Assigning memory to double pointer?

Question

I am having trouble understanding how to assign memory to a double pointer. I want to read an array of strings and store it.

    char **ptr;
    fp = fopen("file.txt","r");
    ptr = (char**)malloc(sizeof(char*)*50);
    for(int i=0; i<20; i++)
    {
       ptr[i] = (char*)malloc(sizeof(char)*50);
       fgets(ptr[i],50,fp);
    }

instead of this I just assign a large block of memory and store the string

  char **ptr;
  ptr = (char**)malloc(sizeof(char)*50*50);

would that be wrong? And if so why is it?

Answer 1

Your second example is wrong because each memory location conceptually would not hold a char* but rather a char. If you slightly change your thinking, it can help with this:

char *x;  // Memory locations pointed to by x contain 'char'
char **y; // Memory locations pointed to by y contain 'char*'

x = (char*)malloc(sizeof(char) * 100);   // 100 'char'
y = (char**)malloc(sizeof(char*) * 100); // 100 'char*'

// below is incorrect:
y = (char**)malloc(sizeof(char) * 50 * 50);
// 2500 'char' not 50 'char*' pointing to 50 'char'

Because of that, your first loop would be how you do in C an array of character arrays/pointers. Using a fixed block of memory for an array of character arrays is ok, but you would use a single char* rather than a char**, since you would not have any pointers in the memory, just chars.

char *x = calloc(50 * 50, sizeof(char));

for (ii = 0; ii < 50; ++ii) {
    // Note that each string is just an OFFSET into the memory block
    // You must be sensitive to this when using these 'strings'
    char *str = &x[ii * 50];
}

Answer 2

 char **ptr;
    fp = fopen("file.txt","r");
    ptr = (char**)malloc(sizeof(char*)*50);
    for(int i=0; i<50; i++)
    {
       ptr[i] = (char*)malloc(sizeof(char)*50);
       fgets(ptr[i],50,fp);
    }

fclose(fp);

may be your typo mistake but your loop should be of 50 instead of 20 if you are looking for 50 x 50 matrix. Also after allocation of memory mentioned above you can access the buffer as ptr[i][j] i.e in the 2D format.