---
name: "dup"
section: "2"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/dup"
---

# dup(2)

duplicate a file descriptor

## NAME

dup, dup2, dup3 - duplicate a file descriptor

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <unistd.h>
```

**int dup(int** *oldfd***);** **int dup2(int** *oldfd***, int** *newfd***);**

**#define \_GNU\_SOURCE**             /\* See [feature\_test\_macros(7)](/feature_test_macros/7.md) \*/ **#include \<fcntl.h\>**              /\* Definition of **O\_\*** constants \*/ **#include \<unistd.h\>**

**int dup3(int** *oldfd***, int** *newfd***, int** *flags***);**

## DESCRIPTION

The **dup**() system call allocates a new file descriptor that refers to the same open file description as the descriptor *oldfd*. (For an explanation of open file descriptions, see [**open**(2)](/open/2.md).) The new file descriptor number is guaranteed to be the lowest-numbered file descriptor that was unused in the calling process.

After a successful return, the old and new file descriptors may be used interchangeably. Since the two file descriptors refer to the same open file description, they share file offset and file status flags; for example, if the file offset is modified by using [**lseek**(2)](/lseek/2.md) on one of the file descriptors, the offset is also changed for the other file descriptor.

The two file descriptors do not share file descriptor flags (the close-on-exec flag). The close-on-exec flag (**FD\_CLOEXEC**; see [**fcntl**(2)](/fcntl/2.md)) for the duplicate descriptor is off.

### dup2()

The **dup2**() system call performs the same task as **dup**(), but instead of using the lowest-numbered unused file descriptor, it uses the file descriptor number specified in *newfd*. In other words, the file descriptor *newfd* is adjusted so that it now refers to the same open file description as *oldfd*.

If the file descriptor *newfd* was previously open, it is closed before being reused; the close is performed silently (i.e., any errors during the close are not reported by **dup2**()).

The steps of closing and reusing the file descriptor *newfd* are performed *atomically*. This is important, because trying to implement equivalent functionality using [**close**(2)](/close/2.md) and **dup**() would be subject to race conditions, whereby *newfd* might be reused between the two steps. Such reuse could happen because the main program is interrupted by a signal handler that allocates a file descriptor, or because a parallel thread allocates a file descriptor.

Note the following points:

- •

  If *oldfd* is not a valid file descriptor, then the call fails, and *newfd* is not closed.

- •

  If *oldfd* is a valid file descriptor, and *newfd* has the same value as *oldfd*, then **dup2**() does nothing, and returns *newfd*.

### dup3()

**dup3**() is the same as **dup2**(), except that:

- •

  The caller can force the close-on-exec flag to be set for the new file descriptor by specifying **O\_CLOEXEC** in *flags*. See the description of the same flag in [**open**(2)](/open/2.md) for reasons why this may be useful.

- •

  If *oldfd* equals *newfd*, then **dup3**() fails with the error **EINVAL**.

## RETURN VALUE

On success, these system calls return the new file descriptor. On error, -1 is returned, and *errno* is set to indicate the error.

## ERRORS

- **EBADF**

  *oldfd* isn't an open file descriptor.

- **EBADF**

  *newfd* is out of the allowed range for file descriptors (see the discussion of **RLIMIT\_NOFILE** in [**getrlimit**(2)](/getrlimit/2.md)).

- **EBUSY**

  (Linux only) This may be returned by **dup2**() or **dup3**() during a race condition with [**open**(2)](/open/2.md) and **dup**().

- **EINTR**

  The **dup2**() or **dup3**() call was interrupted by a signal; see [**signal**(7)](/signal/7.md).

- **EINVAL**

  (**dup3**()) *flags* contain an invalid value.

- **EINVAL**

  (**dup3**()) *oldfd* was equal to *newfd*.

- **EMFILE**

  The per-process limit on the number of open file descriptors has been reached (see the discussion of **RLIMIT\_NOFILE** in [**getrlimit**(2)](/getrlimit/2.md)).

- **ENOMEM**

  Insufficient kernel memory was available.

## STANDARDS

POSIX.1-2024.

## HISTORY

- **dup**() **dup2**()

  4\.3BSD, SVr4, POSIX.1-1988.

- **dup3**()

  POSIX.1-2024. Linux 2.6.27, glibc 2.9.

## NOTES

The error returned by **dup2**() is different from that returned by **fcntl(**..., **F\_DUPFD**, ...**)** when *newfd* is out of range. On some systems, **dup2**() also sometimes returns **EINVAL** like **F\_DUPFD**.

If *newfd* was open, any errors that would have been reported at [**close**(2)](/close/2.md) time are lost. If this is of concern, then—unless the program is single-threaded and does not allocate file descriptors in signal handlers—the correct approach is *not* to close *newfd* before calling **dup2**(), because of the race condition described above. Instead, code something like the following could be used:

```
/* Obtain a duplicate of 'newfd' that can subsequently
   be used to check for close() errors; an EBADF error
   means that 'newfd' was not open.  */
tmpfd = dup(newfd);
if (tmpfd == -1 && errno != EBADF) {
    /* Handle unexpected dup() error.  */
}
/* Atomically duplicate 'oldfd' on 'newfd'.  */
if (dup2(oldfd, newfd) == -1) {
    /* Handle dup2() error.  */
}
/* Now check for close() errors on the file originally
   referred to by 'newfd'.  */
if (tmpfd != -1) {
    if (close(tmpfd) == -1) {
        /* Handle errors from close.  */
    }
}
```

## SEE ALSO

[**close**(2)](/close/2.md), [**fcntl**(2)](/fcntl/2.md), [**open**(2)](/open/2.md), [**pidfd\_getfd**(2)](/pidfd_getfd/2.md)