The most useful user-made C-macros (in GCC, also C99)? [closed]

Answer 1

#define IMPLIES(x, y) (!(x) || (y))

#define COMPARE(x, y) (((x) > (y)) - ((x) < (y)))
#define SIGN(x) COMPARE(x, 0)

#define ARRAY_SIZE(a) (sizeof(a) / sizeof(*a))

#define SWAP(x, y, T) do { T tmp = (x); (x) = (y); (y) = tmp; } while(0)
#define SORT2(a, b, T) do { if ((a) > (b)) SWAP((a), (b), T); } while (0)

#define SET(d, n, v) do{ size_t i_, n_; for (n_ = (n), i_ = 0; n_ > 0; --n_, ++i_) (d)[i_] = (v); } while(0)
#define ZERO(d, n) SET(d, n, 0)

And, of course, various MIN, MAX, ABS etc.

Note, BTW, that none of the above can be implemented by a function in C.

P.S. I would probably single out the above IMPLIES macro as one of the most useful ones. Its main purpose is to facilitate writing of more elegant and readable assertions, as in

void foo(int array[], int n) {
  assert(IMPLIES(n > 0, array != NULL));
  ...

Answer 2

I also like this one:

#define COMPARE_FLOATS(a,b,epsilon) (fabs(a - b) <= epsilon * fabs(a))

And how you macros-haters do fair floating-point comparisons?

Answer 3

If you need to define data multiple times in different contexts, macros can help you avoid have to relist the same thing multiple times.

For example, lets say you want to define an enum of colors and an enum-to-string function, rather then list all the colors twice, you could create a file of the colors (colors.def):

c(red)
c(blue)
c(green)
c(yellow)
c(brown)

Now you can in your c file you can define your enum and your string conversion function:

enum {
#define c(color) color,
# include "colors.def"
#undef c
};

const char *
color_to_string(enum color col)
{
    static const char *colors[] = {
#define c(color) #color,
# include "colors.def"
#undef c
    };
    return (colors[col]);
};

Answer 4

Just the standard ones:

#define LENGTH(array) (sizeof(array) / sizeof (array[0]))
#define QUOTE(name) #name
#define STR(name) QUOTE(name)

but there's nothing too spiffy there.

Answer 5

This one is awesome:

#define NEW(type, n) ( (type *) malloc(1 + (n) * sizeof(type)) )

And I use it like:

object = NEW(object_type, 1);

Answer 6

This one is from linux kernel (gcc specific):

#define container_of(ptr, type, member) ({                  \
const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
    (type *)( (char *)__mptr - offsetof(type,member) ); })

Another missing from other answers:

#define LSB(x) ((x) ^ ((x) - 1) & (x))   // least significant bit