DUP(2)

NAME
dup, dup2 - duplicate an existing file descriptor

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
dup(int oldd);
int
dup2(int oldd, int newd);

DESCRIPTION
Dup() 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 references 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) 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 a close(2) 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.

IMPLEMENTATION NOTES

In the non-threaded library dup() is implemented as the dup syscall.

In the threaded library, the dup syscall is assembled to _thread_sys_dup() and dup() is implemented as a function which locks oldd for read and write, then calls _thread_sys_dup(). Before returning, dup() unlocks oldd.

In the non-threaded library dup2() is implemented as the dup2 syscall.

In the threaded library, the dup2 syscall is assembled to _thread_sys_dup2() and dup2() is implemented as a function which locks both oldd and newd for read and write, then calls _thread_sys_dup2(). Before returning, dup2() unlocks oldd. and newd.

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
Dup() and dup2() fail if:

[EBADF]
    Oldd or newd 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() function calls are expected to conform to p1003.1-90.

HISTORY
A dup() and a dup2() function call appeared in AT&T UNIX.