c语言，这个程序运行后为什么len=20

#include <string.h>
#include <stdio.h>
int main()
{
char dst[100] = {0};
int a = 10;
char src[] = "hello world";
printf("a=%d,src=%s",a,src);
printf("\n");

int len = sprintf(dst,"a=%d,src=%s",a,src);
printf("dst=\"%s\"\n",dst);
printf("len=%d\n",len);

}

因为sprintf的返回值是成功写入的字符串长度
这里dst="a=10,src=hello world"
就是20个字符