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dup(3p) — Linux manual page

PROLOG \|NAME \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|EXAMPLES \|APPLICATION USAGE \|RATIONALE \|FUTURE DIRECTIONS \|SEE ALSO \|COPYRIGHT

DUP(3P)                 POSIX Programmer's ManualDUP(3P)

PROLOG top

       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.

NAME top

       dup, dup2 — duplicate an open file descriptor

SYNOPSIS top

       #include <unistd.h>       int dup(intfildes);       int dup2(intfildes, intfildes2);

DESCRIPTION top

       Thedup() function provides an alternative interface to the       service provided byfcntl() using the F_DUPFD command. The calldup(fildes) shall be equivalent to:           fcntl(fildes, F_DUPFD, 0);       Thedup2() function shall cause the file descriptorfildes2 to       refer to the same open file description as the file descriptorfildes and to share any locks, and shall returnfildes2.  Iffildes2 is already a valid open file descriptor, it shall be       closed first, unlessfildes is equal tofildes2 in which casedup2() shall returnfildes2 without closing it. If the close       operation fails to closefildes2,dup2() shall return -1 without       changing the open file description to whichfildes2 refers. Iffildes is not a valid file descriptor,dup2() shall return -1 and       shall not closefildes2.  Iffildes2 is less than 0 or greater       than or equal to {OPEN_MAX},dup2() shall return -1 witherrno set       to[EBADF].       Upon successful completion, iffildes is not equal tofildes2, the       FD_CLOEXEC flag associated withfildes2 shall be cleared. Iffildes is equal tofildes2, the FD_CLOEXEC flag associated withfildes2 shall not be changed.       Iffildes refers to a typed memory object, the result of thedup2() function is unspecified.

RETURN VALUE top

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

ERRORS top

       Thedup() function shall fail if:EBADFThefildes argument is not a valid open file descriptor.EMFILEAll file descriptors available to the process are currently              open.       Thedup2() function shall fail if:EBADFThefildes argument is not a valid open file descriptor or              the argumentfildes2 is negative or greater than or equal              to {OPEN_MAX}.EINTRThedup2() function was interrupted by a signal.       Thedup2() function may fail if:EIOAn I/O error occurred while attempting to closefildes2.The following sections are informative.

EXAMPLES top

Redirecting Standard Output to a File S       The following example closes standard output for the current       processes, re-assigns standard output to go to the file referenced       bypfd, 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 fromstderr tostdout.           #include <unistd.h>           ...           dup2(1, 2);           ...

APPLICATION USAGE top

       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 callsdup2() with an arbitrary integer forfildes2 risks non-conforming behavior, anddup2() 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 acrossexec.

RATIONALE top

       Thedup() function is redundant. Its services are also provided by       thefcntl() function. It has been included in this volume of       POSIX.1‐2017 primarily for historical reasons, since many existing       applications use it. On the other hand, thedup2() function       provides unique services, as no other interface is able to       atomically replace an existing file descriptor.       Thedup2() function is not marked obsolescent because it presents       a type-safe version of functionality provided in a type-unsafe       version byfcntl().  It is used in the POSIX Ada binding.       Thedup2() function is not intended for use in critical regions as       a synchronization mechanism.       In the description of[EBADF], the case offildes being out of       range is covered by the given case offildes not being valid. The       descriptions forfildes andfildes2 are different because the only       kind of invalidity that is relevant forfildes2 is whether it is       out of range; that is, it does not matter whetherfildes2 refers       to an open file when thedup2() call is made.

FUTURE DIRECTIONS top

       None.

COPYRIGHT top

       Portions of this text are reprinted and reproduced in electronic       form from IEEE Std 1003.1-2017, Standard for Information       Technology -- Portable Operating System Interface (POSIX), The       Open Group Base Specifications Issue 7, 2018 Edition, Copyright       (C) 2018 by the Institute of Electrical and Electronics Engineers,       Inc and The Open Group.  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 athttp://www.opengroup.org/unix/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, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group                2017DUP(3P)

Pages that refer to this page:unistd.h(0p), sh(1p), close(3p), fstatvfs(3p), open(3p), posix_spawn(3p), posix_spawn_file_actions_addclose(3p), posix_spawn_file_actions_adddup2(3p), posix_typed_mem_open(3p), shm_open(3p), write(3p)