C: dup and close-on-exec

In a (German) book about C programming (Linux-UNIX-Programmierung, by Jürgen Wolf) I have found a statement, which translated to English would be as following (sentences numbered by me):

In some cases it can be necessary that you need to duplicate a file descriptor [1]. An example for this would be, if a parent process wants to exchange data with a child process and the child process is overlaid by a new process through the use of exec*() [2]. In such a case, without dup() or dup2(), the close-on-exec flag would be set [3]. When this flag is set, all file descriptors become invalid (since being overlaid by the new process) - that is, they are not present anymore [4]. The communication between the parent and child process would thus be stopped [5]. If on the other hand you duplicate the file descriptor with dup() or dup2(), then the close-on-exec flag is deleted, and the newly overlaid process can use this file descriptor to communicate [6].

I believe that the above paragraph contains several misleading statements or even errors.

In sentence [3], I do not understand why without the use of dup() or dup2() the close-on-exec flag would be set?

Solution

The advice is wrong. Close-on-exec is set only on file descriptors that your program has explicitly asked to be close-on-exec.

The reasons you may choose to use dup2 may be:

The process to be executed expects its I/O to be via particular file descriptors (usually 0, 1, and 2, corresponding to standard input, output and error streams respectively), or
the process will close some file descriptors, for whatever reason, and you need this fd to be outside the range to be closed.

The description is also slightly misleading - it's only the new descriptor (i.e. the return value from dup() or dup2()) which has close-on-exec unset. The close-on-exec state of the original fd is unchanged.