c-extensions.md

C Extensions

1. GCC / CLANG. auto cleanup on scope exit.

#define auto_free_cstr __attribute__((cleanup(f_cleanup_cstr)))

void f_cleanup_cstr(char **s) { 
  if(s && *s) 
    free(*str); 
}

int main(void) {
  auto_free_cstr char *s = malloc(10);
  return 0;                     // Great! s is freed when it exit its scope   
}

1. GCC / CLANG / ICC. labels as values / indirect goto. This can be used as dispatch tables.

int main(){
  int value  = 2;
  const void *labels[] = {&&val_0, &&val_1, &&val_2};
  goto *labels[value];
  val_0:
      printf("The value is 0\n");
      goto end;
  val_1:
      printf("The value is 1\n");
      goto end;
  val_2:
      printf("The value is 2\n");
      goto end;
  end:
  return 0;
}

1. GCC / CLANG. statement expression

#define MAX(a,b)                    \
    ({                              \
    typeof (a) _a = (a);            \
    typeof (b) _b = (b);            \
    _a > _b ? _a : _b; })

int b=56;
int c= ({int a; a=sin(b); a});

1. GCC / CLANG. __builtin_expect optimization flag

#define likely(x)     __builtin_expect(!!(x), 1)
#define unlikely(x)   __builtin_expect(!!(x), 0)

int foo(int i){
  if(likely(i == 0))
      return 1;
  return 2;
}

1. GCC / CLANG. __builtin_prefetch optimization flag

__builtin_prefetch(&data[offset], 1); /* 0 - read only, 1 - rw */

1. GCC / CLANG?. __attribute__((weak)) linker level dynamic overriding

main.c :

void __attribute__((weak)) f();
int main(void) {
  if (f)
    f();
  return 0;
}

lib_f.c :

#include <stdio.h>
void __attribute__((weak)) f() {
  printf("lib f..\n");
}

ovrd_f.c :

#include <stdio.h>
void f() {
  printf("overridden f!\n");
}

gcc main.c -o main && ./main : (nothing but no linker error)

gcc main.c lib_f.c -o main && ./main : lib f..

gcc main.c ovrd_f.c -o main && ./main : overridden f!

gcc main.c lib_f.c ovrd_f.c -o main && ./main : overridden f!

Note, you can have only one strong reference, but you can have multiple weak reference. In the case with multiple-weak reference and no strong reference a random weak function will be chosen.