Programming Forums > Application Development > C

cCj · Apr 26th, 2006, 11:24 PM

Hi,
strcpy is a one way to copy strings but what if I need to make sure that the dest string (buffer) is large enough to hold the source string, is there a function for that?
I could just check the length of the source string can compare that to the predefined size of destination buffer but I assume there is already function for this.

Thank you.

Mad_guy · Apr 27th, 2006, 12:02 AM

IIRC, there's no function to do that, if you're dealing with a buffer obtained from malloc you can just realloc it to the appropriate size, though. If you're not afraid of just truncating the value you can do:

(Toggle Plain Text)

strncpy(destination,source,sizeof(source)-1);
source[sizeof(source)-1] = 0;

That'll make sure that the string is copied safely and that the string is null terminated (many str functions like strncat and strncpy don't ensure null termination which can still lead to arbitrary code execution or nothing based on adjacent buffers.)

The Dark · Apr 27th, 2006, 12:32 AM

Quote:

Originally Posted by Mad_guy

IIRC, there's no function to do that, if you're dealing with a buffer obtained from malloc you can just realloc it to the appropriate size, though. If you're not afraid of just truncating the value you can do:

(Toggle Plain Text)

strncpy(destination,source,sizeof(source)-1);
source[sizeof(source)-1] = 0;

That'll make sure that the string is copied safely and that the string is null terminated (many str functions like strncat and strncpy don't ensure null termination which can still lead to arbitrary code execution or nothing based on adjacent buffers.)

I think you mean

(Toggle Plain Text)

strncpy(destination,source,sizeof(destination)-1);
destination[sizeof(destination)-1] = 0;

Also, this will only work if destination is an array, not a pointer. If it is a pointer, sizeof will return 4 (for a 32 bit system).

DaWei · Apr 27th, 2006, 7:32 AM

You can use 'strdup' if you accept responsibility for freeing the memory when you're through.

cCj · Apr 27th, 2006, 7:57 AM

Thanks for the replies but I know that option but you see what I'm doing is receiving IRC messages from clients but if the client sends very long messages I don't want to just allocate more space. I could just reject the over sized (flood) messages.

But anyway, thanks again, I can continue from my own from here.

Narue · Apr 27th, 2006, 8:20 PM

>but if the client sends very long messages
It's generally a bad idea to have built-in limitations.

>I could just reject the over sized (flood) messages.
You could terminate the program too, and get about the same reaction from your user base.

cCj · Apr 28th, 2006, 10:40 PM

Quote:

Originally Posted by Narue

>I could just reject the over sized (flood) messages.
You could terminate the program too, and get about the same reaction from your user base.

But then if one client sends an oversized message all the client apps would be terminated since none could handle it.

Narue · Apr 29th, 2006, 8:38 AM

>But then if one client sends an oversized message all the client apps would be
>terminated since none could handle it.
Methinks you missed the point entirely.

cCj · Apr 30th, 2006, 9:08 AM

Quote:

Originally Posted by Narue

>But then if one client sends an oversized message all the client apps would be
>terminated since none could handle it.
Methinks you missed the point entirely.

Lol and I still don't know.

Any hints?

jayme · Apr 30th, 2006, 10:21 AM

I think he means rejecting data would be the same as removing the client from the server, so why keep the client in the server?

Apr 26th, 2006, 11:24 PM	#1
cCj Newbie Join Date: Mar 2006 Posts: 11 Rep Power: 0	Safe string copy? Hi, strcpy is a one way to copy strings but what if I need to make sure that the dest string (buffer) is large enough to hold the source string, is there a function for that? I could just check the length of the source string can compare that to the predefined size of destination buffer but I assume there is already function for this. Thank you.

Apr 27th, 2006, 12:02 AM	#2
Mad_guy Hobbyist Programmer Join Date: Oct 2004 Location: Sandstorm, Techno Club Posts: 239 Rep Power: 5	IIRC, there's no function to do that, if you're dealing with a buffer obtained from malloc you can just realloc it to the appropriate size, though. If you're not afraid of just truncating the value you can do: (Toggle Plain Text) strncpy(destination,source,sizeof(source)-1); source[sizeof(source)-1] = 0; strncpy(destination,source,sizeof(source)-1); source[sizeof(source)-1] = 0; That'll make sure that the string is copied safely and that the string is null terminated (many str functions like strncat and strncpy don't ensure null termination which can still lead to arbitrary code execution or nothing based on adjacent buffers.) __________________ os: mac os 10.5.4 revision control: git editor: emacs site

Apr 27th, 2006, 7:32 AM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You can use 'strdup' if you accept responsibility for freeing the memory when you're through. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Apr 27th, 2006, 8:20 PM	#6
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	>but if the client sends very long messages It's generally a bad idea to have built-in limitations. >I could just reject the over sized (flood) messages. You could terminate the program too, and get about the same reaction from your user base. __________________ Even if the voices aren't real, they have some pretty good ideas.

Apr 29th, 2006, 8:38 AM	#8
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	>But then if one client sends an oversized message all the client apps would be >terminated since none could handle it. Methinks you missed the point entirely. __________________ Even if the voices aren't real, they have some pretty good ideas.

Apr 27th, 2006, 7:57 AM	#5
cCj Newbie Join Date: Mar 2006 Posts: 11 Rep Power: 0	Thanks for the replies but I know that option but you see what I'm doing is receiving IRC messages from clients but if the client sends very long messages I don't want to just allocate more space. I could just reject the over sized (flood) messages. But anyway, thanks again, I can continue from my own from here.

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Programming Forums > Application Development > C

Safe string copy?