dup2

dup2 is a POSIX system call and C library function that creates a duplicate of an existing file descriptor oldfd and places it in the descriptor number newfd. If newfd is currently open, dup2 closes it before duplicating oldfd. If oldfd and newfd are the same, the call succeeds and simply returns newfd.

After successful completion, both descriptors refer to the same open file description; the file offset and status flags are shared, meaning reads and writes observe the same position and flags such as O_APPEND are shared by both descriptors. The close-on-exec flag is inherited by the new descriptor.

Return value and errors: On success, dup2 returns newfd. On error, it returns -1 and sets errno. Common errors include EBADF when oldfd is not a valid open file descriptor, EINTR if the call is interrupted by a signal, and EINVAL if newfd is invalid or out of range.

Usage and portability: dup2 is widely available on Unix-like systems and is often used to redirect standard input, output, or error streams in shells and daemon programs before an exec call. It is the POSIX-compliant variant of duplicating descriptors; on Windows, similar functionality exists under different APIs.