How are multi-dimensional arrays formatted in memory?

by

A static two-dimensional array looks like an array of arrays – it’s just laid out contiguously in memory. Arrays are not the same thing as pointers, but because you can often use them pretty much interchangeably it can get confusing sometimes. The compiler keeps track properly, though, which makes everything line up nicely. You do have to be careful with static 2D arrays like you mention, since if you try to pass one to a function taking an int ** parameter, bad things are going to happen. Here’s a quick example:

int array1[3][2] = {{0, 1}, {2, 3}, {4, 5}};

In memory looks like this:

0 1 2 3 4 5

exactly the same as:

int array2[6] = { 0, 1, 2, 3, 4, 5 };

But if you try to pass array1 to this function:

void function1(int **a);

you’ll get a warning (and the app will fail to access the array correctly): 

warning: passing argument 1 of ‘function1’ from incompatible pointer type

Because a 2D array is not the same as int **. The automatic decaying of an array into a pointer only goes “one level deep” so to speak. You need to declare the function as:

void function2(int a[][2]);

or

void function2(int a[3][2]);

To make everything happy.

This same concept extends to n-dimensional arrays. Taking advantage of this kind of funny business in your application generally only makes it harder to understand, though. So be careful out there.

More Related Contents:

  1. Find the Biggest Number in C, BUT with characters
  2. How dangerous is it to access an array out of bounds?
  3. How do I determine the size of my array in C?
  4. Getting a stack overflow exception when declaring a large array
  5. Is using flexible array members in C bad practice?
  6. Arrays, heap and stack and value types
  7. Pointer address in a C multidimensional array
  8. Measuring Cache Latencies
  9. Using Dynamic Memory allocation for arrays
  10. Why would you ever want to allocate memory on the heap rather than the stack? [duplicate]
  11. dynamic allocating array of arrays in C
  12. Why does “memset(arr, -1, sizeof(arr)/sizeof(int))” not clear an integer array to -1?
  13. Resizing an array with C
  14. How many chars can be in a char array?
  15. C-programming board game [duplicate]
  16. Returning an array using C
  17. Determine size of dynamically allocated memory in C
  18. Why does a C-Array have a wrong sizeof() value when it’s passed to a function? [duplicate]
  19. Passing a multidimensional variable length array to a function
  20. Does stack grow upward or downward?
  21. Allocate memory 2d array in function C
  22. Array Type – Rules for assignment/use as function parameter
  23. Obtaining peak bandwidth on Haswell in the L1 cache: only getting 62%
  24. reading a text file into an array in c
  25. C – Difference between “char var[]” and “char *var”?
  26. What do square brackets mean in array initialization in C?
  27. Literal string initializer for a character array
  28. How can I work with dynamically-allocated arbitrary-dimensional arrays?
  29. C Function to Convert float to byte array
  30. C split a char array into different variables

Leave a Comment