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+'\" et
+.TH DUP "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
+.SH PROLOG
+This manual page is part of the POSIX Programmer's Manual.
+The Linux implementation of this interface may differ (consult
+the corresponding Linux manual page for details of Linux behavior),
+or the interface may not be implemented on Linux.
+
+.SH NAME
+dup,
+dup2
+\(em duplicate an open file descriptor
+.SH SYNOPSIS
+.LP
+.nf
+#include <unistd.h>
+.P
+int dup(int \fIfildes\fP);
+int dup2(int \fIfildes\fP, int \fIfildes2\fP);
+.fi
+.SH DESCRIPTION
+The
+\fIdup\fR()
+function provides an alternative interface to the service provided by
+\fIfcntl\fR()
+using the F_DUPFD command. The call
+.IR dup ( fildes )
+shall be equivalent to:
+.sp
+.RS 4
+.nf
+\fB
+fcntl(fildes, F_DUPFD, 0);
+.fi \fR
+.P
+.RE
+.P
+The
+\fIdup2\fR()
+function shall cause the file descriptor
+.IR fildes2
+to refer to the same open file description as the file descriptor
+.IR fildes
+and to share any locks, and shall return
+.IR fildes2 .
+If
+.IR fildes2
+is already a valid open file descriptor, it shall be closed first, unless
+.IR fildes
+is equal to
+.IR fildes2
+in which case
+\fIdup2\fR()
+shall return
+.IR fildes2
+without closing it. If the close operation fails to close
+.IR fildes2 ,
+\fIdup2\fR()
+shall return \(mi1 without changing the open file description to which
+.IR fildes2
+refers. If
+.IR fildes
+is not a valid file descriptor,
+\fIdup2\fR()
+shall return \(mi1 and shall not close
+.IR fildes2 .
+If
+.IR fildes2
+is less than 0 or greater than or equal to
+{OPEN_MAX},
+\fIdup2\fR()
+shall return \(mi1 with
+.IR errno
+set to
+.BR [EBADF] .
+.P
+Upon successful completion, if
+.IR fildes
+is not equal to
+.IR fildes2 ,
+the FD_CLOEXEC flag associated with
+.IR fildes2
+shall be cleared. If
+.IR fildes
+is equal to
+.IR fildes2 ,
+the FD_CLOEXEC flag associated with
+.IR fildes2
+shall not be changed.
+.P
+If
+.IR fildes
+refers to a typed memory object, the result of the
+\fIdup2\fR()
+function is unspecified.
+.SH "RETURN VALUE"
+Upon successful completion a non-negative integer, namely the file
+descriptor, shall be returned; otherwise, \(mi1 shall be returned
+and
+.IR errno
+set to indicate the error.
+.SH ERRORS
+The
+\fIdup\fR()
+function shall fail if:
+.TP
+.BR EBADF
+The
+.IR fildes
+argument is not a valid open file descriptor.
+.TP
+.BR EMFILE
+All file descriptors available to the process are currently open.
+.P
+The
+\fIdup2\fR()
+function shall fail if:
+.TP
+.BR EBADF
+The
+.IR fildes
+argument is not a valid open file descriptor or the argument
+.IR fildes2
+is negative or greater than or equal to
+{OPEN_MAX}.
+.TP
+.BR EINTR
+The
+\fIdup2\fR()
+function was interrupted by a signal.
+.P
+The
+\fIdup2\fR()
+function may fail if:
+.TP
+.BR EIO
+An I/O error occurred while attempting to close
+.IR fildes2 .
+.LP
+.IR "The following sections are informative."
+.SH EXAMPLES
+.SS "Redirecting Standard Output to a File" S
+.P
+The following example closes standard output for the current processes,
+re-assigns standard output to go to the file referenced by
+.IR pfd ,
+and closes the original file descriptor to clean up.
+.sp
+.RS 4
+.nf
+\fB
+#include <unistd.h>
+\&...
+int pfd;
+\&...
+close(1);
+dup(pfd);
+close(pfd);
+\&...
+.fi \fR
+.P
+.RE
+.SS "Redirecting Error Messages"
+.P
+The following example redirects messages from
+.IR stderr
+to
+.IR stdout .
+.sp
+.RS 4
+.nf
+\fB
+#include <unistd.h>
+\&...
+dup2(1, 2);
+\&...
+.fi \fR
+.P
+.RE
+.SH "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
+\fIdup2\fR()
+with an arbitrary integer for
+.IR fildes2
+risks non-conforming behavior, and
+\fIdup2\fR()
+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, an application should consider opening all file descriptors
+with the FD_CLOEXEC bit set unless the file descriptor is intended to
+be inherited across
+.IR exec .
+.SH RATIONALE
+The
+\fIdup\fR()
+function is redundant. Its services are also provided by the
+\fIfcntl\fR()
+function. It has been included in this volume of POSIX.1\(hy2008 primarily for historical reasons,
+since many existing applications use it. On the other hand, the
+\fIdup2\fR()
+function provides unique services, as no other interface is able to
+atomically replace an existing file descriptor.
+.P
+The
+\fIdup2\fR()
+function is not marked obsolescent because it presents a type-safe
+version of functionality provided in a type-unsafe version by
+\fIfcntl\fR().
+It is used in the POSIX Ada binding.
+.P
+The
+\fIdup2\fR()
+function is not intended for use in critical regions as a
+synchronization mechanism.
+.P
+In the description of
+.BR [EBADF] ,
+the case of
+.IR fildes
+being out of range is covered by the given case of
+.IR fildes
+not being valid. The descriptions for
+.IR fildes
+and
+.IR fildes2
+are different because the only kind of invalidity that is relevant for
+.IR fildes2
+is whether it is out of range; that is, it does not matter whether
+.IR fildes2
+refers to an open file when the
+\fIdup2\fR()
+call is made.
+.SH "FUTURE DIRECTIONS"
+None.
+.SH "SEE ALSO"
+.IR "\fIclose\fR\^(\|)",
+.IR "\fIfcntl\fR\^(\|)",
+.IR "\fIopen\fR\^(\|)"
+.P
+The Base Definitions volume of POSIX.1\(hy2008,
+.IR "\fB<unistd.h>\fP"
+.SH COPYRIGHT
+Portions of this text are reprinted and reproduced in electronic form
+from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
+-- Portable Operating System Interface (POSIX), The Open Group Base
+Specifications Issue 7, Copyright (C) 2013 by the Institute of
+Electrical and Electronics Engineers, Inc and The Open Group.
+(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
+event of any discrepancy between this version and the original IEEE and
+The Open Group Standard, the original IEEE and The Open Group Standard
+is the referee document. The original Standard can be obtained online at
+http://www.unix.org/online.html .
+
+Any typographical or formatting errors that appear
+in this page are most likely
+to have been introduced during the conversion of the source files to
+man page format. To report such errors, see
+https://www.kernel.org/doc/man-pages/reporting_bugs.html .