substr.c

// Extract a substring from an existing string, return the length of the substring
// dst: receive the substring
// src: the source string to extract from
// start: extract start with the 'src' + 'start' char, zero based; no bound check
// len: length of the substring; may be shorter than expected.
// Ansi string only!
 
int substr(char* dst, char* src, int start, int len)
{
	char* d = dst;
	char* s = src + start;
	while (len-- > 0)
	{
		if (*s == '\0')
		{
			*d = '';
			return d - dst; 
		}
		*d++ = *s++;
	}
	*d = '\0';
	return d - dst;
}
 
int main()
{
	char s[1024] = { '\0' };
	printf("%dt%sn", substr(s, "0123456789", 3, 10), s);
}