Some common C language library function implementation

Often in the written examination and interview, the company will have some C language library function for the interviewer to do, these functions look very simple, in fact, still have to consider a lot. Here are some simple implementations of several commonly used functions.

1. String copy function

String assignment function
char *strcpy (char *strdestination, const char *strsource)
{
	//
	assert (strdestination!= Null && strsource!= null);
	int i = 0;
	while (*strsource!= ' ")
	{
		strdestination[i++] = *strsource++;
	}
	Strdestination[i] = ' the ';
	return strdestination;
}

2. Memory copy function

void *memcpy (void *dest, const void *SRC, size_t count)
{
	assert (dest!= null && src!= null);
	Char *tmp = (char*) dest;
	const char *s = (const char*) src;

	while (count--)
	{
		*tmp = *s;
	}
	return dest;
}

3. Memory Setup function

void *memset1 (void *dest, int c, size_t count)
{
	assert (dest!= NULL);
	char* tmp = (char*) dest;
	int i = 0;
	while (count-)
	{
		tmp[i++] = c;
	}
	return dest;
}

4. Converts a string to an integer

int atoi (const char *string)
{
	char *p = (char*) string;
	char c;
	int   i = 0;
	while (c = *p++)
	{
		if (c>= ' 0 ' && c<= ' 9 ')
		{
			i = i*10 + (c ' 0 ');
		}
		else
			return-1;                     Invalid string
	} return
	i;
}

To achieve so much first.