Doubt in memcpy() and memset()

[Shhnwz.a]

Hi,
I want to know some of the situations , when to use memcpy() and when
to memset();

Thanx in Advance..

[Tom St Denis]

Shhnwz.a wrote:

Hi,
I want to know some of the situations , when to use memcpy() and when
to memset();

Thanx in Advance..

NAME
memcpy - copy memory area

NAME
memset - fill memory with a constant byte

HOMEWORKALITY!

[hint: man pages are fun]

Tom

[Chris Dollin]

Shhnwz.a wrote:

I want to know some of the situations , when to use memcpy() and when
to memset();

Can't you tell from their descriptions?

memcpy( void *s1, const void *s2, size_t n );

... copies n characters from the object pointed to by s2
into the object pointed to by s1. If copying takes place
between objects that overlap, the behaviour is undefined.

memset( void *s, int c, size_t n );

... copies the value of c (converted to an unsigned char)
into each of the first n characters of the object pointed
to by s.

Use whichever one fits your situation.

--
Chris "Perikles triumphant" Dollin
"No-one here is exactly what he appears." G'kar, /Babylon 5/

[Dead Loop]

Chris Dollin wrote:

>
memset( void *s, int c, size_t n );

... copies the value of c (converted to an unsigned char)
into each of the first n characters of the object pointed
to by s.

By the way, I'm puzzled about the type of the second parameter.
Why not use (unsigned char) type?
memset(void *s, unsigned char c, size_t n);

Thanks.

[Lew Pitcher]

Dead Loop wrote:

Chris Dollin wrote:

memset( void *s, int c, size_t n );

... copies the value of c (converted to an unsigned char)
into each of the first n characters of the object pointed
to by s.

By the way, I'm puzzled about the type of the second parameter.
Why not use (unsigned char) type?
memset(void *s, unsigned char c, size_t n);

Backwards compatibility.

Both memcpy() and memset() were defined back in the day of K&R C. In
the K&R C definition, char was automatically promoted to int when used
as a function argument.

[Random832]

2006-12-06 <11************ **********@j72g 2000cwa.googleg roups.com>,
Lew Pitcher wrote:

>
Dead Loop wrote:

>Chris Dollin wrote:

>
memset( void *s, int c, size_t n );

... copies the value of c (converted to an unsigned char)
into each of the first n characters of the object pointed
to by s.

By the way, I'm puzzled about the type of the second parameter.
Why not use (unsigned char) type?
memset(void *s, unsigned char c, size_t n);

Backwards compatibility.

Both memcpy() and memset() were defined back in the day of K&R C. In
the K&R C definition, char was automatically promoted to int when used
as a function argument.

yeah, but the size argument changed types. certainly nothing promotes to
a size_t, and there's no guarantee that size_t doesn't promote to
something.

[Tom St Denis]

Random832 wrote:

yeah, but the size argument changed types. certainly nothing promotes to
a size_t, and there's no guarantee that size_t doesn't promote to
something.

Does it matter? Most platforms won't let you push a char on the stack
anyways (disregarding the fact the standard makes no mention of
"stack").

So from a performance point of view it's moot.

Tom

[Kenneth Brody]

Tom St Denis wrote:

>
Random832 wrote:

yeah, but the size argument changed types. certainly nothing promotes to
a size_t, and there's no guarantee that size_t doesn't promote to
something.

Does it matter? Most platforms won't let you push a char on the stack
anyways (disregarding the fact the standard makes no mention of
"stack").

So from a performance point of view it's moot.

No, but if you are passing an int rather than unsigned char, it
may need to zero-fill the rest of the int before passing it.

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>

[Tom St Denis]

Kenneth Brody wrote:

No, but if you are passing an int rather than unsigned char, it
may need to zero-fill the rest of the int before passing it.

unsigned char x;
int ix;

x = 4;
ix = x;

What am I missing?

totally legal conversion provided you keep the values in x to the
portable range.

Tom

[Keith Thompson]

"Tom St Denis" <to********@gma il.comwrites:

Kenneth Brody wrote:

>No, but if you are passing an int rather than unsigned char, it
may need to zero-fill the rest of the int before passing it.

unsigned char x;
int ix;

x = 4;
ix = x;

What am I missing?

totally legal conversion provided you keep the values in x to the
portable range.

Certainly it's legal. The question is performance (though in practice
I don't think it's a significant concern).

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.