PTR = malloc (0 );

# Include <stdio. h> # include <stdlib. h> # include <string. h>/* to avoid errors: The size is dynamically allocated after being determined. Why? Malloc has different implementations on different compilers. Some apply for 0 to return NULL, and some can return an address. It seems that the latter is more common. When malloc allocates memory, in addition to allocating memory blocks of the specified size, it also allocates additional memory to store our memory block information for maintaining the memory block. In fact, the memory is sharded, which is equivalent to malloc (10); instead of giving you 10 bytes, It is a memory shard */intmain () {char * PTR = NULL; if (PTR = (char *) malloc (0) = NULL) printf ("null pointer \ n"); elseprintf ("valid pointer \ n ");} /* [[email protected] test_class] #. /. out valid pointer */