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Qry : Behaviour of fgets -- ?


Machine 1 :
bash-3.00$ uname -a
SunOS <hostname5.10 Generic_118822-30 sun4u sparc SUNW,Sun-Fire-280R

bash-3.00$ gcc -v
Reading specs from /usr/local/lib/gcc-lib/sparc-sun-solaris2.8/2.95.3/
specs
gcc version 2.95.3 20010315 (release)
Machine 2:
bash-2.05b$ uname -a
Linux <hostname2.4. 21-4.EL #1 Fri Oct 3 18:13:58 EDT 2003 i686 i686
i386 GNU/Linux

bash-2.05b$ gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/3.2.3/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --
infodir=/usr/share/info --enable-shared --enable-threads=posix --
disable-checking --with-system-zlib --enable-__cxa_atexit --host=i386-
redhat-linux
Thread model: posix
gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-20)

bash-2.05b$ cat fgets_fail_test .c
#include<stdio. h>
#include <string.h>
#include<stdlib .h>

/* Demo fgets failed by Raxit Sheth & Gaurav Gupta */

int main()
{
char spcontent[100],sendrepfile[100];
FILE *spfp=NULL;
memset(sendrepf ile,0,100);
strcpy(sendrepf ile,"/tmp/filenotexists") ;
if ((spfp = fopen(sendrepfi le, "r")) == (FILE *)NULL)
{
printf("error in opening file %s",sendrepfile );fflush(stdout );
}
memset(spconten t, 0, sizeof(spconten t));

while (fgets(spconten t,40, spfp)!=NULL)
{
printf("The value of spcontent is
[%s]",spcontent);ff lush(stdout);;
}
return 0;
}
<simillar core dump occurs on both the system>
bash-2.05b$ gcc -Wall fgets_fail_test .c -o test
bash-2.05b$ ./test
error in opening file /tmp/filenotexistsSe gmentation fault (core
dumped)
bash-2.05b$ gdb ./test ./core.27887
GNU gdb Red Hat Linux (5.3.90-0.20030710.40rh )
Copyright 2003 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and
you are
welcome to change it and/or distribute copies of it under certain
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for
details.
This GDB was configured as "i386-redhat-linux-gnu"...(no debugging
symbols found)...Using host libthread_db library "/lib/tls/
libthread_db.so .1".

Core was generated by `./test'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /lib/tls/libc.so.6...(no debugging symbols
found)...done.
Loaded symbols for /lib/tls/libc.so.6
Reading symbols from /lib/ld-linux.so.2...(n o debugging symbols
found)...done.
Loaded symbols for /lib/ld-linux.so.2
#0 0xb7506444 in fgets () from /lib/tls/libc.so.6
(gdb) where
#0 0xb7506444 in fgets () from /lib/tls/libc.so.6
#1 0x0804854b in main ()
(gdb) frame 0
#0 0xb7506444 in fgets () from /lib/tls/libc.so.6
(gdb) print errno
Cannot access memory at address 0xb74a7008
char *fgets(char *s, int n, FILE *stream)
Qry : What is the Behaviour if stream is NULL. ?

As per our understanding fgets should return NULL < and internally
gets should put check on stream, if it is not NULL then only should do
additional access of stream.

please ignore if it is known and would be great if you can point out
in the implementation detail.
--Gaurav Gupta & Raxit Sheth
http://www.barcamp.org/BarCampMumbaiOct2007 <----BarCampMumbaiOc t2007

Sep 6 '07
285 8754
Chris Dollin <ch**********@h p.comwrites:
jacob navia wrote:
[...]
>In the case of fgets this implies:
o Testing for NULL.
o Testing for a bad value of n (<= 0)
o Testing for a NULL value in the receiving buffer.

o Testing for a bad value of n (maximum possible line length)
[...]

If the system has a maximum possible line length (say, 2000
characters), calling fgets() with n == 3000 is perfectly valid as long
as the buffer is at least 3000 byte long.
o Testing that the non-null buffer points to a legal C object
containing at least the `n` characters required

o Testing that the non-null stream points to a legal C object
Of course these last two may be impossible to check unless the
implementation keep track of *all* objects, which would impose a
tremendous overhead.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Sep 7 '07 #41
Rainer Weikusat <rw*******@mssg mbh.comwrites:
Jonathan Leffler <jl******@earth link.netwrites:
>Rainer Weikusat wrote:
[...]
>>Any statement ascribing properties to areas not defined by the
C-standard is a statement about something which is not part of C, as
defined by this standard.

Read up on 'Nasal Demons' (Google - 4th entry when I looked). In
comp.std.c land, undefined behaviour means exactly that.

Well, I wasn't about writing 'preferred phantasies of comp.std.c
regulars' but about C. And from the perpsective of the standard text,
claiming that

a = 3;
a = ++a;

results in any particular value for a is as wrong as claiming that it
may cause your next door neighbour to turn into a piano. If it may can
be determined empirically and both the answer and the means for
determining it are not part of the C programming language.
We don't claim (except jokingly) that executing those statements
*will* cause demons to fly out of your nose. The claim is merely that
nasal demons are one of the infinitely many behaviors permitted by the
standard. If you compile and run a program containing those
statements and demons in fact do fly out of your nose, you can
certainly complain to your compiler vendor, but you have no basis for
claiming that the compiler violates the standard.

It's entirely possible that we're sometimes insufficiently precise on
that point.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Sep 7 '07 #42
Rainer Weikusat wrote:
Jonathan Leffler <jl******@earth link.netwrites:
>>
Read up on 'Nasal Demons' (Google - 4th entry when I looked). In
comp.std.c land, undefined behaviour means exactly that.

Well, I wasn't about writing 'preferred phantasies of comp.std.c
regulars' but about C. And from the perpsective of the standard text,
claiming that

a = 3;
a = ++a;

results in any particular value for a is as wrong as claiming that it
may cause your next door neighbour to turn into a piano.
The standard does indeed not require that UB causes my next door
neighbour to turn into a piano, but that appears to be exactly the
behaviour of my DeathStation 9000.
Are you claiming that this behaviour is not correct?

Bart v Ingen Schenau
--
a.c.l.l.c-c++ FAQ: http://www.comeaucomputing.com/learn/faq
c.l.c FAQ: http://www.eskimo.com/~scs/C-faq/top.html
c.l.c++ FAQ: http://www.parashift.com/c++-faq-lite/
Sep 7 '07 #43
jacob navia wrote:
Jack Klein wrote:
.... snip ...
>>
"7.1.4 Use of library functions
1 Each of the following statements applies unless explicitly stated
otherwise in the detailed descriptions that follow: If an argument to
a function has an invalid value (such as a value outside the domain of
the function, or a pointer outside the address space of the program,
or a null pointer, or a pointer to non-modifable storage when the
correspondin g parameter is not const-qualifed) or a type (after
promotion) not expected by a function with variable number of
arguments, the behavior is undefined."

Nothing is said about the return value or behavior of any error.
The behavior is UNDEFINED.

Can't you read?

That is what I said!
No you didn't. You quoted something which said a NULL was
returned, without bothering to identify the source of the quote.
Then, in this immediate reply, you got ugly and rude rather than
accepting the accurate correction and a quote from the standard.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

Sep 7 '07 #44
jacob navia wrote:
Casper H.S. Dik wrote:
.... snip ...
>>
Dumping core is a good thing; you have a bug, it was caught.
Now go and fix your code.

Casper

Why error analysis is necessary?

Error analysis means trying to have a defined an in all cases
identical reaction to program errors.

This means that for each function we write, we try to
return a specified error report value if things go wrong.

In the case of fgets this implies:
o Testing for NULL.
o Testing for a bad value of n (<= 0)
o Testing for a NULL value in the receiving buffer.
NULLs are allowed in the receiving buffer, and don't matter since
they will either be beyond the line end or overwritten. It helps
to get the objective right before trying to code for it.

One is also allowed to snip obvious signatures, even though the
originator neglected to include a proper sig. marker. There are no
prizes for extreme laziness or rudeness.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

Sep 7 '07 #45
jacob navia wrote:
>
.... snip ...
>
The standard doesn't supply an bool isvalid_file(FI LE *);

This routine would be very easy and cheap to implement for the
folks that wrote the standard library but it wasn't added to C99.
No, it isn't easy. This is one more of your silly statements. C
doesn't have files, it has streams. They are controlled by the
FILE type.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

Sep 7 '07 #46
CBFalconer wrote:
jacob navia wrote:
>Jack Klein wrote:
... snip ...
>>"7.1.4 Use of library functions
1 Each of the following statements applies unless explicitly stated
otherwise in the detailed descriptions that follow: If an argument to
a function has an invalid value (such as a value outside the domain of
the function, or a pointer outside the address space of the program,
or a null pointer, or a pointer to non-modifable storage when the
correspondi ng parameter is not const-qualifed) or a type (after
promotion) not expected by a function with variable number of
arguments, the behavior is undefined."
Nothing is said about the return value or behavior of any error.
The behavior is UNDEFINED.

Can't you read?

That is what I said!

No you didn't. You quoted something which said a NULL was
returned,
I quoted the standard. That is the "something" .
without bothering to identify the source of the quote.

No. I started my message with
>The C99 standard of C says:

7.19.7.2 The fgets function
Synopsis
[snip]
>Then, in this immediate reply, you got ugly and rude rather than
accepting the accurate correction and a quote from the standard.
No. I did not insult anyone or whatever. You are dreaming things up.
I just wanted to argument that error checking is lacking in many
functions that *could* reasonably have more error returns specified.

This is a valid opinion, and I am free to argue it.

Sep 7 '07 #47
CBFalconer <cb********@yah oo.comwrites:
jacob navia wrote:
[...]
>In the case of fgets this implies:
o Testing for NULL.
o Testing for a bad value of n (<= 0)
o Testing for a NULL value in the receiving buffer.

NULLs are allowed in the receiving buffer, and don't matter since
they will either be beyond the line end or overwritten. It helps
to get the objective right before trying to code for it.
[...]

Let's be clear what we're talking about.

Here's the declaration of fgets:

char *fgets(char * restrict s, int n, FILE * restrict stream);

You're interpreting jacob's third point to mean null characters ('\0')
in the array pointed to by 's'. I don't believe that's what he meant;
the initial contents of the array are irrelevant, and if he were
talking about null characters he probably wouldn't have used the term
NULL, which is a macro that expands to a null pointer constant.

I think he was referring to checking in fgets whether s == NULL.
That's a reasonable check to make, but it's not required, and it
misses a plethora of other possible invalid values.

As a programmer, of course, I can't depend on any such checks, since
not all implementations perform them, and that's going to remain the
case for a long time. It's still entirely *my* responsibility to
avoid passing invalid arguments to fgets. If the implementation helps
me out by catching certain errors, that's great (unless it imposes too
much of a burden on *correct* calls), but I can't count on it. If I
don't trust myself to get this right, I can always write a wrapper
that checks the arguments before calling fgets.

A side note: Above I referred to "the array pointed to by 's'". Since
's' is a char*, it doesn't point to an array; it points to a char
(which presumably is the first element of an array). The standard
allows a char* to be a "pointer to a string", but there's no such
wording that allows it to be called a "pointer to an array", since a
pointer to an array is a valid and distinct concept. However, the
standard itself refers to the "the array pointed to by s" (C99
7.19.7.2p2). I suppose it's clear enough from context.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Sep 7 '07 #48
On Sep 7, 5:40 am, jacob navia <ja...@jacob.re mcomp.frwrote:
Chris Dollin wrote:
If you're /serious/ about checking for errors, "3 integer comparisions"
is a substantial underestimate.

No, not in this case. You argument seems to be:

If we are going to test exhaustively all errors, that would be
impossible.
Consequence:
We do not test anything at all.
Methinks that is a reasonable stance for an implementor to take.

* Conform to the standard.
* Provide value-adding features like useful and meaningful
diagnostics. Kudos if you can do semantic code analysis at compile
time to detect possible UB at runtime.
* Provide any extensions deemed useful in a way that it doesn't break
anything conforming.

- Anand

Sep 7 '07 #49
In article <87************ @fever.mssgmbh. com>,
Rainer Weikusat <rw*******@mssg mbh.comwrote:
>Yes - undefined behaviour means anything can happen,
>This statement is untrue (and obvious nonsense for any conceivable
real-world example). Undefined behaviours is 'behaviour [...] upon
which this international standard imposes no requirements'. This means
whatever the behaviour might or might not be is not relevant for
determining if the implementation is conforming to ISO/IEC 9899:1999,
ie follows the requirements imposed by it, or not.
Ok, let me rephrase it. Undefined behaviour means anything can happen
*without violating the standard*. No real implementation will make
DMR's head explode, but if it did it wouldn't be a standard violation.

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.
Sep 7 '07 #50

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