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?
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 char
s.
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];
}