Puzzling behavior from echo area: truncates prompt string at 100 characters?

[Tom Davey <tdavey@gmail.com>]

Hello folks,

I'm using Emacs 24.0.50.1. The following "message" command, with an
arbitrarily chosen argument string of exactly 150 characters --

(message "0123456789 123456789 223456789 323456789 423456789 523456789
623456789 723456789 823456789 923456789 023456789 123456789 223456789
323456789 423456789")

-- displays the entire length of the string (all 150 characters) in
the minibuffer's echo area, as one would expect. In fact, I find that
the echo area frame will expand in height to display even much longer
messages. (The variable "message-truncate-lines" is nil.)

However, if the same 150-character string becomes an argument to an
"(interactive)" declaration in a defun() function, like this --

(interactive "c0123456789 123456789 223456789 323456789 423456789
523456789 623456789 723456789 823456789 923456789 023456789 123456789
223456789 323456789 423456789")

-- then the display of the prompt string in the echo area is
truncated. Only the first 99 characters display; the remainder of the
echo area (to the right) is unused and empty. Nor does the echo area's
frame expand in height, as it does with the "message" function.

Naturally, in a real-world application my prompt string would not be
composed of integers but of helpful alphanumeric text. However, no
matter what form of a prompt string I supply to the "interactive"
declaration, the echo area truncates it at the 100th character.

Does anybody have an idea how I might get a long prompt in an
interactive declaration to fully display in the echo area?

Thanks,
Tom Davey

-- 
Tom Davey
tom@tomdavey.com
New York NY USA