The MH and nmh mail clients used to directly look into FILE internals. If you look for LINUX_STDIO in this old version of the relevant file you can see the kind

The MH and nmh mail clients used to directly look into FILE internals. If you look for LINUX_STDIO in this old version of the relevant file you can see the kind of ugliness that resulted: https://cgit.git.savannah.gnu.org/cgit/nmh.git/tree/sbr/m_getfld.c?id=ffbc91b2b571f18eb8c1ed3a90e76e5d0ad65bd1 https://cgit.git.savannah.gnu.org/cgit/nmh.git/tree/sbr/m_ge... It's basically searching an email file to find the contents of either a given header or the mail body. These days there is no need to go under the hood of libc for this (and this code got ripped out over a decade ago), but back when the mail client was running on elderly VAXen this ate up significant time. Sneaking in and reading directly from the internal stdio buffer lets you avoid copying all the data the way an fread would. The same function also used to have a bit of inline vax assembly for string searching... The only reason this "works" is that traditionally the FILE struct is declared in a public header so libc can have some of its own functions implemented as macros for speed, and that there was not (when this hack was originally put in in the 1980s) yet much divergence in libc implementations.