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mystrcpy

Started by luasitdown Jan 26, 2006 at 1:02 AM 7 replies 1.3k views

Original Post

Jan 26, 2006 at 1:02 AM


char *mystrcpy(char *strDest, const char *strSrc)
{
	char *address=strDest;
	while(*strDest++=*strSrc++);
	*strDest++='\0';
	return address;
}

int _tmain(int argc, _TCHAR* argv[])
{
	char a[]="awp";
	char b[]="abcdefghigklmn";
	out<<mystrcpy(a,b);
}

it is answer by a doctor to write a char *mystrcpy(char *strDest, const char *strSrc). I think it is dangous, if strSrc < strDest, strSrc will be overlay. [Edited by - LessBread on January 26, 2006 3:54:40 AM]

erissian

Jan 26, 2006 at 1:37 AM

I'm not sure what you are asking.

If you are running into a problem with this, it is because of this:

char a[]="awp";char b[]="abcdefghigklmn";

You are trying to copy 15 characters (including \0) from b into an area of memory that only has 4 bytes reserved. You should be getting segmentation faults whenever you try it.

You solution could be to reallocate memory for 'a' before writing to it, or to measure it and truncate 'b' appropriately.

LessBread

Jan 26, 2006 at 3:54 AM

You could probably get away with 15 characters due to the compiler padding the string to a default length, but that doesn't make the usage safe. It's up to the programmer to make certain the destination is large enough. The code for mystrcpy isn't very different from the code for strcpy.

If the length of strSrc < the length of strDest, there won't be any overlay due to the null termination of the string. Whatever was in strDest beyond the length of strSrc will simply be lost.

At any rate, these routines aren't truly safe anyway.

iMalc

Jan 26, 2006 at 12:17 PM

Quote:
Original post by Anonymous Poster
also note that the code isn't minimal. the third line within the cpy function is redundant. if you're expecting the second argument to be a C null terminated string.

Yes the destination string is double-null terminated, being more dangerous than terminating with 1 null char because the programmer may only have a buffer big enough for one null char.

I have written my own strcpy routines before and it was for a valid purpose. I was working with an embedded C compiler and using a mixture of near and far pointers. althought there were functions like strcpy_n_f for near to far etc, they didn't actually work! Yes were were using them correctly, and yes we were calling the right ones.

Sneftel

Jan 26, 2006 at 7:04 PM

Quote:
Original post by Anonymous Poster
Why do people keep calling C-strings "null-terminated"? The name of the '\0' character is NUL, and nothing else. So, the correct name is nul-terminated.

Programmers these days....

Because "NUL" is simply the abbreviation for the word "NULL" which was used in the ASCII standard character set reference table. Similarly, it's called backspace, not just BS; a carriage return, not just a CR; and the thing that old terminal printers used to make noise was a bell, not a BEL. Apparently, programmers these days are less prone to nitpicking.

LessBread

Jan 26, 2006 at 7:07 PM

#define NULL (void *)0

Ok. So NULL is a void pointer and NUL is a char. They both evaluate to 0.

I suppose it's because programmers these days aren't as familiar with ASCII code as were programmers from days of yore... [smile]

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