The header file unistd.h contains several functions that are useful for porting a program from UNIX. These functions are similar to the functions in many UNIX libraries. However, since the UNIX and Macintosh operating systems have some fundamental differences, they cannot be identical. The descriptions of the functions tell you what the differences are.

These facilities in unistd.h are:

• "chdir" change the directory
• "close" close a file opened with open
• "cuserid" retrieves the current user's ID
• "exec" executes programs from within a program
• "getcwd" gets the current working directory
• "getlogin" returns a login name
• "getpid" returns the process ID
• "isatty" determines if a file ID is attached to a terminal
• "lseek" seek when opened with open
• "read" read when opened with open
• "rmdir" removes a directory or folder
• "sleep" pauses a program
• "ttyname" determines a terminal id
• "unlink" deletes a file