Lesser known tricks, quirks and features of C(blog.joren.ga)

  // traditional syntax:
  boxes[products[myorder.product].box].weight
  // index[array]:
  myorder.product[products].box[boxes].weight

   typedef void func(int);
   func f;
   void f(int) {}

    int foo();
    decltype(foo) bar;

  int n = 3, m = 4;
  int (*matrix_NxM)[n][m] = malloc(sizeof *matrix_NxM); // `n` and `m` are variables with dimensions known at runtime, not compile time
  if (matrix_NxM) {
      // (*matrix_NxM)[i][j] = ...;
      free(matrix_NxM);
  }

  #ifndef UTIL_H
  #define UTIL_H

  #include <errno.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>

  #define LEN(arr) (sizeof (arr) / sizeof (arr)[0])

  #define NEW_ARRAY(ptr, n) \
     (ptr) = malloc((n) * sizeof (ptr)[0]); \
     if ((ptr) == NULL) { \
        fprintf(stderr, "Memory allocation failed: %s\n", strerror(errno)); \
        exit(EXIT_FAILURE); \
     }

  #define NEW(ptr) NEW_ARRAY(ptr, 1)

  #endif

  int (*ap3)[900000] = malloc(sizeof *ap3);

  int *a = malloc(900000 * sizeof *a);