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From: pfalstad@phoenix.Princeton.EDU (Paul Falstad)
Newsgroups: comp.unix.wizards
Subject: Re: dup2
Message-ID: <6050@idunno.Princeton.EDU>
Date: 7 Feb 91 22:16:32 GMT
References: <27B1CA1C.22559@ics.uci.edu>
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schmidt@ics.uci.edu (Doug Schmidt) wrote:
>  I'm curious, is it possible to implement the dup2() system
>call using only routines available in the standard C library and other
>existing system calls?

Two ways, at least.

1: close and dup only

int dup2(int fd1,int fd2)
{
   close(fd2);
   fd1 = dup(fd1);
   if (fd1 != fd2)
      fd1 = movefd(fd1,fd2);
   return fd1;
}

int movefd(int fd1,int fd2)
{
int fe;

   if (fd1 == -1)
      return -1;
   if ((fe = dup(fd1)) != fd2)
      fe = movefd(fe);
   close(fd1);
   return fe;
}

2: fcntl

int dup2(int fd1,int fd2)
{
   close(fd2);
   return(fcntl(fd1,F_DUPFD,fd2));
}

--
Paul Falstad, pfalstad@phoenix.princeton.edu PLink:HYPNOS GEnie:P.FALSTAD
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