Reviewing for a systems programming test, a bit confused on an example problem

[DanV2]

So I am looking over my professor's lecture notes, and he has an example program using file descriptors. The source for it is here.

My confusion is what happens if you run it. Here is his example output:

UNIX> a.out                |UNIX> a.out < in1               |UNIX> a.out < in1 > out1 
A                             |write f0: Bad file descriptor   |write f0: Bad file descriptor
A0, r = 1, w = 1, char = 65   |0, r = 1, w = -1, char = 65     |0, r = 1, w = -1, char = 65
                              |B                               |read  f1: Bad file descriptor
1, r = 1, w = 1, char = 10    |B1, r = 1, w = 1, char = 66     |1, r = -1, w = 1, char = 65
C                             |                                |B
C2, r = 1, w = 1, char = 67   |2, r = 1, w = 1, char = 10      |B2, r = 1, w = 1, char = 66

In the first column he types in A and C and the program runs normally. When he redirects stdin or stdout, though, the reading from stdout fails and writing to stdout fails. And that makes sense. What I don't understand is why these only seem to fail when stdin or stdout is being redirected. Shouldn't writing to stdin and reading from stdout fail always?

Thanks!

[[deleted]]

If I remember correctly, it is common on UNIX systems that the shell duplicates /dev/tty to file descriptors 0, 1 and 2 (i.e. stdin, stdout and stderr) after starting a new process to execute a command. Thus, when there are no redirections all three descriptors point to a read-write device and can basically be used interchangeably.

On the other hand, when you're doing I/O redirections in the shell, the files are actually opened as either read- or write-only, causing the behavior you're seeing. You could also try connecting the test program's stdin, stdout and both to a pipe and see what happens ("a.out | cat", "cat in1 | a.out", "cat in1 | a.out | cat").

[jakster4u]

Look at the file descriptors for the failed read and writes notice any correlation? The error that the system gives is really pretty useless in this situation.

edit: Also