A builtin that needs its argument, written bare to an exec, is EINVAL as through ctl

Through a ctl the bare word was refused up front (EINVAL); through exec it
ran, failed with EIO or asked for the argument. The exec now refuses it
the same way before it runs.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 84b27b1f..4b0214c3 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -262,7 +262,10 @@ refused here has done nothing. A line that then fails as it runs fails the
 write with the error the editor reports for it and the line, e.g. `Mount:
 already mounted "Mount peer /tmp/s"` (EIO), and `control message needs its
 argument "Save"` (EINVAL), for a builtin that would have asked at a prompt
-(a `Save` on a scratch) rather than open one nobody is there to answer. The
+(a `Save` on a scratch) rather than open one nobody is there to answer. A
+builtin that means nothing without its argument (`Mount`, `Msg`, `Find`),
+written bare to an `exec` or `/tagexec`, is refused the same way, `wrong
+#args in control message "Mount"` (EINVAL), before it runs. The
 lines before a failing one have taken effect and those after it never run,
 which is what acme's ctl loop does (editors/acme/xfid.c:600-790). An error
 that only happens as the editor performs what a line asked for -- a `Save`