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#include <stdlib.h>  /* For malloc() and size_t */
 
struct flexArrayStruct {
  int num;
  int data[1];
};

/* ... */

size_t array_size;
size_t i;

void func(void) {
  /* Initialize array_size */
  /* Space is allocated for the struct. */
  struct flexArrayStruct *structP
    = (struct flexArrayStruct *)
     malloc(sizeof(struct flexArrayStruct)
          + sizeof(int) * (array_size - 1));
  if (structP == NULL) {
    /* Handle malloc failure */
  }
  structP->num = 0;

  /* Access data[] as if it had been allocated
   * as data[array_size] */
  for (i = 0; i < array_size; i++) {
    structP->data[i] = 1;
  }

}

#include <stdlib.h>  /* For malloc() and size_t */
 
struct flexArrayStruct{
  int num;
  int data[];
};

/* ... */

size_t array_size;
size_t i;

void func(void) {
  /* Initialize array_size */
  /* Space is allocated for the struct. */
  struct flexArrayStruct *structP = (struct flexArrayStruct *)
     malloc(sizeof(struct flexArrayStruct) + sizeof(int) * array_size);
  if (structP == NULL) {
    /* Handle malloc failure */
  }

  structP->num = 0;

  /* Access data[] as if it had been allocated
   * as data[array_size]
   */
  for (i = 0; i < array_size; i++) {
    structP->data[i] = 1;
  }

}