where is variable dump_file assigned?

[wang tom via gcc-help]

Hello,
there are a lot of places to use dump_file, such as the following in ipa_inline.c:
 if(dump_file)  fprintf (dump_file, "\nFlattening functions:\n");
And I don't know where is the dump_file being assigned a file name.Can I directly assign the file name and do fprintf() to output data?Or, is there something, like the format, I need to pay attention for?