fd1

    The file associated with this file descriptor in the parent
    process is associated with file descriptor number 0 (stdin) in
    the child process. If -1 is specified, the file associated with
    the parent's file descriptor number 0 is used (the default).

 fd2

    The file associated with this file descriptor in the parent
    process is associated with file descriptor number 1 (stdout) in
    the child process. If -1 is specified, the file associated with
    the parent's file descriptor number 1 is used (the default).

 fd3

    The file associated with this file descriptor in the parent
    process is associated with file descriptor number 2 (stderr) in
    the child process. If -1 is specified, the file associated with
    the parent's file descriptor number 2 is used (the default).