a)); \
else \
printf(“%d\n”, (b));

Notice here that when a macro definition is long enough not to fit in one line, break it over several lines, ending each line except the last with back slash ‘\’.

And now use the new definition of MAX(a,b),

int main(void)
{
    printf("Larger of %d, %d is %d\n", a, b, MAX(a, b));
}