Issue with memcpy

**vandrea** · 08-17-2009

Hi guys,

I had an issue wrt memcpy in my code...Pls find the snippet of code below

Code:

#define ARR_LEN 100

char arr_ca[ARR_LEN + 1 ];
arr_ca[ARR_LEN]= {'\0'}; // Initialising the last element of the string
memset(arr_ca,0x00,sizeof(arr_ca));
memset(arr_ca, "Checking the text", ARR_LEN);

When i debug the above prgm and check the value of arr_ca it gives
"Checking the text \00\00\00 asjdlkjewiruioiuoiuasdsd .(some junk value).."

I am not getting as to why am i getting junk value since i have already initiliased arr_ca using 0x00 in step 3...Can anyone explain me on this!!!

**Kennedy** · 08-17-2009

What do you get when you execute this:

Code:

printf("Sizeof arr_ca is %i\n", sizeof(arr_ca));

**vandrea** · 08-17-2009

Code:
printf("Sizeof arr_ca is %i\n", sizeof(arr_ca));

I get 101

**robwhit** · 08-17-2009

memset isn't memcpy. why do you have memcpy in your title?

what are you trying to do?

**bithub** · 08-17-2009

Code:

memset(arr_ca, "Checking the text", ARR_LEN);

That is where your junk values are coming from. Read up on memset(), and what it does.

**vandrea** · 08-17-2009

memset isn't memcpy. why do you have memcpy in your title?

what are you trying to do?

Sorry this should have been memcpy on line no 4 i.e.

memcpy(arr_ca, "Checking the text", ARR_LEN); instead of memset

I am assigning the character array with sum value!!
I have used memcpy in my code, but then too it gives junk value...any idea?

**bithub** · 08-17-2009

Originally Posted by vandrea

Sorry this should have been memcpy on line no 4 i.e.

memcpy(arr_ca, "Checking the text", ARR_LEN); instead of memset

I am assigning the character array with sum value!!

That code is not valid (it is a buffer overrun). "Checking the text" is 18 bytes, but you are telling memcpy() to copy 100 bytes from this location. This means you copy in whatever random data is after the "Checking the text" data. I think you meant to use strcpy() instead of memcpy().

**vandrea** · 08-17-2009

Thanks bithub for ur reply ..

That code is not valid (it is a buffer overrun). "Checking the text" is 18 bytes, but you are telling memcpy() to copy 100 bytes from this location. This means you copy in whatever random data is after the "Checking the text" data. I think you meant to use strcpy() instead of memcpy().

I did intend to use memcpy to know its behaviour while copying the data . But how is the random value coming since I would have done memset earlier and all the data in array would be initialised with

Code:

{'\0'}.

So then when I do

Code:

memcpy

I would just copy those 18 characters in first 18 places right without modifying the rest of the data right??? Pls enlighten me on this I might be wrong in understanding!!

Also which ones better strcpy or memcpy in C?

**itCbitC** · 08-17-2009

Why are you initialising the array arr_ca[] so many times.

Code:

arr_ca[ARR_LEN]= {'\0'}; /* don't need this as it is handled by the memset() following it */

**tabstop** · 08-17-2009

Originally Posted by vandrea

Thanks bithub for ur reply ..

I did intend to use memcpy to know its behaviour while copying the data . But how is the random value coming since I would have done memset earlier and all the data in array would be initialised with

Code:

{'\0'}.

So then when I do

Code:

memcpy

I would just copy those 18 characters in first 18 places right without modifying the rest of the data right??? Pls enlighten me on this I might be wrong in understanding!!

Also which ones better strcpy or memcpy in C?

Your memcpy statement says -- go get 100 bytes from the address given by "Checking the text" and copy it to this location. The rub being, there's not necessarily 100 bytes at that address for you to copy.

Strcpy is used to copy strings, memcpy is used to copy chunks of memory. (Obviously that's a somewhat crude dichotomy, but it works surprisingly well.) Since in this case you don't want to copy a chunk of memory, but rather a string, memcpy is not the tool for you.

**vandrea** · 08-17-2009

Thanks guys!

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