0 vs Null

**chico1st** · 04-24-2009

I am a little confused about the difference between zeros and nulls in C. My programming friend (who uses C++ if that matters) says that in C they are the same thing. But if i was reading values out of an array or reading an array wouldnt the array stop being read at a null? If they were the same my array would stop being read at a zero.

A problem arose when I was passing an array from another language into a C library. If I passed in zeros the C function would not write to the array but if i passed in ones for instance everything was fine.

Does this make any sense?

**BEN10** · 04-24-2009

Originally Posted by chico1st

I am a little confused about the difference between zeros and nulls in C. My programming friend (who uses C++ if that matters) says that in C they are the same thing. But if i was reading values out of an array or reading an array wouldnt the array stop being read at a null? If they were the same my array would stop being read at a zero.

A problem arose when I was passing an array from another language into a C library. If I passed in zeros the C function would not write to the array but if i passed in ones for instance everything was fine.

Does this make any sense?

the NULL has ASCII value of 0.on the other hand 0 is just an integer and array indexing starts with 0 in C.

**MK27** · 04-24-2009

Originally Posted by chico1st

I am a little confused about the difference between zeros and nulls in C. My programming friend (who uses C++ if that matters) says that in C they are the same thing.

They are the same thing in this sense: if you test the "truth" of a variable, it is considered false if the value is 0 or NULL, eg:

Code:

int X=0;
char *ptr=NULL;
if (X) {won't happen, because X is false};
if (!(ptr)) {will happen, because "ptr" is not(!) true}

But if i was reading values out of an array or reading an array wouldnt the array stop being read at a null? If they were the same my array would stop being read at a zero.

If you are talking about an array of ints, no, 0 is not null or ignored. It is the integer 0. If you are talking about a character string, '\0' (the null terminator) has a value of 0, and that is where the string is considered to end. So string functions will stop reading the array there. But the character 0 ('0', not '\0') is not the same thing (it actually has a ASCII value of 48).

**samf** · 04-24-2009

Originally Posted by chico1st

I am a little confused about the difference between zeros and nulls in C. My programming friend (who uses C++ if that matters) says that in C they are the same thing. But if i was reading values out of an array or reading an array wouldnt the array stop being read at a null? If they were the same my array would stop being read at a zero.

A problem arose when I was passing an array from another language into a C library. If I passed in zeros the C function would not write to the array but if i passed in ones for instance everything was fine.

Does this make any sense?

You may want to take a look at this:

Cprogramming.com FAQ > NULL, 0, \0 and nul?

**cpjust** · 04-24-2009

I believe NULL used to be defined as:

Code:

#define NULL  (void*)0

I'm not sure if that's still the case though.

**Snafuist** · 04-24-2009

Originally Posted by cpjust

I believe NULL used to be defined as:

Code:

#define NULL  (void*)0

I'm not sure if that's still the case though.

NULL can always be replaced with 0, as C guarantees to implicitly convert 0 to a void pointer to 0. Hence it's perfectly valid to define NULL this way:

Code:

#define NULL 0

The standard talks explicitly about that:

An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant.

When defining 0 as (void*)0, I'd suggest putting parentheses around the expression, i.e. ((void*)0).

Greets,
Philip

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