Some obscure C features


If you spent a few years programming in C, you're probably much more confident about your knowledge of the language than if you spent as much time working with C++ or java.

Both the C language and its standard library are quite close to the smallest they could be.

The current most used version of the language, c99, brought a bunch of new features, many of which are completely unknown to most C programmers (Older specifications obviously also have some dark corners).

Here are the ones I know about:

Sizeof may have side effects

int main(void) {
    return sizeof(int[printf("ooops\n")]);

sizeof on Variadic types require evaluating arbitrary code!

Hexadecimal float with an exponent

int main() {
  return assert(0xap-1 == 5.0);

p stands for power, and is followed by a base 10 encoded signed two exponent. The expression has type double, but you can change it to float by appending a f to the literal.

Compatible declarations and array function parameters

#include <stdio.h>

void a(); // 1
void a(long story, int a[*], int b[static 12][*][*]); // 2
void a(long story, int a[42], int b[*][*][64]);       // 3
void a(long story, int a[*], int b[const 42][24][*]); // 4
// void a(long story, int a[*], int b[*][666][*]);    // 5
// void a(long story, int a[*], int b[*][*][666]);    // 6

void a(long story, int a[42], int b[restrict 0 * story + a[0]][24][64]) {
    printf("%zu\n", sizeof(a));
    printf("%zu\n", sizeof(b));

int main() {
    a(0, 0, 0);
    return 0;

There are plenty of things going on there:

  • One can declare multiple times the same function as long as their declarations are compatible, which means that if they have parameters, both declarations must have compatible ones. Declaration must also consistently use ....
  • If the size of some array dimension is unknown at declaration time, one can write [*] instead.
  • You can enclose type qualifiers inside the array brackets, to add some informations about the properties of the array. If the keyword static is present, the array dimension size is interpreted as an actual minimum size, instead of being ignored. Type qualifiers and static can only be inside the first array dimension's brackets.
  • The compiler should use new declarations to fill in missing informations about the function's prototype. That's why uncommenting any of declaration 5 and 6 should trigger an error: 666 isn't the known array dimension size. CLang ignores this. In fact, it doesn't seem to care at all about declaration merging.
  • The size of the first dimension doesn't actually matter, so it gets ignored by the compiler. That's why declaration 2 and 4 do not conflict, even though their first dimension doesn't have the same size.

Compile-time tree structures

struct bin_tree {
    int value;
    struct bin_tree *left;
    struct bin_tree *right;

#define NODE(V, L, R) &(struct bin_tree){V, L, R}

const struct bin_tree *tree = \
         NODE(2, NULL, NULL),
              NODE(5, NULL, NULL),

This feature is called compound literals. You can do plenty of other funny tricks with these.

VLA typedef

int main() {
    int size = 42;
    typedef int what[size];
    what the_fuck;
    printf("%zu\n", sizeof(the_fuck));

This is standard since C99. I have no clue how this could ever be useful.

Array designators

struct {
    int a[3], b;
} w[] = {
    [0].a = {
        [1] = 2
    [0].a[0] = 1,

int main() {
    printf("%d\n", w[0].a[0]);
    printf("%d\n", w[0].a[1]);

You can iteratively define a structure member using a designator.

Preprocessor is a functional language

#define OPERATORS_CALL(X)  \
    X(negate, 20, !)       \
    X(different, 70, !=)   \
    X(mod, 30, %)

struct operator {
    int priority;
    const char *value;

#define DECLARE_OP(Name, Prio, Op)       \
    struct operator operator_##Name = {  \
        .priority = Prio,                \
        .value = #Op,                    \


You can pass a macro as a parameter to another macro.

You can interleave switches and regular code

#include <stdio.h>
#include <stdlib.h>
#include <err.h>

int main(int argc, char *argv[]) {
    if (argc != 2)
        errx(1, "Usage: %s DESTINATION", argv[0]);

    int destination = atoi(argv[1]);

    int i = 0;
    switch (destination) {
        for (; i < 2; i++) {
        case 0: puts("0");
        case 1: puts("1");
        case 2: puts("2");
        case 3: puts("3");
        case 4: puts("4");
    return 0;

These things are known as Duff's devices. Among other things, they enable easy manual loop unrolling.

Typedef is almost a storage class

typedef works almost like inline or static.

You should be able to write

void typedef name;

a[b] is a syntactic sugar

I know, I know, nothing crazy. But funny nonetheless!

a[b] is literally equivalent to *(a + b). You can thus write some absolute madness such as 41[yourarray + 1].

Macro calls in #include

This is valid preprocessor:

#define ARCH x86
#define ARCH_SPECIFIC(file) <ARCH/file>
#include ARCH_SPECIFIC(test.h)

Awkward pointer declaration

int (*b);
int (*b)(int);
int (*b)[5];   // 1
int *b[5];     // 2

All of these are valid declarations.

The parenthesis are useful for disambiguation:

  • declaration 1 is a pointer to an array of 5 ints
  • declaration 2 is an array of 5 pointers to int

A single # is valid preprocessor

It does nothing.


int main() {
    return 0;

That's all I got!

I found most of these reading the specification, some others while reading production code.

Happy C adventures :)

EDIT: I don't even know how I managed to forget Duff's devices. Thanks to reddit user needadvicebadly for bringing this up

Written by multun on Wed 21 August 2019, in software

Tagged as C, programming
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