The OP code is currently a string and should be binary numbers. Also the size of RRQ and WRQ looks wrong.

[satiros12]

RRQ and WRQ should have 4+file_name_size + mode_name_size bytes. Because first 2 are for OP code and the other 2 are zero bytes between the file name and after the mode name.

[jrrk]

Likewise, strings containing \000 bytes cannot be used because the string terminates at this point, and in addition when using malloc() the size to be allocated needs one added when any of the str... functions are used due to 0 termination.