How to recover stdin overwritten by dup2?

Question

I am trying to replace stdin with another pipe, then place the original stdin back to fd #0.

e.g.

dup2(p, 0); // p is a pre-existing fd of a pipe
exec(/* some commands */);

//what will be here in order to have the original stdin back?

scanf(...) //continue processing with original stdin.

Ezraezri

Simply dup2() stdin to a temporary filedescriptor first and dup2() it back again later – Zoubek

@Zoubek Is it possible to keep stdin without temporary duplicate it? – Ezraezri

Not "keep" it, you could reopen it if it is possible. Depending on what your stdin is, this might be not possible however. – Zoubek

The 'exec..()` commands do not return, unless they failed. So the only reasonable statements after an exec command are: perror( "exec.. failed" ); followed by exit( EXIT_FAILURE ); – Sexual

Answer 1

You can't recover the original once it has been overwritten (closed). What you can do is save a copy of it before you overwrite it (which requires planning ahead, of course):

int old_stdin = dup(STDIN_FILENO);

dup2(p, STDIN_FILENO);
close(p);               // Usually correct when you dup to a standard I/O file descriptor.

…code using stdin…

dup2(old_stdin, STDIN_FILENO);
close(old_stdin);       // Probably correct
scanf(…);

However, your code mentions exec(…some commands…); — if that is one of the POSIX execve() family of functions, then you won't reach the scanf() (or the second dup2()) call unless the exec*() call fails.

Bergquist

Without duplicate the stdin to temporary fd, would you be able to reopen stdin? – Ezraezri

In general, no. The original standard input might be a pipe, a socket, or some other fancy and ephemeral file type, which cannot be reopened. Plus you don't have a name to (re)open it with. After the dup2(p, STDIN_FILENO); operation succeeds, /dev/stdin is no help; it yields the file that p was opened for, not the original standard input. – Bergquist

Also, while I think about it, note that you may run into problems with buffering of input if you dink with file descriptor while the standard I/O library has some input read from the kernel but not delivered to the application. (Similar problems could occur with outputs too.) – Bergquist