NAME

dup, dup2, dup3 — duplicate an open file descriptor

SYNOPSIS

#include <unistd.h>

int dup(int fildes);
int dup2(int fildes, int fildes2);
int dup3(int fildes, int fildes2, int flag);


DESCRIPTION

The dup() function provides an alternative interface to the service provided by fcntl() using the F_DUPFD command. The call dup(fildes) shall be equivalent to:

fcntl(fildes, F_DUPFD, 0);

The dup2() function shall cause the file descriptor fildes2 to refer to the same open file description as the file descriptor fildes and to share any locks, and shall return fildes2. If fildes2 is already a valid open file descriptor, it shall be closed first, unless fildes is equal to fildes2 in which case dup2() shall return fildes2 without closing it. If the close operation fails to close fildes2, dup2() shall return -1 without changing the open file description to which fildes2 refers. If fildes is not a valid file descriptor, dup2() shall return -1 and shall not close fildes2. If fildes2 is less than 0 or greater than or equal to {OPEN_MAX}, dup2() shall return -1 with errno set to [EBADF].

Upon successful completion, if fildes is not equal to fildes2, the FD_CLOEXEC and FD_CLOFORK flags associated with fildes2 shall be cleared. If fildes is equal to fildes2, the FD_CLOEXEC and FD_CLOFORK flags associated with fildes2 shall not be changed.

The dup3() function shall be equivalent to the dup2() function, except that it shall be an error if fildes is equal to fildes2, and the state of FD_CLOEXEC and FD_CLOFORK on the fildes2 file descriptor shall be determined solely by the flag argument, which can be constructed from a bitwise-inclusive OR of flags from the following list:

O_CLOEXEC

Atomically set the FD_CLOEXEC flag on fildes2.

O_CLOFORK

Atomically set the FD_CLOFORK flag on fildes2.

^[TYM] If fildes refers to a typed memory object, the result of the dup2() or dup3() functions is unspecified.

RETURN VALUE

Upon successful completion a non-negative integer, namely the file descriptor, shall be returned; otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS

The dup() function shall fail if:

[EBADF]

The fildes argument is not a valid open file descriptor.

[EMFILE]

All file descriptors available to the process are currently open.

The dup2() and dup3() functions shall fail if:

[EBADF]

The fildes argument is not a valid open file descriptor or the argument fildes2 is negative or greater than or equal to {OPEN_MAX}.

[EINTR]

The function was interrupted by a signal.

The dup3() function shall fail if:

[EINVAL]

The fildes and fildes2 arguments are equal.

The dup2() and dup3() functions may fail if:

[EIO]

An I/O error occurred while attempting to close fildes2.

The dup3() function may fail if:

[EINVAL]

The value of the flag argument is invalid.

EXAMPLES

Redirecting Standard Output to a File

The following example closes standard output for the current processes, re-assigns standard output to go to the file referenced by pfd, and closes the original file descriptor to clean up.

#include <unistd.h>
...
int pfd;
...
close(1);
dup(pfd);
close(pfd);
...

Redirecting Error Messages

The following example redirects messages from stderr to stdout.

#include <unistd.h>
...
dup2(1, 2);
...

APPLICATION USAGE

Implementations may use file descriptors that must be inherited into child processes for the child process to remain conforming, such as for message catalog or tracing purposes. Therefore, an application that calls dup2() with an arbitrary integer for fildes2 risks non-conforming behavior, and dup2() can only portably be used to overwrite file descriptor values that the application has obtained through explicit actions, or for the three file descriptors corresponding to the standard file streams. In order to avoid a race condition of leaking an unintended file descriptor into a child process or executed program, an application should consider opening all file descriptors with the FD_CLOFORK or FD_CLOEXEC flag, or both flags, set unless the file descriptor is intended to be inherited by child processes or executed programs, respectively.

RATIONALE

The dup() function is redundant. Its services are also provided by the fcntl() function. It has been included in this volume of POSIX.1-2024 primarily for historical reasons, since many existing applications use it. On the other hand, the dup2() function provides unique services, as no other interface is able to atomically replace an existing file descriptor.

The dup2() function is not marked obsolescent because it presents a type-safe version of functionality provided in a type-unsafe version by fcntl(). It is used in the POSIX Ada binding.

The dup2() function is not intended for use in critical regions as a synchronization mechanism.

In the description of [EBADF], the case of fildes being out of range is covered by the given case of fildes not being valid. The descriptions for fildes and fildes2 are different because the only kind of invalidity that is relevant for fildes2 is whether it is out of range; that is, it does not matter whether fildes2 refers to an open file when the dup2() call is made.

The dup3() function with the O_CLOEXEC and O_CLOFORK flags is necessary to avoid a data race in multi-threaded applications. Without O_CLOFORK, a file descriptor is leaked into a child process created by one thread in the window between another thread creating a file descriptor with dup2() and then using fcntl() to set the FD_CLOFORK flag. Without O_CLOEXEC, a file descriptor intentionally inherited by child processes is similarly leaked into an executed program if FD_CLOEXEC is not set atomically. The safe counterpart for avoiding the same race with dup() is the use of the F_DUPFD_CLOFORK or F_DUPFD_CLOEXEC action of the fcntl() function.

FUTURE DIRECTIONS

None.

SEE ALSO

close , fcntl , open

XBD <unistd.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 7

SD5-XSH-ERN-187 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0075 [149,428] and XSH/TC1-2008/0076 [149] are applied.

Issue 8

Austin Group Defects 411, 1318, 1483, and 1577 are applied, adding dup3() and FD_CLOFORK.