How can I mimic the head Unix command in my own program?

The head utility shall copy its input files to the standard output, ending the output for each file at a designated point. Copying shall end at the point in each input file indicated by the -n number option. The optionargument number shall be counted in units of lines. If no file operand is specified, or when file is -, then standard input is assumed. -n number: The first number lines of each input file shall be copied to standard output. The application shall ensure that the number option-argument is a positive decimal integer. When a file contains less than number lines, it shall be copied to standard output in its entirety. This shall not be an error. If no options are specified, head shall act as if -n 10 had been specified. I need to use low-level io, such as read() and write(). I need to know the logic behind how to do this with varying buffersizes.