Date: Sun, 15 Dec 1996 13:17:48 +0200 (EET) From: jau@iki.fi (Jukka Ukkonen) To: hackers@freebsd.org, questions@freebsd.org Subject: funny unstable behaviour in the stdio library... Message-ID: <199612151117.NAA24559@jau.thunderbolt.fi>
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Hi everybody!
Has anyone noticed the following unexpected behaviour in the
stdio library? When one reads the input using fgetc() and
echoes the read character immediately using fputc() there is
no problem.
If one instead reads the input input using fgetc(stdin) and tries
to output something using fprintf(stdout, ...) (or printf) there
appears some weird extra garbage on the out (usually an 'a' in my
case) just as if the printf machinery left some trash in the input
for the fgetc (stdin) to find. If I fseek() the stdin even without
trying any real movement within the stream as in
fseek (stdin, 0, SEEK_CUR);
before calling fgetc(stdin) the next time everything seems to
be almost alright. But even then it only works correctly for
one character at the time input, because fseek (stdin, ...)
flushes the input stream after the one character that was just
read and processed.
You could try the following short piece of code to reproduce
the problem.
------------------------------ clip clip ------------------------------
/*
* stdio-echo-test
*/
#include <stdio.h>
#undef fputc
int
main (ac, av)
int ac;
char **av;
{
register char ch;
for (ch = fgetc (stdin); ! feof (stdin); ch = fgetc (stdin)) {
fprintf (stdout, "ch = %x %c\n", ch, ch);
#ifdef SEEK_STDIN
fseek (stdin, 0, SEEK_CUR);
#endif
}
return (0);
}
------------------------------ clip clip ------------------------------
When everything works as expected one would expect it produce
something like this...
input: XYZ
output: ch = 58 X
output: ch = 59 Y
output: ch = 5a Z
What I get without the additional call to fseek() is...
input: XYZ
output: ch = 58 X
output: ch = 59 Y
output: ch = 5a Z
output: ch = a
I have no idea whether this oddity is present already in the
original BSD version of stdio library or has someone tried to
fix or enhance something and broken the original on the way.
I assume though that this is probably a problem in the original
code because also Linux and DEC's OSF/1 both seem to behave
exactly the same way.
Anyhow one does not (And should not have to!!) expect the stdin
change when calling one of the printf functions on the stdout
stream, if the input stream does not change also when using
the fputc() family of functions. The same weird behaviour seems
to be present at least on FreeBSD versions from 2.1.0-RELEASE
to 2.2-960501-SNAP. Does anyone have good ideas about why is
this as it is, and what could be done about it if anything?
Cheers,
// jau
------
/ Jukka A. Ukkonen, Telemedia / Finnish Telecom Ltd.
/__ M.Sc. (sw-eng & cs) (Phone) +358-2040-4025
/ Internet: Jukka.Ukkonen@tele.fi (Fax) +358-2040-2712
/ Internet: jau@iki.fi (Mobile) +358-400-606671
v Internet: ukkonen@nic.funet.fi (Home&Fax) +358-9-6215280
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