Write a C program with a focus on system calls. So only use read() and write() for obtaining input and creating output - do not use printf or fopen or similar functions. Use open() and close() and other system call functions.

The programs should be insensitive to the number of input characters (i.e. input from STDIN should terminate with a Control-D). Be sure that every process "cleans-up'' before terminating.

flip: This program reads characters from STDIN, converts them based on flag arguments passed to the program (example: "./flip -l -u"), and writes the characters to STDOUT. The character stream may contain any ASCII value.

A "-l" flag means alphabetic characters should be all output in lowercase.

A "-u" flag means alphabetic characters should be all output in uppercase.

If both flags are present, uppercase characters should be output in lowercase and lowercase characters should be output in uppercase.