C - memmove() function - How many bytes am I moving in this implementation?

Question

This seems to be a great place. My question is, what value (or how many bytes) am I moving in this implementaion of memmove()?

int main ()
{
char str[] = "memmove can be very useful......";
memmove (str+15,str+20,/*?*/);
puts (str);
return 0;
}

In the next example it says I am moving 11 bytes. But what makes it 11 bytes? Could somebody explain?

int main ()
{
char str[] = "memmove can be very useful......";
memmove (str+20,str+15,11); //source and destination are reversed
puts (str);
return 0;
}

Thanks!

Edit: BTW, the string length is 33 including the terminating null character.

`int main()` should be `int main(void)`. More important, you need `#include ` and `#include `. If your compiler didn't warn you about this, crank up its warning level until it does. — Keith Thompson, Oct 29 '11 at 02:24
What you're showing us isn't an "implementation of memmove()", it's merely a program that calls `memmove()`. An *implementation* of `memmove()` would be the code in the runtime library that actually copies the bytes. — Keith Thompson, Oct 29 '11 at 02:25
The 11 makes it 11 bytes. Duh. In the first example you are not moving anything until you put a number in place of /*?*/. Are you confusing memmove with strcpy? — Jim Rhodes, Oct 29 '11 at 02:50

score 1 · Answer 1 · answered Oct 29 '11 at 02:22

1

The third parameter of memmove specifies the number of bytes to move, so in your second example you are moving 11 bytes. Your first example should not compile because you will have a syntax error on the line that calls memmove.

answered Oct 29 '11 at 02:22

David Grayson

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score 0 · Answer 2 · edited Oct 29 '11 at 02:23

0

The final argument to memmove() is the number of bytes to move - in this case 11

edited Oct 29 '11 at 02:23

Keith Thompson

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answered Oct 29 '11 at 02:21

Martin Beckett

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@Keith Thompson Do you know how many bytes it should be in the first example? – Mike mmm Oct 30 '11 at 02:28
@Mikemmm: What are you trying to do? – Keith Thompson Oct 30 '11 at 02:36
@Keith Thompson I am trying to left-shift elements of an array, over-writing the intial one with the one to it's right and subsequently all the elements to the right of it. Kind of like deleteing from a queue. – Mike mmm Oct 30 '11 at 03:13

Suroot · Answer 3 · 2011-10-29T02:36:04.430

0

Third parameter defines how many bytes to copy; in the first example you should be defining how many bytes to copy.

edited Oct 29 '11 at 02:36

answered Oct 29 '11 at 02:27

Suroot

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Incorrect. `str+15` is the same as `&str[15]`; both expressions are of type `char*`. – Keith Thompson Oct 29 '11 at 02:30
I removed my downvote after the edit (though this is still redundant with other answers). – Keith Thompson Oct 29 '11 at 04:15

C - memmove() function - How many bytes am I moving in this implementation?

3 Answers3