String functions like strlen,memcpy not working on unsigned char buffer which contains data read from binary file on sd-card

string functions like strlen,memcpy not working on unsigned char buffer which contains data read from binary file on sd-card. Please suggest help, i am stuck here.??

example:
***************example
unsigned char Readbuffer[1024]={0x00};
f_open(&file,“15_12_12.bin”,FA_READ|FA_OPEN_EXISTING);
f_read(&file,Readbuffer,1002,&bytesread);
strlen(Readbuffer);------->>>>>here it is not working gives 0 lenght & .bin file contains short int data written using f_write function

*********end

waiting for suggestions…!!!
kindly help.