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dup(2) [ultrix man page]

dup(2)								System Calls Manual							    dup(2)

Name
       dup, dup2 - duplicate an open file descriptor

Syntax
       newd = dup(oldd)
       int newd, oldd;

       dup2(oldd, newd)
       int oldd, newd;

Description
       The  system  call  duplicates  an  existing  object descriptor.	The argument oldd is a small non-negative integer index in the per-process
       descriptor table.  The value must be less than the size of the table, which is returned by The new descriptor, newd, returned by  the  call
       is the lowest numbered descriptor that is not currently in use by the process.

       The object referenced by the descriptor does not distinguish between references using oldd and newd in any way.	Thus, if newd and oldd are
       duplicate references to an open file, and calls all move a single pointer into the file.  If a separate pointer into the file is desired, a
       different object reference to the file must be obtained by issuing an additional call.

       In the second form of the call, specify the value of newd needed. If this descriptor is already in use, the descriptor is first deallocated
       as if a call had been done.

Return Values
       The value -1 is returned if an error occurs in either call.  The external variable errno indicates the cause of the error.

Diagnostics
       The and system calls fail under the following conditions:

       [EBADF]	      The oldd or newd is not a valid active descriptor.

       [EMFILE]       Too many descriptors are active.

       [EINTR]	      The or function was terminated prematurely by a signal.

See Also
       accept(2), close(2), getdtablesize(2), lseek(2), open(2), pipe(2), read(2), socket(2), socketpair(2), write(2)

																	    dup(2)

Check Out this Related Man Page

DUP(2)							      BSD System Calls Manual							    DUP(2)

NAME
dup, dup2 -- duplicate an existing file descriptor LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> int dup(int oldd); int dup2(int oldd, int newd); DESCRIPTION
The dup() system call duplicates an existing object descriptor and returns its value to the calling process (newd = dup(oldd)). The argument oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table, which is returned by getdtablesize(2). The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process. The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate refer- ences to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) system call. The close-on-exec flag on the new file descriptor is unset. In dup2(), the value of the new descriptor newd is specified. If this descriptor is already in use and oldd != newd, the descriptor is first deallocated as if the close(2) system call had been used. If oldd is not a valid descriptor, then newd is not closed. If oldd == newd and oldd is a valid descriptor, then dup2() is successful, and does nothing. RETURN VALUES
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error. ERRORS
The dup() and dup2() system calls fail if: [EBADF] The oldd or newd argument is not a valid active descriptor [EMFILE] Too many descriptors are active. SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2) STANDARDS
The dup() and dup2() system calls are expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
The dup() and dup2() functions appeared in Version 7 AT&T UNIX. BSD
June 4, 1993 BSD
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