[pmwiki-devel] Testing empty string (was:php logic question)

[christian.ridderstrom at gmail.com]

On Wed, 14 Mar 2007, Hans wrote:

> Wednesday, March 14, 2007, 9:00:29 AM, christian wrote:
>
>> but as it's not NULL
>
> BTW, what is the difference between NULL and "" (and 0 for that matter)?

Historically speaking, i.e. the language C, NULL and 0 are the same. 
That's for C though, it's probably different in other languages.

In C, it's used with pointers to signify that the pointer is invalid. If 
you for instance requested some memory using

 	char *p;		/* Declare a pointer to a character */
 	p = malloc(100);	/* Try to allocate some memory */

 	/* Check if we got the memory */
 	if(p == NULL) { /* Error */ }

 	free(p);		/* Release the memory */

In C, NULL and 0 are equivalent, so it's ok[*] to write

 	if(p == 0) { /* Error */ }

To confuse things, strings are byte sequences that are terminated by the 
byte 0. :-) So an empty string in C would be a pointer (non-zero!) that 
points to a byte that is zero.

As a curiosity, the memory area with offset (0) when a C-program runs 
typically contains four zeros. The reason is so that the program can 
detect if (non-zero) data has accidentally been written using a NULL 
pointer.

/Christian

[*] Some older compilers and memory models (think DOS!) might require '0L' 
for a long zero instead of just '0', depending on the exact type of your 
pointer (near, far, huge), but this is getting into the details.

-- 
Christian Ridderström, +46-8-768 39 44               http://www.md.kth.se/~chr