编写一个函数，从一个字符串中提取一个子字符串

#include<stdio.h>#include<string.h>int substr(char dst[],char src[],int start,int len){char* sc = src+start;int n = strlen(sc);int i = 0;if(n < len){len = n;}while(len){dst[i] = sc[i];len--;i++;}dst[i]='\0';return len;}int main(){char dst[10];char src[]="bit-tech";substr(dst,src,3,4);printf("%s\n",dst);return 0;}