gistfile1.c

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>

size_t check_malloc_size(void* p, size_t n) {
  int fd[2];
  pid_t child;

  pipe(fd);
  child = fork();
  if (child == 0) {
    void* orig;
    void* dummy;
    dummy = malloc(1);

    for (orig = p; orig == p; p = realloc(p, ++n))
      ;

    free(dummy);
    write(fd[1], &n, sizeof(n));
    exit(EXIT_SUCCESS);
  } else {
    int status;
    read(fd[0], &n, sizeof(n));
    waitpid(child, &status, 0);
    return n - 1;
  }
  assert(0);
}

int main(void) {
  void* p;
  unsigned n;
  for (;;) {
    if (scanf("%u", &n) != 1)
      break;
    p = malloc(n);
    printf("%u\n", (unsigned) check_malloc_size(p, n));
    free(p);
  }

  return 0;
}