casting char[][] to char** causes segfault?

Question

Ok my C is a bit rusty but I figured I\'d make my next(small) project in C so I could polish back up on it and less than 20 lines in I already have a seg fault.

This

Answer 1

When you do this declaration:

char main_map[ROWS][COLS+1]={
  "a.bb",
  "a.c.",
  "adc.",
  ".dc."};

You create an array-of-arrays-of-char. An array-of-char is just a block of chars, and an array-of-arrays is just a block of arrays - so overall, main_map is just a whole heap of chars. It looks like this:

| 'a' | '.' | 'b' | 'b' | 0 | 'a' | '.' | 'c' | '.' | 0 | ... | 'd' | 'c' | '.' | 0 |

When you pass main_map to print_map(), it is evaluating main_map as a pointer to the first element of the array - so this pointer is pointing at the start of that block of memory. You force the compiler to convert this to type char **.

When you evaluate map[0] within the function (eg. for the first iteration of the loop), it fetches the char * value pointed to by map. Unfortunately, as you can see from the ASCII-art, map doesn't point to a char * - it points to a bunch of plain char s. There's no char * values there at all. At this point, you load some of those char values (4, or 8, or some other number depending on how big char * is on your platform) and try to interpret those as a char * value.

When puts() then tries to follow that bogus char * value, you get your segmentation fault.