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). The returned file descriptor will have neither the close-on-exec (FD_CLOEXEC) or close-on-fork (FD_CLOFORK) flags set on it, regardless of what is set on fildes.

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. The flags argument allows for control of the returned file descriptor's flags. Valid values are the bitwise-inclusive-OR of:

O_CLOEXEC

The returned file descriptor will have the close-on-exec flag, FD_CLOEXEC set on it. The file descriptor will be automatically closed when the process calls the exec(2) family of functions.

O_CLOFORK

The returned file descriptor will have the close-on-fork flag, FD_CLOFORK set on it. The file descriptor will be automatically closed in any child processes created with the fork(2) family of functions. It will remain open in the parent.

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:

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 the bitwise-inclusive-OR of O_CLOEXEC and O_CLOFORK. fildes and fildes2 point to the same file descriptor.

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

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

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

June 21, 2024 OmniOS