Redirect printf to TWO streams

Question

I am extending an existing C project that prints all information to stdout with printf. I would like to have this information printed both to stdout AND to a log file.

If I were a contributor to the original project, I would just substitute all printf calls with my custom log function. Alas, I am not, so here's my question:

Is it possible to redirect printf so that a single call prints both to stdout and to file?

I know this is a long shot but, if it were possible, I could obtain what I want without having to modify the original code.

EDIT: Thank you for the answer and comments about the tee command. However, I am looking for a way to do it directly in the C code, in an automated fashion, so that users won't have to bother using tee.

Thank you for your attention!

Answer 1

You're looking for the tee command:

./prog | tee file

This will show the output of ./prog in stdout and it will also store it in file. Think of tee as a tee fixture in plumbing :)

UPDATE

If you don't want to force users to think about using tee, I would just make a shell script that does precisely what I showed above - invoke the program with tee - and have users interact with the script only.

If that doesn't work for you, and you really want to change the source, I don't see any immediate easy solution. You could write a printf() wrapper that writes into the two streams, but then you'd have to go ahead and replace every call to printf() to your wrapper.