renaudcerrato

int a[5] = { [3] = 42, [1] = 29 };
int b[] = { [0 ... 9 ] = 1, [10 ... 19] = 4, [20] = 16 }; // GCC extension

struct point { int x, y; };
struct point p1 = { .x = 0, .y = 1 };
struct point p[10] = { [5].y = 1, [4].x = -1 };

union foo { int i; double d; };
union foo f = { .d = 4 };

renaudcerrato

union foo { int i; double d; };
extern void hacky(union foo);

int x = 5;
union foo u = (union foo) x; 
hacky((union foo) x);
hacky((union foo) 5);

double d = 42.0;
union foo u = (union foo) d;

renaudcerrato

#include <stddef.h>

struct S {
  int i;
  double d;
};

printf("The first element is at offset %lu, "
       "the second element is at offset %lu.",
       offsetof(S, i), offsetof(S, d));

renaudcerrato

void goodbye(char **s) {
  printf("Goodbye %s!\n", *s);
}

int main(int argc, char *argv[]) {
  char *name __attribute__((cleanup(goodbye)));
  name = argc > 1 ? argv[1] : "World";
  printf("Hello %s!\n", name);
  return 0;
}

renaudcerrato

typedef union {
  float value;
  struct {
    unsigned int mantissa:23;
    unsigned int exponent:8;
    unsigned int negative:1;
  };
} ieee754_float_t;
  
ieee754_float_t f = (ieee754_float_t) 42.f;

renaudcerrato

struct {
  int a;
  union { int b; float c; };
  int d;
} foo;

foo.b = 42; // OK

renaudcerrato

extern void mylib_user_defined_hook(void) __attribute__((weak));

int mylib_do_something(void) {
  ...
  if(mylib_user_defined_hook) {
    mylib_user_defined_hook();
  }
  ...
}

renaudcerrato

void log_default(char *fmt, ...) {
  va_list arglist;
  va_start(arglist, fmt);
  vprintf(fmt, arglist);
  va_end(arglist);
}

extern void log_debug(const char *fmt, ...) __attribute__((weak, alias("log_default")));
extern void log_info(const char *fmt, ...) __attribute__((weak, alias("log_default")));
extern void log_warning(const char *fmt, ...) __attribute__((weak, alias("log_default")));

renaudcerrato

__attribute__((constructor)) 
void hello() {
  printf("Hello!\n");
}

__attribute__((destructor))
void goodbye() {
  printf("Goodbye!\n");
}

renaudcerrato

#define STATIC_ASSERT(COND,MSG)   typedef char static_assertion_##MSG[(!!(COND))*2-1]
#define COMPILE_TIME_ASSERT3(X,L) STATIC_ASSERT(X,static_assertion_at_line_##L)
#define COMPILE_TIME_ASSERT2(X,L) COMPILE_TIME_ASSERT3(X,L)
#define COMPILE_TIME_ASSERT(X)    COMPILE_TIME_ASSERT2(X,__LINE__)

COMPILE_TIME_ASSERT(sizeof(long) == 8);

int main() {
  COMPILE_TIME_ASSERT(sizeof(int) == 4); 
}