请问串口发送数据运行不正常是什么原因呢

rt_uint16_t n,i,crc;
rt_uint8_t str[50] = {0}, string[100] = {0};
switch(mode)
{
case 1:
sprintf(string,"{"addr":%d,"control":%d,"ctime":"000000"",addr,mode);
break;
default:
sprintf(string,"{"addr":%d,"control":%d,"comNum":%d,"comIndex":%d,"ctime":"000000"",addr,mode,num,index);
break;
}
//计算crc不需要前面的大括号，现在把大括号写进去了，i=1，string+1越过大括号
i=0;while(string[i++]!=0);
crc=modbus_crc16((unsigned char*)string+1,i-2);//多减去一个
crc=(crc%256)*256+(crc/256);
// sprintf(string,","crc":%d}",crc);
sprintf((char*)str,","crc":%d}",crc);
strcat((char*)string, (char*)str);
LOG_D("%s",string);
rt_uint8_t len = strlen((char*)string);
rt_device_write(env_dev, 0, string, len);
n=0;

这是通过日志打印出来的字符串的内容 string字符串的内容

{"addr":1,"control":2,"comNum":7,"comIndex":0,"ctime":"000000","crc":8999}

这是通过串口发送的内容，最后几位变成了 0

rt_device_write(env_dev, 0, string, len);
{"addr":1,"control":2,"comNum":7 "comIndex":0,"ctime":"000000","crc":899\0\0

我一开始以为是strlen长度不够，但是字符串里没有0啊，单步调试，strlen的结果也和字符串的长度一致。

通过这个串口发送其他内容的时候，都挺正常的，发送这个不正常