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FreeBSD 11.0 - man page for dup2 (freebsd section 2)

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 new descriptor returned by the call is the lowest num- bered 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
These calls return the new file descriptor if successful; otherwise the value -1 is returned and the external variable errno is set to indi- cate the cause of the error.
ERRORS
The dup() system call fails if: [EBADF] The oldd argument is not a valid active descriptor [EMFILE] Too many descriptors are active. The dup2() system call fails if: [EBADF] The oldd argument is not a valid active descriptor or the newd argument is negative or exceeds the maximum allowable descriptor number
SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3)
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 1, 2013 BSD