Is setting a FILE* equal to stdout portable?

I've got a function that needs to be able to write to either stdout, or to a file, depending on what the user wants. It defaults to standard out though. To accomplish this, I'm doing the following (minus error checking etc):

FILE* out;
if (writeToFile) { /*Code to open file*/; }

else
    out = stdout;

// ...rest of the function goes here

if (out != stdout)
    fclose(out);

This certainly does the trick, but I have no idea how portable it is. And if it's not, and/or there's another problem with it, how should I go about this?