[C/C++] What is the quickest/most efficient way to clear an array?

[A4B4]

So far I've been using:

memset(myarray, 0, sizeof(myarray));

Most of the time, myarray is just a character array, so I fill it full of null characters. However, is there a better way to do this? (It almost seems this is more suited to clearing structures or objects.)

Should I be using a string function? Or, should I use strlen() instead of sizeof()?

Or, at the very least is memset any more or any less efficient that a standard for loop to clear an array? I ask this because I might want to set myarray to a value other than null within the program.

Thanks in advance!

Edit: And yes, performance is quite important for my application.

[XerXis]

delete [] arrayname;

?

that's how i would clear an array :p

oh forgot: if you need to fill a static array with some value i would use std::fill instead of memset

Edited March 8, 2008 by XerXis

[Sterling Christensen]

The only way to be sure is to try both and compare benchmarks. If these strings are ridiculously huge then memset should always win, because memset is usually optimized (it doesn't do it one byte at a time). It depends on a lot of things like whether the compiler you're using inlines memset or not.

BTW: if these character arrays are strictly null terminated strings, then setting only the first character to null should be enough and much faster.

Edited March 8, 2008 by Sterling Christensen

[Sterling Christensen]

oh forgot: if you need to fill a static array with some value i would use std::fill instead of memset

With gcc's standard library, std::fill reduces to the alternatives he's already considering anyway - either a for loop or memset:

* This function fills a range with copies of the same value. For one-byte

* types filling contiguous areas of memory, this becomes an inline call to

* @c memset.

See http://www.cs.brown.edu/~jwicks/libstdc++/...rce.html#l00516

It's probably similar with other compilers like Microsoft's.

[Mike]

if its a char array, why clear it all?

if you're going to be keeping "strings" in it, you'd be add a null terminating character after the string anyway so I normally just set the first byte to null.

[Brandon Live Veteran]

Do you explicitly need to zero out the buffer? Or is it enough to simply free it?

[Mike]

Do you explicitly need to zero out the buffer? Or is it enough to simply free it?

surely to free it, it would have to have been malloc/calloc/realloc'd and so would just be a simple point and so his current method using sizeof() wouldn't work

[Brandon Live Veteran]

if its a char array, why clear it all?

if you're going to be keeping "strings" in it, you'd be add a null terminating character after the string anyway so I normally just set the first byte to null.

Uhh, that doesn't work if you allocated the array on the heap... and it doesn't actually free anything up on the stack, since that entire memory block is still allocated on the stack until it goes out of scope. The only reason to do that is if you have a null-terminated string on the stack that you want to make "empty." It neither frees it nor zeros it out.

surely to free it, it would have to have been malloc/calloc/realloc'd and so would just be a simple point and so his current method using sizeof() wouldn't work

If freeing it is all that you care about...

If it's allocated on the heap, you need to free it using whatever allocator you allocated it with. If you used new, you need to use delete[].

If it's allocated on the stack, it's there until it goes out of scope...

In either case, if you want to explicitly zero it out, that's another matter (since free'ing it doesn't actually reset the bits until somebody else allocated that memory and uses it).

[Mike]

Uhh, that doesn't work if you allocated the array on the heap... and it doesn't actually free anything up on the stack, since that entire memory block is still allocated on the stack until it goes out of scope. The only reason to do that is if you have a null-terminated string on the stack that you want to make "empty." It neither frees it nor zeros it out.

He didn't ask about freeing it, he wants to clear it. He wants the memory allocation there, just not the contents. What i said will work for a null terminated string. When working with a string you'd be using strlen and so setting the first byte to null will make strlen return 0 and be no different to if the whole char array was full of 0s.

If freeing it is all that you care about...

If it's allocated on the heap, you need to free it using whatever allocator you allocated it with. If you used new, you need to use delete[].

If it's allocated on the stack, it's there until it goes out of scope...

In either case, if you want to explicitly zero it out, that's another matter (since free'ing it doesn't actually reset the bits until somebody else allocated that memory and uses it).

I know that, when you use *alloc to allocate memory on the heap you only get a pointer back which, when used with sizeof(), won't give you how much memory is allocated. Seeing as he is using that method to get the size, it is more likely to be a simple char[] on the stack and so won't be able to free it (if that is even what he wanted, which I don't think it is...)

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[Brandon Live Veteran]

He didn't ask about freeing it, he wants to clear it. He wants the memory allocation there, just not the contents. What i said will work for a null terminated string. When working with a string you'd be using strlen and so setting the first byte to null will make strlen return 0 and be no different to if the whole char array was full of 0s.

But it is different, and that's why I asked about his usage. If he believed there was sensitive data there that he wanted to clear our of memory as soon as possible (encryption keys / passwords, or who knows what else) then he may in fact desire to zero the entire buffer.