strcat() crashes if using same array as both parameters

Question

char r[40];
strcpy(r,"abcdef");
strcat(r,r);

My program crashes at the third line?

Replacing strcat(r,r); by strcat(r,"abcdef"); works fine though.... why is that?

Answer 1

According to strcat(3):

The strcat() function appends the src string to the dest string, overwriting the terminating null byte ('\0') at the end of dest, and then adds a terminating null byte. The strings may not overlap, and the dest string must have enough space for the result.