Struct help...

MKirstensen · 07-01-2008, 12:44 PM

Hello World!

(struct sockaddr *)&my_addr

What does this line do?... I am still trying to learn C

MacGyver · 07-01-2008, 12:46 PM

It casts the address of my_addr to type struct sockaddr *. By itself, it's useless.

dwks · 07-01-2008, 05:38 PM

It's for network programming. If you're interested in learning more, I suggest you read Beej's network tutorial. http://beej.us/guide/bgnet/

It's rather advanced stuff, though. Don't try it until you have some experience with C.

MKirstensen · 07-02-2008, 06:00 AM

Quote:

Originally Posted by MacGyver

It casts the address of my_addr to type struct sockaddr *. By itself, it's useless.

So it takes the address of my_addr and place it in the structure sockaddr... Am i right?

MacGyver · 07-02-2008, 06:04 AM

No, you're wrong.

Elysia · 07-02-2008, 06:24 AM

(struct sockaddr*)&my_addr

Green = Tells the compiler it's a cast
Red = Tells the compiler what you're casting to
Blue = Address of operator: takes the address of a variable
Orange = Tells the compiler what you want to take the address of

The cast converts the expression on the right to the type specified.

MKirstensen · 07-02-2008, 07:45 AM

Thx alot! Elysia

slingerland3g · 07-02-2008, 08:31 AM

A bit of an advanced concept for just starting out learning C. If you are interested in network programming. "Unix Network Programming - The sockets network API" is a good read.

MKirstensen · 07-02-2008, 09:03 AM

Quote:

Originally Posted by MacGyver

It casts the address of my_addr to type struct sockaddr *. By itself, it's useless.

I THINK i get it now, it makes &my_addr act like struct sockaddr * , but how can a memory address act like a structure ?

Elysia · 07-02-2008, 09:05 AM

The thing is that your struct is just a block of raw memory in the eyes of the CPU or the computer.
The compiler just manages that memory for you.
And because memory is "raw," there are actually no "types". Your struct is just a piece of memory. The compiler keeps track of its type.
But you can tell the compiler that it's another type than it is. It works because it's just a block of memory.

MKirstensen · 07-02-2008, 09:26 AM

Quote:

Originally Posted by Elysia

The thing is that your struct is just a block of raw memory in the eyes of the CPU or the computer.
The compiler just manages that memory for you.
And because memory is "raw," there are actually no "types". Your struct is just a piece of memory. The compiler keeps track of its type.
But you can tell the compiler that it's another type than it is. It works because it's just a block of memory.

So i places the value of &my_addr in the struct sockaddr memory block.... right? and whats i the "*" good for?

Elysia · 07-02-2008, 10:26 AM

Again, because it's a pointer.
It simply tells the compiler to treat the data at the address in the pointer as something else.
There's a difference:

Code:

float f = 1.0f;
int n = (int)f; /* Converts the data inside f to an integer - result is 1. */
int* pN = (int*)&f; /* Tells the compiler to treat the data at the address where f resides as 
an int. The result will not be 1, but something much else. */

MKirstensen · 07-02-2008, 11:09 AM

Quote:

Originally Posted by Elysia

Again, because it's a pointer.
It simply tells the compiler to treat the data at the address in the pointer as something else.
There's a difference:

Code:

float f = 1.0f;
int n = (int)f; /* Converts the data inside f to an integer - result is 1. */
int* pN = (int*)&f; /* Tells the compiler to treat the data at the address where f resides as 
an int. The result will not be 1, but something much else. */

Why is the result not 1?...

MacGyver · 07-02-2008, 11:12 AM

Look, in all honesty, you're asking questions for things that require way more knowledge than you have at the moment and will likely have for some time. You need to buckle down and get a book or a tutorial and start from the basics and work your way up.

Elysia · 07-02-2008, 11:14 AM

Quote:

Originally Posted by MKirstensen

Why is the result not 1?...

Because, simply put, floats are not stored the same way ints are. That's why.

07-01-2008, 12:44 PM	#1
MKirstensen Registered User Join Date: Jul 2008 Location: Denmark Posts: 22	Struct help... Hello World! (struct sockaddr *)&my_addr What does this line do?... I am still trying to learn C
MKirstensen is offline

07-01-2008, 12:46 PM	#2
MacGyver Deathray Engineer Join Date: Mar 2007 Posts: 3,211	It casts the address of my_addr to type struct sockaddr *. By itself, it's useless. __________________
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07-01-2008, 05:38 PM	#3
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,629	It's for network programming. If you're interested in learning more, I suggest you read Beej's network tutorial. http://beej.us/guide/bgnet/ It's rather advanced stuff, though. Don't try it until you have some experience with C. __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, etc. New project: nort
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07-02-2008, 06:04 AM	#5
MacGyver Deathray Engineer Join Date: Mar 2007 Posts: 3,211	No, you're wrong. __________________
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07-02-2008, 07:45 AM	#7
MKirstensen Registered User Join Date: Jul 2008 Location: Denmark Posts: 22	Thx alot! Elysia
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07-02-2008, 08:31 AM	#8
slingerland3g Registered User Join Date: Jan 2008 Location: Seattle Posts: 476	A bit of an advanced concept for just starting out learning C. If you are interested in network programming. "Unix Network Programming - The sockets network API" is a good read.
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07-02-2008, 11:12 AM	#14
MacGyver Deathray Engineer Join Date: Mar 2007 Posts: 3,211	Look, in all honesty, you're asking questions for things that require way more knowledge than you have at the moment and will likely have for some time. You need to buckle down and get a book or a tutorial and start from the basics and work your way up. __________________
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