one cause, it *really* should have an `int` as the return value for the error
code.
+- libc system calls typically return -1 on error (with the error code in
+ `errno`), and >= 0 on success. Use the RET_NERRNO() helper if you are looking
+ for a simple way to convert this libc style error returning into systemd
+ style error returning. e.g.
+
+ ```c
+ …
+ r = RET_NERRNO(unlink(t));
+ …
+ ```
+
+ or
+
+ ```c
+ …
+ r = RET_NERRNO(open("/some/file", O_RDONLY|O_CLOEXEC));
+ …
+ ```
+
- Do not bother with error checking whether writing to stdout/stderr worked.
- Do not log errors from "library" code, only do so from "main program"