Thread: how sizeof works and how to useinet_pton faster?

Hello,

please be easy with me because I have struggling learning the C language and bad english.

sizeof question:

I don't understand how to figure this without compile.

Code:

char x[4];
sizeof(x)    -> gives 4
sizeof(&x)  -> gives 8, why?

Code:

struct sockaddr_in sa;
sizeof(sa)   -> gives 16, why?
sizeof(&sa) -> gives 8, why?

Code:

unsigned long x;
sizeof(x)   -> gives 8, why?
sizeof(&x) -> gives 8, why?

how is it possible that they are same?


inet_pton: which one of them is the faster? :

my goal is given ipv4 address is valid and for that I found more than one solution using inet_pton function, I want to use the fatest solution.


Option 1:
Sending address of (unsigned long) to third parameter in inet_pton function.
Like shown here: C - IP address validation

Option 2
Sending the address of sockaddr_in to third parameter in inet_pton function.
Like shown here: Determine if a string is a valid IPv4 address in C - Stack Overflow

Option 3:
Creating an array of char (4 size), and sending it to third parameter in inet_pton function.
Beause it's doing memset 4 times: inet_pton.c source code [glibc/resolv/inet_pton.c] - Woboq Code Browser

Which one of the options is faster?

I ran benchmark using perf tool and it appears that Option 2 is the fatest as it run less time.

.

Originally Posted by laserlight

Pointers to objects don't necessarily have to be of different suzes: a pointer to a char could be the same size as a pointer to a struct object containing an array of a thousand chars, as long as the pointer can point to the memory occupied by the objects.


You seem to have answered your own question.

I don't have any reputation here but that is a derogatory comment

Originally Posted by Teeth TriHard

I don't have any reputation here but that is a derogatory comment

Derogatory?! Why?

Pointers have, indeed, in most platforms, the same size. A pointer holds a memory address...
And, if you did the test with perf and found an answer, you really seem to have answered your own question, didn't you?

Originally Posted by laserlight

Well, if you're not satisfied by the result of your own measurement, and would like a definitive answer from the standard in which inet_pton is defined, then the answer is: none of them are guaranteed to be the fastest; it depends on the implementation.

I tested with perf again and the elapsed time is not constant, running it couple times doesn't give me a close result to the previous one.


I'm still struggling with the first question. I understood that using & on a variable will give me the addess of it but not with the sizeof, how it works, why those are different? Could you explain it ? if not point a references please?

Code:

char x[4];
sizeof(x)    -> gives 4
sizeof(&x)  -> gives 8, why?

Code:

struct sockaddr_in sa;
sizeof(sa)   -> gives 16, why?
sizeof(&sa) -> gives 8, why?

Code:

unsigned long x;
sizeof(x)   -> gives 8, why?
sizeof(&x) -> gives 8, why?

Code:

// 4 byte object, 8 byte (64bit) address
char x[4];
sizeof(x)    -> gives 4
sizeof(&x)  -> gives 8

// 16 byte object, 8 byte address
struct sockaddr_in sa;
sizeof(sa)   -> gives 16
sizeof(&sa) -> gives 8

// 8 byte object, 8 byte address
unsigned long x;
sizeof(x)   -> gives 8
sizeof(&x) -> gives 8

Originally Posted by Guest

Code:

// 4 byte object, 8 byte (64bit) address
char x[4];
sizeof(x)    -> gives 4
sizeof(&x)  -> gives 8

// 16 byte object, 8 byte address
struct sockaddr_in sa;
sizeof(sa)   -> gives 16
sizeof(&sa) -> gives 8

// 8 byte object, 8 byte address
unsigned long x;
sizeof(x)   -> gives 8
sizeof(&x) -> gives 8

Thanks! That is much clearer