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DUP(2) System Calls Manual DUP(2)

dup, dup2duplicate an existing file descriptor

#include <unistd.h>

int
dup(int oldd);

int
dup2(int oldd, int newd);

dup() duplicates an existing object descriptor and returns its value to the calling process (newd = dup(oldd)). The argument oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table, which is returned by getdtablesize(3). The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.

The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd 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.

In dup2(), the value of the new descriptor newd is specified. If this descriptor is already in use, it is first deallocated as if a close(2) call had been done first. When newd equals oldd, dup2() just returns without affecting the close-on-exec flag.

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

dup() and dup2() fail if:

[]
oldd or newd is not a valid active descriptor.
[]
Too many descriptors are active.

accept(2), close(2), fcntl(2), open(2), pipe(2), socket(2), socketpair(2), getdtablesize(3)

dup() and dup2() are expected to conform to IEEE Std 1003.1-1988 (“POSIX.1”).

May 31, 2007 OpenBSD-5.1