how to allocate large arrayes using C in linux

Question

Is there a way to allocate an array with this size:

unsigned long M[2000][900000] ;

This is what I get when I run the program (no errors during compilation).

Processus arrêté (Process stopped)  

Answer 1

unsigned long (*pM)[2000][900000] = malloc(sizeof *pM);

does the job.

Use it like this

#define ROWS_MAX (2000)
#define COLUMNS_MAX (900000)

...

unsigned long (*pM)[ROWS_MAX][COLUMNS_MAX] = malloc(sizeof *pM);

/* 1st test whether the allocation succeeded! */
if (NULL == pM)
{
  perror("malloc() failed");
  exit(EXIT_FAILURE);
}

/* Then initialise the array. */
for (size_t row = 0; row < ROWS_MAX; ++row)
{
  for (size_t column = 0; column < COLUMNS_MAX; ++column)
  {
    (*pM)[row][column] = 42;
  }
}

/* Do something ... */
...

/* Deallocate, free the memory. */
free(pM);

An alternative approach using more than one block or memory would be using a scattered/sparse array:

unsigned long ** ppM = malloc(ROWS_MAX * sizeof *ppM);
if (NULL == ppM)
{
  perror("malloc() for row pointers failed");
  exit(EXIT_FAILURE);
}

for (size_t row = 0; row < ROWS_MAX; ++row)
{
  ppM[row] = malloc(COLUMNS_MAX * sizeof *ppM[row]);
  if (NULL == ppM[row])
  {
    perror("malloc() for a column failed");
    exit(EXIT_FAILURE);
    /* If not exiting the process here (but probably return from the function
       we are in), we need to perform a clean-up on what had been allocated 
       so far. See below code for free()ing it as a hint how to approach this. */
  }
}

/* Then initialise the array. */
for (size_t row = 0; row < ROWS_MAX; ++row)
{
  for (size_t column = 0; column < COLUMNS_MAX; ++column)
  {
    ppM[row][column] = 42; /* Note the difference how to access the array. */
  }
}

/* Do something ... */
...

/* Deallocate, free the memory. */
/* Free columns. */
for (size_t row = 0; row < ROWS_MAX; ++row)
{
  free(ppM[row]);
}

/* Free row pointers. */
free(ppM);