GNU
troff
operates in “safer” mode by default;
to mitigate risks from untrusted input documents,
the
cf
,
pi
,
and
sy
requests are disabled.
GNU
troff
’s
-U
option enables “unsafe mode”,
restoring their function and enabling additional
groff
extension requests,
open
,
opena
,
and pso
.
See I/O.