Thread: open bracket and underscore before function opening bracket

Hi I've seen this around and I'm not sure what it is for. I've seen it in other projects and it compiles fine but when I tried:

Code:

file_chooser_directory = gtk_file_chooser_button_new (_("Select a directory"),
                                                              GTK_FILE_CHOOSER_ACTION_OPEN);

I recieved the following from gcc:

Code:

/tmp/ccbrQALM.o: In function `main':
/media/disk/cleaner/src/main.c:175: undefined reference to `_'
collect2: ld returned 1 exit status

Yet if I remove "_", gcc has no complaints. Any insight is appreciated. Thanks.