man/man3c/dup2.3c

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 DUP2 3C "February 17, 2023"
 NAME
dup2, dup3 - duplicate an open file descriptor
 SYNOPSIS
#include <unistd.h>

int dup2(int fildes, int fildes2);


int dup3(int fildes, int fildes2, int flags);


 DESCRIPTION
The dup2() function causes the file descriptor fildes2 to refer to
the same file as fildes. The fildes argument is a file descriptor
referring to an open file, and fildes2 is a non-negative integer less
than the current value for the maximum number of open file descriptors allowed
the calling process. See getrlimit(2). If fildes2 already refers
to an open file, not fildes, it is closed first. If fildes2 refers
to fildes, or if fildes is not a valid open file descriptor,
fildes2 will not be closed first.

The dup2() function is equivalent to fcntl(fildes,
F_DUP2FD, fildes2).

The dup3() function works similarly to the dup2() function with
two exceptions. If fildes and fildes2 point to the same file
descriptor, -1 is returned and errno set to EINVAL. If flags
is O_CLOEXEC, then filedes2 will have the FD_CLOEXEC flag
set causing the file descriptor to be closed during any future call of
exec(2).
 RETURN VALUES
Upon successful completion a non-negative integer representing the file
descriptor is returned. Otherwise, -1 is returned and errno is
set to indicate the error.
 ERRORS
The dup2() and dup3() functions will fail if:
EBADF

The fildes argument is not a valid open file descriptor.


EBADF

The fildes2 argument is negative or is not less than the current resource
limit returned by getrlimit(RLIMIT_NOFILE, .\|.\|.).


EINTR

A signal was caught during the dup2() call.


EMFILE

The process has too many open files. See fcntl(2).


Additionally, the dup3() function will fail if:
EINVAL

flags has a value other than 0 or O_CLOEXEC or fildes and
fildes2 point to the same file descriptor.


 ATTRIBUTES
See attributes(7) for descriptions of the following attributes:


ATTRIBUTE TYPE ATTRIBUTE VALUE
Interface Stability Standard
MT-Level Async-Signal-Safe


 SEE ALSO
 close (2),  creat (2),  exec (2),  fcntl (2),  getrlimit (2),  open (2),  pipe (2),  attributes (7),  standards (7)