2 file descriptors for the same file

Question

I have a file descriptor >0 for a file that is already opened. I want to add a second file descriptor to that file. I know this is possible if I open the file with the second file descriptor again but the problem is that in that point of my code I don't know the name of the file.

So I was wondering if I can just do that: fd2 = fd1;

Answer 1

You may duplicate the file descriptor, returning a new, distinct handle to the same open file description, with either dup

#include <unistd.h>
int fd2 = dup(fd1);

or with fcntl / F_DUPFD

#include <fcntl.h>
int fd2 = fcntl(fd1, F_DUFPD, 0); // consider F_DUPFD_CLOEXEC !

Because fd2 and fd1 now refer to the same open file description , they "share" certain attributes of the underlying open file description:

• status flags (nonblocking, append-only, etc.)
• access mode (read-only, read-write, write-only)
• file position (reading/seeking on one will be reflected in the other)
• record locks (as with POSIX struct flock locks)

If you change one of the above on fd2 , that change will be visible in fd1 since, again, both refer to the same underlying I/O construct. The same thing happens when file descriptors are duplicated across processes ("inherited").

The descriptors themselves have only one interesting attribute of their own, FD_CLOEXEC, which controls whether or not the descriptor will be preserved (inherited) across an execve call. This may differ for each descriptor.