3.12 dup and dup2 Functions
An existing file descriptor is duplicated by either of the following functions:
#include <unistd.h> int dup(int filedes); int dup2(int filedes, int
filedes2); Both return: new file
descriptor if OK, -1 on error
The new file descriptor returned by dup is guaranteed to be the lowest numbered available file descriptor. With dup 2 we specify the value of the new descriptor with the filedes2 argument. If filedes2 is already open, it is first closed. If filedes equals filedes2, then dup 2 returns filedes2 without closing it.
The new file descriptor that is returned as the value of the functions shares the same file table entry as the filedes argument. We show this in Figure 3.4.
Figure 3.4. Kernel data structures after dup(1).
In this figure we're assuming that the process executes
newfd = dup(1);
when it's started. We assume the next available descriptor is 3 (which it probably is, since 0, 1, and 2 are opened by the shell). Since both descriptors point to the same file table entry they share the same file status flags (read, write, append, etc.) and the same current file offset.
Each descriptor has its own set of file descriptor flags. As we describe in the next section, the close-on-exec file descriptor flag for the new descriptor is always cleared by the dup functions.
Another way to duplicate a descriptor is with the fcntl function, which we describe in the next section. Indeed, the call
is equivalent to
fcntl(filedes, F_DUPFD, 0);
and the call
is equivalent to
close(filedes2); fcntl(filedes, F_DUPFD, filedes2);
In this last case, the dup2 is not exactly the same as a close followed by an fcntl.
The differences are
dup2 is an atomic operation, while the alternate form involves two function calls. It is possible in the latter case to have a signal catcher called between the close and fcntl that could modify the file descriptors. (We describe signals in Chapter 10.)
There are some errno differences between dup2 and fcntl.
The dup2 system call originated with Version 7 and propagated through the BSD releases. The fcntl method for duplicating file descriptors appeared with System III and continued with System V. SVR3.2 picked up the dup2 function and 4.2BSD picked up the fcntl function and the F_DUPFD functionality. POSIX.1 requires both dup2 and the F_DUPFD feature of fcntl.