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DUP(2) System Calls Manual DUP(2)

dup, dup2, dup3 — duplicate an existing file descriptor

#include <unistd.h>

int
dup(int oldd);

int
dup2(int oldd, int newd);

#include <fcntl.h>
#include <unistd.h>

int
dup3(int oldd, int newd, int flags);

() 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 and close-on-fork flags on the new file descriptor are unset.

In (), 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 or close-on-fork flags.

In (), the value of the new descriptor and the close-on-exec and close-on-fork flags on the new file descriptor are all specified: newd specifies the value and the O_CLOEXEC and O_CLOFORK bits in flags specify the close-on-exec and close-on-forks flag, respectively. Unlike dup2(), if oldd and newd are equal then dup3() fails. Otherwise, if flags is zero then dup3() is identical to a call to dup2().

Upon successful completion, the value of the new descriptor is returned. The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.

dup() will fail if:

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

dup2() and dup3() will fail if:

[]
oldd is not a valid active descriptor or newd is negative or greater than or equal to the process's RLIMIT_NOFILE limit.
[]
A race condition with accept(2) or open(2) has been detected.
[]
An interrupt was received.
[]
An I/O error occurred while writing to the file system.

In addition, dup3() will return the following error:

[]
oldd is equal to newd or flags is invalid.

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

The dup(), dup2(), and dup3() functions conform to IEEE Std 1003.1-2024 (“POSIX.1”).

The dup() system call first appeared in Version 3 AT&T UNIX, dup2() in Version 7 AT&T UNIX, and dup3() in OpenBSD 5.7.

August 4, 2025 OpenBSD-current