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 is my complete code:

#define ROWS 4
#define COLS 4

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

 void print_map(char** map){
  int i;
  for(i=0;i<ROWS;i++){
    puts(map[i]); //segfault here
  }
 }



int main(){
  print_map(main_map); //if I comment out this line it will work.
  puts(main_map[3]);
  return 0;
}

I am completely confused as to how this is causing a segfault. What is happening when casting from [][] to **!? That is the only warning I get.

rushhour.c:23:3: warning: passing argument 1 of ‘print_map’ from incompatible pointer type
rushhour.c:13:7: note: expected ‘char **’ but argument is of type ‘char (*)[5]’

Are [][] and ** really not compatible pointer types? They seem like they are just syntax to me.

Answer 1

A char[ROWS][COLS+1] cannot be cast into a char**. The input argument of print_map should be

void print_map(char map[][COLS+1])

or

void print_map(char (*map)[COLS+1])

---

The difference being that a char** means to point to something that can be dereferenced like this:

   (char**)map
       |
       v
  +--------+--------+------+--------+-- ...
  | 0x1200 | 0x1238 | NULL | 0x1200 |
  +----|---+----|---+--|---+----|---+-- ...
       v        |      =        |
    +-------+   |               |
    | "foo" | <-----------------'
    +-------+   |
                v
             +---------------+
             | "hello world" |
             +---------------+

While a char(*)[n] is a points to a continuous memory region like this

   (char(*)[5])map
       |
       v
  +-----------+---------+---------+-------------+-- ...
  | "foo\0\0" | "hello" | " worl" | "d\0\0\0\0" |
  +-----------+---------+---------+-------------+-- ...

If you treat a (char(*)[5]) as a (char**) you get garbage:

   (char**)map
       |
       v
  +-----------+---------+---------+-------------+-- ...
  | "foo\0\0" | "hello" | " worl" | "d\0\0\0\0" |
  +-----------+---------+---------+-------------+-- ...
      force cast (char[5]) into (char*):
  +----------+------------+------------+------------+-- ...
  | 0x6f6f66 | 0x6c686500 | 0x77206f6c | 0x646c726f |
  +----|-----+---------|--+------|-----+------|-----+-- ...
       v               |         |            |
    +---------------+  |         |            v
    | "hsd®yœâñ~22" |  |         |       launch a missile
    +---------------+  |         |
                       v         v
               none of your process memory
                        SEGFAULT