CODEWITHSUNDEEP
cunixfile-descriptordup2
Yi Lin Liu
Yi Lin Liu

Reputation: 169

How to recover stdin overwritten by dup2?

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.

Upvotes: 5

Views: 3438

Answers (1)

Jonathan Leffler
Jonathan Leffler

Reputation: 754910

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.

Upvotes: 9

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