Question

The question is how to get the length of dynamically allocated 2D Arrays in C? I thought the code below should get the number of rows, but it doesn’t.

char** lines;
/* memory allocation and data manipulation */

int length; //the number of rows

length = sizeof(lines)/sizeof(char*);

Any thoughts on this?

Answer 1

You can’t get the length of dynamically allocated arrays in C (2D or otherwise). If you need that information save it to a variable (or at least a way to calculate it) when the memory is initially allocated and pass the pointer to the memory and the size of the memory around together.

In your test case above sizeof is returning the size of the type of lines, and thus your length calculation is equivalent to sizeof(char**)/sizeof(char*) and is likely to have the trivial result of 1, always.