A question about sizeof()

[yuzhangoscar]

Hi all:

I need to measure the size of an alphanumeric string. But I found out when I used sizeof() on the following strings:

1: VIEW1
2: VIEW10

The result I got was 4 for both of them. Can anyone point out why and which function I should use to measure the size of an alphanumeric string.

Thank you

[QuantumPete]

You want strlen(). Sizeof gives you the size of the variable. I'm guessing you call it on a char*, and on a 32bit system, pointers are 4 bytes. Strlen() runs through your char* until it encounters a \0 character and then returns the number of characters read

QuantumPete

[yuzhangoscar]

You got it exactly right.
I was so confused.
I am very glad to have helpful people like you around.

Cheers

[MK27]

sizeof measures the size of a data type, not a specific instance. It exists because the size required to store, eg. a long int, may differ from place to place (altho I think it rarely does) and, more importantly, for checking the size of unusual data types that may be part of a non-standard library (such as a widget). Since a "char" is four bytes, you will always get the same answer.

However, testing a char array with sizeof does return the number of characters:

char this[]="blah blah blah";
int x = sizeof(this);

x should be 15, because it includes the null terminator (\0).

But I think the normative way to measure a string is with strlen:

char this[]="blah blah blah";
int x = strlen(this);

x should be 14, because it doen't include that extra character.

[zacs7]

> Since a "char" is four bytes, you will always get the same answer.
You mean 1 byte ? The standard defines, sizeof char will always result in 1.

> a long int, may differ from place to place (altho I think it rarely does)
It does on 64-bit Linux vs 64-bit Windows (speaking generically, of the 'typical' compilers).

[QuantumPete]

However, testing a char array with sizeof does return the number of characters:

Not true. It only works for the example you've given where you initialize a variable size array with a string literal. If I declare a 1000 character array to hold user input, calling sizeof on that will always return 1000 no matter how long/short the user input.

QuantumPete

[MK27]

> Since a "char" is four bytes, you will always get the same answer.
You mean 1 byte ? The standard defines, sizeof char will always result in 1.

Oh yeah. I suppose I meant sizeof(char*)

QuantumPete makes an key point too:
If I declare a 1000 character array to hold user input, calling sizeof on that will always return 1000 no matter how long/short the user input.

[laserlight]

Not true. It only works for the example you've given where you initialize a variable size array with a string literal. If I declare a 1000 character array to hold user input, calling sizeof on that will always return 1000 no matter how long/short the user input.

I'd say MK27 is right: "testing a char array with sizeof does return the number of characters". Testing a null terminated string (whether given a char array or a pointer) with sizeof, on the other hand, does not return the number of characters in the string (except for the special case where it is equal to the sizeof the pointer).

Incidentally, the array is not a variable size (length) array, but one that is fixed with a size of 15 at compile time.

[QuantumPete]

I'd say MK27 is right: "testing a char array with sizeof does return the number of characters".

It returns the number of possible characters, but not the number of characters in your string (if we consider the string ending with its NULL terminator).

QuantuMPete

[laserlight]

It returns the number of possible characters, but not the number of characters in your string (if we consider the string ending with its NULL terminator).

A char array is not necessarily a null terminated string.

[QuantumPete]

A char array is not necessarily a null terminated string.

My point exactly; since the OP wanted to find the size of his string, he needs to call strlen and not sizeof.

QuantumPete

[laserlight]

My point exactly; since the OP wanted to find the size of his string, he needs to call strlen and not sizeof.

Yes, but your other point was that MK27 was wrong to say that "testing a char array with sizeof does return the number of characters", which is incorrect. Rather, MK27 was wrong to regard the set of char arrays and the set of null terminated strings as equivalent.