dup2

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  neces-
       sary.

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  condi-
	      tion 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 EINVAL 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)