将指针转换为 C 中的二维字符数组

Question

This probably has been asked already, but I'm unable to find anything on it.

I have a string array, where the numbers of strings in it is determined at runtime (the max string length is known, if that helps). Since I need global access to that array, I used a pointer and malloc'ed enough space to it when I actually know how much has to fit in there:

char *global_strings;

void some_func(int strings_nr, int strings_size)
{
    global_strings = (char*) malloc(strings_nr* strings_size* sizeof(char));
}

What would be the correct way in C to use this pointer like a two-dimensional char array equivalent to global_strings[strings_nr][strings_size] ?

Answer 1

As a global pointer to 2D data, whose N*M characteristics defined at run-time, I'd recommend a helper function to access the strings rather than directly use it. Make it inline or as a macro if desired.

char *global_strings = NULL;
size_t global_strings_nr = 0;
size_t global_strings_size = 0;

// Allocation -  
// OK to call again, but prior data may not be organized well with a new string_size
// More code needed to handle that.
void some_func(int strings_nr, int strings_size) {
  global_strings_nr = strings_nr;      // save for later use
  global_strings_size = strings_size;  // save for later use
  global_strings = realloc(global_strings,
      sizeof *global_strings * strings_nr * strings_size);
  if (global_strings == NULL) {
    global_strings_nr = global_strings_size = 0;
  }
}

// Access function
char *global_strings_get(size_t index)  {
  if (index >= global_strings_nr) {
    return NULL;
  }
  return global_strings + index*global_strings_size;
}

#define GLOBAL_STRINGS_GET_WO_CHECK(index) \
   (global_strings + (index)*global_strings_size)

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Better to use size_t for array indexing and sizing than int .

Casts not needed.

Memory calculations should begin with a size_t rather than int * int * size_t .