autotmp.c

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

struct tmpvar_hook {
  void *ptr;
  struct tmpvar_hook *next;
};

static void tmpvar_hook_free(struct tmpvar_hook **hook) {
  struct tmpvar_hook *h = *hook;

  while (h) {
    printf("freeing ptr %p\n", h->ptr);
    free(h->ptr);
    h = h->next;
  }
}

#define TMPVAR_ALLOC_MALLOC_THRESHOLD 64

#define TMPVAR_ALLOC(hook_, size_)                                             \
  ({                                                                           \
    void *__h_ptr__ = NULL;                                                    \
    if ((size_) <= TMPVAR_ALLOC_MALLOC_THRESHOLD) {                            \
      __h_ptr__ = alloca(size_);                                               \
      printf("%zu bytes is below thresh, alloca()->%p\n", (size_), __h_ptr__); \
    } else {                                                                   \
      __h_ptr__ = malloc(size_);                                               \
      if (__h_ptr__) {                                                         \
        struct tmpvar_hook *__h_tmp__ = alloca(sizeof(struct tmpvar_hook));    \
        __h_tmp__->ptr = __h_ptr__;                                            \
        __h_tmp__->next = hook_;                                               \
        hook_ = __h_tmp__;                                                     \
        printf("%zu bytes is *above* thresh, malloc()->%p\n", (size_),         \
               __h_ptr__);                                                     \
      }                                                                        \
    }                                                                          \
    __h_ptr__;                                                                 \
  })

int main(void) {
  struct tmpvar_hook *hook __attribute__((cleanup(tmpvar_hook_free))) = NULL;

  for (int i = 1; i < 1000; i += 10) {
    void *ptr = TMPVAR_ALLOC(hook, i);
  }
}