practical examples use dup or dup2

Question

Hi,

I know what does dup or dup2 do ,but I have no idea when it would be used. Any practical examples? Thanks.

Answer 1

One example use would be I/O redirection. For this you fork a child process and close the stdin or stdout file descriptors (0 and 1) and then you do a dup() on another filedescriptor of your choice which will now be mapped to the lowest available file descriptor, which is in this case 0 or 1.

Using this you can now exec any child process which is possibly unaware of your application and whenever the child writes on the stdout (or reads from stdin, whatever you configured) the data gets written on the provided filedescriptor instead.

Shells use this to implement commandos with pipes, e.g. /bin/ls | more by connecting the stdout of one process to the stdin of the other.

Answer 2

When you are curious about POSIX functions, especially those that seem to duplicate themselves its generally good to check the standard itself. At the bottom you will usually see examples, as well as reasoning behind the implementation (and existence) of both.

In this case:

The following sections are informative.
EXAMPLES
Redirecting Standard Output to a File

The following example closes standard output for the current processes, re-assigns standard output to go to the file referenced by pfd, and closes the original file descriptor to clean up.
#include <unistd.h>
...
int pfd;
...
close(1);
dup(pfd);
close(pfd);
...

Redirecting Error Messages

The following example redirects messages from stderr to stdout.
#include <unistd.h>
...
dup2(1, 2);
...

APPLICATION USAGE

None.
RATIONALE

The dup() and dup2() functions are redundant. Their services are also provided by the fcntl() function. They have been included in this volume of IEEE Std 1003.1-2001 primarily for historical reasons, since many existing applications use them.

While the brief code segment shown is very similar in behavior to dup2(), a conforming implementation based on other functions defined in this volume of IEEE Std 1003.1-2001 is significantly more complex. Least obvious is the possible effect of a signal-catching function that could be invoked between steps and allocate or deallocate file descriptors. This could be avoided by blocking signals.

The dup2() function is not marked obsolescent because it presents a type-safe version of functionality provided in a type-unsafe version by fcntl(). It is used in the POSIX Ada binding.

The dup2() function is not intended for use in critical regions as a synchronization mechanism.

In the description of [EBADF], the case of fildes being out of range is covered by the given case of fildes not being valid. The descriptions for fildes and fildes2 are different because the only kind of invalidity that is relevant for fildes2 is whether it is out of range; that is, it does not matter whether fildes2 refers to an open file when the dup2() call is made.
FUTURE DIRECTIONS

None.
SEE ALSO

close(), fcntl(), open(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Answer 3

One practical example is redirecting output messages to some other stream like some log file. Here is a sample code for I/O redirection.
Please refer to original post here

#include <stdio.h>

main()
{
    int    fd;
    fpos_t pos;

    printf("stdout, ");

    fflush(stdout);
    fgetpos(stdout, &pos);
    fd = dup(fileno(stdout));
    freopen("stdout.out", "w", stdout);

    f();

    fflush(stdout);
    dup2(fd, fileno(stdout));
    close(fd);
    clearerr(stdout);
    fsetpos(stdout, &pos);        /* for C9X */

    printf("stdout again\n");
}

f()
{
printf("stdout in f()");
}