DUP2(3C) Standard C Library Functions DUP2(3C)

dup2, dup3 - duplicate an open file descriptor

#include <unistd.h>
int dup2(int fildes, int fildes2);

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

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).

Ths 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).

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.

The dup2() and dup3() functions will fail if:


The fildes argument is not a valid open file descriptor.


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


A signal was caught during the dup2() call.


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

Additionally, the dup3() function will fail if:


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

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

Interface Stability Standard
MT-Level Async-Signal-Safe

close(2), creat(2), exec(2), fcntl(2), getrlimit(2), open(2), pipe(2), attributes(7), standards(7)
February 4, 2020 OmniOS