You can duplicate a file descriptor, or allocate another file
descriptor that refers to the same open file as the original. Duplicate
descriptors share one file position and one set of file status flags
(see File Status Flags), but each has its own set of file descriptor
flags (see Descriptor Flags).
The major use of duplicating a file descriptor is to implement
redirection of input or output: that is, to change the
file or pipe that a particular file descriptor corresponds to.
You can perform this operation using the fcntl function with the
F_DUPFD command, but there are also convenient functions
dup and dup2 for duplicating descriptors.
The fcntl function and flags are declared in fcntl.h,
while prototypes for dup and dup2 are in the header file
— Function: int dup (int old)
This function copies descriptor old to the first available
descriptor number (the first number not currently open). It is
equivalent to fcntl (old, F_DUPFD, 0).
— Function: int dup2 (int old, int new)
This function copies the descriptor old to descriptor number
If old is an invalid descriptor, then dup2 does nothing; it
does not close new. Otherwise, the new duplicate of old
replaces any previous meaning of descriptor new, as if new
were closed first.
If old and new are different numbers, and old is a
valid descriptor number, then dup2 is equivalent to:
fcntl (old, F_DUPFD, new)
However, dup2 does this atomically; there is no instant in the
middle of calling dup2 at which new is closed and not yet a
duplicate of old.
— Macro: int F_DUPFD
This macro is used as the command argument to fcntl, to
copy the file descriptor given as the first argument.
The form of the call in this case is:
fcntl (old, F_DUPFD, next-filedes)
The next-filedes argument is of type int and specifies that
the file descriptor returned should be the next available one greater
than or equal to this value.
The return value from fcntl with this command is normally the value
of the new file descriptor. A return value of -1 indicates an
error. The following errno error conditions are defined for
The old argument is invalid.
The next-filedes argument is invalid.
There are no more file descriptors available—your program is already
using the maximum. In BSD and GNU, the maximum is controlled by a
resource limit that can be changed; see Limits on Resources, for
more information about the RLIMIT_NOFILE limit.
ENFILE is not a possible error code for dup2 because
dup2 does not create a new opening of a file; duplicate
descriptors do not count toward the limit which ENFILE
indicates. EMFILE is possible because it refers to the limit on
distinct descriptor numbers in use in one process.
Here is an example showing how to use dup2 to do redirection.
Typically, redirection of the standard streams (like stdin) is
done by a shell or shell-like program before calling one of the
exec functions (see Executing a File) to execute a new
program in a child process. When the new program is executed, it
creates and initializes the standard streams to point to the
corresponding file descriptors, before its main function is
So, to redirect standard input to a file, the shell could do something