dup - phpMan

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



NAME
       dup, dup2 - duplicate a file 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 descriptor oldfd.

       After  successful  return  of  dup or dup2, the old and new descriptors may be used
       interchangeably. They share locks, file position pointers and flags;  for  example,
       if  the  file  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 necessary.

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

ERRORS
       EBADF  oldfd  isn’t  an  open file descriptor, or newfd is out of the allowed range
              for file descriptors.

       EMFILE The process already has the maximum number  of  file  descriptors  open  and
              tried to open a new one.

       EINTR  The dup2 call was interrupted by a signal.

       EBUSY  (Linux  only)  This  may  be  returned  by dup2 during a race condition with
              open() and dup().

WARNING
       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 EIN-
       VAL like F_DUPFD.

BUGS
       If newfd was open, any errors that would have been reported at  close()  time,  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 conditions.  POSIX.1 adds EINTR.  The EBUSY return is Linux-specific.

SEE ALSO
       fcntl(2), open(2), close(2)



Linux 1.1.46                      1994-08-21                            DUP(2)

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