Comment by tcas

Nope, neither will segfault on the initial access.

  char ** argv

is a null terminated array in C according to the C standard. (EDIT: see clarification below) C doesn't care about types at all and you can abuse this by casting pointers to different (sometimes incompatible types). The second argument to main (argv) it set up by the initialization code and has the same space as a pointer regardless if it's a string, array of strings, struct, integer etc...

The existing structure will simply be interpreted as that new type like so:

                String1       String2    String3     String4
  char ** a2: |0x41414141 |0x42424242 |0x43434343 |0x00000000 |
              +-----------+-----------+-----------+-----------+
  char * a1 : |41|41|41|41|42|42|42|42|43|43|43|43|00|00|00|00|
  
  printf("%s\n", a1);
  > AAAABBBBCCCC

That being said, some compilers (clang) will emit an error on the improper type, like so:

  test.c:3:5: error: second parameter of 'main' (argument array) must be of type
        'char **'
  int main(int argc, char * argv)
      ^
  1 error generated.