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dup(2) - dup, dup2, dup, dup2 - duplicate a file descriptor - man 2 dup

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DUP(2)                     Linux Programmer's Manual                    DUP(2)



NAME
       dup, dup2 - duplicate a file(1,n) descriptor

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       dup and dup2 create a copy of the file(1,n) descriptor oldfd.

       After successful return of dup or dup2, the old and new descriptors may
       be used interchangeably. They share locks, file(1,n) position  pointers  and
       flags;  for example, if(3,n) the file(1,n) position is modified by using lseek on
       one of the descriptors, the position is also changed for the other.

       The two descriptors do not share the close-on-exec flag, however.

       dup uses the lowest-numbered unused descriptor for the new  descriptor.

       dup2  makes  newfd  be the copy of oldfd, closing newfd first if(3,n) neces-
       sary.

RETURN VALUE
       dup and dup2 return the new descriptor, or -1 if(3,n) an error(8,n) occurred  (in(1,8)
       which case, errno is set(7,n,1 builtins) appropriately).

ERRORS
       EBADF  oldfd  isn't  an  open(2,3,n)  file(1,n)  descriptor, or newfd is out of the
              allowed range for file(1,n) descriptors.

       EBUSY  (Linux only) This may be returned by dup2 during a  race  condi-
              tion with open(2,3,n)() and dup().

       EINTR  The dup2 call was interrupted by a signal.

       EMFILE The  process  already has the maximum number of file(1,n) descriptors
              open(2,3,n) and tried to open(2,3,n) a new one.

WARNING
       The  error(8,n)  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.

BUGS
       If newfd was open(2,3,n), any errors that would have been reported at  close(2,7,n)()
       time(1,2,n),  are lost. A careful programmer will not use dup2 without closing
       newfd first.

CONFORMING TO
       SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional EINTR and
       ENOLINK  error(8,n)  conditions.   POSIX.1  adds EINTR.  The EBUSY return is
       Linux-specific.

SEE ALSO
       close(2,7,n)(2), fcntl(2), open(2,3,n)(2)



Linux 1.1.46                      1994-08-21                            DUP(2)

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