how to use the system call dup? - c

Question

I am trying to understand how the system call dup() works. I am asking this question because I am writing a shell in C and I need to redirect the STDOUT to a file. Is this the right way to do it?
If for example I have the following code:
remember = dup(STDOUT_FILENO);
fileDescriptor = open("file.txt",O_RDONLY);
then everything that writes to the stdout will now write to the opened file?
As soon as the following line is executed:
remember = dup(STDOUT_FILENO);
STDOUT_FILENO is removed from the table of file descriptors leaving the first spot empty. When a new file is opened, the earliest empty file descriptor will be appointed to this new opened file, so in this case 1.

Answer 1

Nope. You just duplicate the file descriptor for stdout.
With the code you have so far, you could now do a write to remember, and the output would go to console, too:
char str = "this now goes to console, too!";
write(remember, str, strlen(str));
If you want to redirect console output, you yet have to do this:
dup2(fileDescriptor, STDOUT_FILENO);
This will close STDOUT_FILENO (but you have a duplicate in remember to restore it, if need be) and overwrite it with fileDescriptor – and from now on, console output goes to file...
If you don't ever consider to restore outputting to console, you can ommit the first call to dup entirely...
Edit (in response to your edit):
STDOUT_FILENO is removed from the table of file descriptors leaving the first spot empty. When a new file is opened, the earliest empty file descriptor will be appointed to this new opened file, so in this case 1.
This applies for close(STDOUT_FILENO)!
So back to if not wanting to restore: You could then do, too:
close(STDOUT_FILENO);
fileDescriptor = open("file.txt",O_WRONLY | O_CREAT);
// fileDescriptor will be 1 now
By the way: You must open your file with write access enabled (O_WRONLY or O_RDWR), as you want to write to that file (redirect output to)!
And you need the O_CREAT flag for the case the file does not exist yet. If you do not want to clear the file, but append to, add the O_APPEND flag (see open).

Answer 2

No dup is used to duplicate an existing descriptor and returning a duplicate whose value is the less possible among free descriptors (some docs says ``The new descriptor returned by the call is the lowest
numbered descriptor currently not in use by the process.''), so:
fileDescriptor = open("file.txt",O_RDONLY); // get new desc.
close(STDIN_FILENO); // close 0
dup(fileDescriptor); // dup new desc to 0 (less possible free desc).
// here fileDescriptor and 0 are aliases to the same opened file
close(fileDescriptor); // free unused desc.