MyroC Commands with Responses

C procedure:    write (int file_descriptor, const void * buffer, size_t num_bytes)
asks the operating system to send num_bytes from the character array buffer through channel file_descriptor.

C procedure:   read (int file_descriptor, void * buffer, size_t num_bytes)
asks the operating system for up to num_bytes from the file_descriptor, and the data are copied to the buffer.

How to use read to obtain the full num_bytes of data desired?