Question

Tell me please why this function may take '\0' as second argument, compile and crash?

char var[9];
/*some initialization of var */
strcat(var, '\0');

Answer 1

That function takes a const char * as the second parameter. You’re passing an int.

char *strcat(char *restrict s1, const char *restrict s2);

You’re basically tricking strcat into dereferencing \0 as a pointer – it dereferences NULL.