Strlen Acting Weird and I don't know how to solve it!

[Hirls]

Hi everyone, I'm currently developing a mesh network to send sensor data.

I'm stumble on a problem which is very confusing to me.

What I'm trying to do is send the bytes of float by accessing it through union.

typedef  union{
  double lat;
  uint8_t ch[8];
}union_send; 

int main(){
    lat_compress.lat = 10.823929100;
    SEGGER_RTT_printf(0,"length of packets %d\n",sizeof(lat_compress.ch));
    SEGGER_RTT_printf(0,"length of packets %d\n",sizeof(lat_compress.lat));
    SEGGER_RTT_printf(0,"length of packets 2 %d\n",strlen(lat_compress.ch));

}

But when I'm try different compiler, it's appeared different from my board.

typedef union{
    double lat;
     uint8_t ch[8];
}lat_send;
int main() {
    // Write C code here
    
    lat_send lat_s;
    lat_s.lat=10.823929100213;
    printf("str len: %d\n",strlen(lat_s.ch));
    printf("size of: %d\n",sizeof(lat_s.ch));
}

Regards, 

Hoang