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Manual page for DUP(2)

dup, dup2 - duplicate a descriptor

SYNOPSIS

int dup(int fd);

int dup2(int fd, int fd2);

DESCRIPTION

The dup function duplicates an existing object descriptor. The argument fd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table. The size of the table for POSIX programs is OPEN_MAX; for other programs the size of the table is the value returned by getdtablesize.2 The new descriptor returned by dup is the lowest numbered descriptor that is not currently in use by the process.

The object referenced by the descriptor does not distinguish, in any way, between references using fd and the new descriptor returned. Thus, if the returned descriptor and are duplicate references to an open file, read.2 write.2 and lseek.2 calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open.2 call. The close-on-exec flag on the new file descriptor is unset.

With dup2, fd2 specifies the desired value of the new descriptor. If the descriptor fd2 is already in use, it is first deallocated as if a close.2 call had been performed on it.

RETURN VALUE

The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.

ERRORS

The dup and dup2 functions fail if:
[EBADF]
fd or fd2 is not a valid active descriptor
[EMFILE]
Too many descriptors are active.

Additionally, dup2 fails if:

[EINTR]
The dup2 function was interrupted by a signal.

SEE ALSO

close(2), creat(2), exec(2), fcntl(2), open(2), pipe(2)

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