why the usage of gets() is dangerous.

[jayapal]

Hi all,

Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

regards,
jayapal.

[user923005]

On Nov 16, 9:04 am, jayapal <jayapal...@gma il.comwrote:

Hi all,

Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

For the same reason that failure to read the C-FAQ is dangerous.
Because you will look like a nincompoop due to the omission of common
sense.

12.23: Why does everyone say not to use gets()?

A: Unlike fgets(), gets() cannot be told the size of the buffer
it's to read into, so it cannot be prevented from overflowing
that buffer. As a general rule, always use fgets(). See
question 7.1 for a code fragment illustrating the replacement
of
gets() with fgets().

References: Rationale Sec. 4.9.7.2; H&S Sec. 15.7 p. 356.

[Paul Hsieh]

On Nov 16, 9:04 am, jayapal <jayapal...@gma il.comwrote:

Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

No set of program control can prevent gets() from having undefined
behavior. In fact, basically all C compilers implement gets() to have
undefined behavior. Because of this, the function has been slated to
be deprecated in the next C standard. I have made a safe
implementation of gets() that you can find as the first example here:

http://www.pobox.com/~qed/userInput.html

Please feel free to use it in lieu of the upcoming standard which will
make its usage obsolete.

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/

[Eric Sosman]

Paul Hsieh wrote On 11/16/07 14:43,:

On Nov 16, 9:04 am, jayapal <jayapal...@gma il.comwrote:

>>Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

No set of program control can prevent gets() from having undefined
behavior. In fact, basically all C compilers implement gets() to have
undefined behavior. Because of this, the function has been slated to
be deprecated in the next C standard. I have made a safe
implementation of gets() that you can find as the first example here:

http://www.pobox.com/~qed/userInput.html

Please feel free to use it in lieu of the upcoming standard which will
make its usage obsolete.

Isn't there a buffer overrun vulnerability in the
fgetstralloc() function? Look carefully at the second
argument of the first call to getInputFrag().

--
Er*********@sun .com

[jameskuyper]

CJ wrote:

On 16 Nov 2007 at 18:41, jacob navia wrote:

jayapal wrote:

Hi all,

Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

regards,
jayapal.

That function is deprecated and will disappear shortly.
Its usage is not recommended because there is no way to
limit the input that it will receive, and it can overflow
the input buffer.

This sort of absolute prohibition on gets() is completely wrong-headed.
It's completely fine to use gets(), as long as you use it properly. To
use it properly, *you* need to be in control of the data that gets()
reads.

Keep in mind, of course, that there's absolutely nothing you can do
within strictly conforming C code that would give you the kind of
control you need to have to use gets() safely. The safety of such
usage depends upon something outside the C standard, and more likely
than not, something outside of the program itself.

It's trivial to replace any call to gets() with a similar call to
fgets() with minor modifications to the surrounding code, and that
change is sufficient to completely avoid the buffer overruns. I don't
know of any legitimate reason for not doing so.

[CJ]

On 16 Nov 2007 at 20:47, ja*********@ver izon.net wrote:

>

CJ wrote:

>On 16 Nov 2007 at 18:41, jacob navia wrote:

jayapal wrote:
Hi all,

Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

regards,
jayapal.

That function is deprecated and will disappear shortly.
Its usage is not recommended because there is no way to
limit the input that it will receive, and it can overflow
the input buffer.

This sort of absolute prohibition on gets() is completely wrong-headed.
It's completely fine to use gets(), as long as you use it properly. To
use it properly, *you* need to be in control of the data that gets()
reads.

Keep in mind, of course, that there's absolutely nothing you can do
within strictly conforming C code that would give you the kind of
control you need to have to use gets() safely. The safety of such
usage depends upon something outside the C standard, and more likely
than not, something outside of the program itself.

It's trivial to replace any call to gets() with a similar call to
fgets() with minor modifications to the surrounding code, and that
change is sufficient to completely avoid the buffer overruns. I don't
know of any legitimate reason for not doing so.

It's much more typing!

[Ben Pfaff]

jacob navia <ja***@nospam.c omwrites:

jayapal wrote:

>Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

That function is deprecated and will disappear shortly.

It is not listed in the current C standard as deprecated.
--
"A lesson for us all: Even in trivia there are traps."
--Eric Sosman

[=?iso-2022-kr?q?=1B=24=29CHarald_van_D=0E=29=26=0F]

On Fri, 16 Nov 2007 13:06:52 -0800, Ben Pfaff wrote:

jacob navia <ja***@nospam.c omwrites:

>jayapal wrote:

>>Whenever I use the gets() function, the gnu c compiler gives a warning
that it is dangerous to use gets(). why...?

That function is deprecated and will disappear shortly.

It is not listed in the current C standard as deprecated.

http://open-std.org/JTC1/SC22/WG14/www/docs/dr_332.htm

The status from the summary page is "closed, published in TC 3", and you
can find the added paragraph in n1256. Admittedly, I'm not sure if TC 3
is official just yet.

However, of course being deprecated does not mean it will actually be
removed any time soon.

[Paul Hsieh]

On Nov 16, 12:20 pm, Eric Sosman <Eric.Sos...@su n.comwrote:

Paul Hsieh wrote On 11/16/07 14:43,:

On Nov 16, 9:04 am, jayapal <jayapal...@gma il.comwrote:

>Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

No set of program control can prevent gets() from having undefined
behavior. In fact, basically all C compilers implement gets() to have
undefined behavior. Because of this, the function has been slated to
be deprecated in the next C standard. I have made a safe
implementation of gets() that you can find as the first example here:

http://www.pobox.com/~qed/userInput.html

Please feel free to use it in lieu of the upcoming standard which will
make its usage obsolete.

Isn't there a buffer overrun vulnerability in the
fgetstralloc() function? Look carefully at the second
argument of the first call to getInputFrag().

Its 64. getInputFrag(*, 64,*,*,*) never writes to more than 64 chars
(the extra '\0' only comes when the input is <= 64 in length; unlike
strncat, this is ok because the length read is always explicitly
returned), and the buffer passed (char blk[64]) in is 64 chars in
length. So ... what am I missing?

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/

[Keith Thompson]

jacob navia wrote:

jayapal wrote:

>Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

That function is deprecated and will disappear shortly.
Its usage is not recommended because there is no way to
limit the input that it will receive, and it can overflow
the input buffer.

It's true that gets() has been declared obsolescent and deprecated.
This is reflected in TC3 and in the latest standard draft, n1256.pdf.
This just happened within the last couple of months.

But please don't make the mistake of thinking that it "will disappear
shortly". It has not been removed from the C99 standard. In fact, any
conforming C99 implementation *must* provide gets(), undefined behavior
and all (though any implementation is free to warn about it).

Deprecation means that it will most likely be removed from the *next* C
standard, which is still a number of years away. Consider that the C99
standard is 8 years old, and still has not been fully implemented by the
vast majority of compilers. It will likely be decades, if ever, before
a significant number of implementations conform to a new C20YZ standard.
And even then, compilers will be free to continue to provide it in a
non-conforming mode, perhaps for backward compatibility.

I'm afraid that gets() is going to be around for a very long time. It's
still up to each of us, as programmers, to avoid using it.

jacob, if you really thing gets() will "disappear shortly", I'd be
interested in your reasoning.

--
Keith Thompson (The_Other_Keit h) <ks***@mib.or g>
Looking for software development work in the San Diego area.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[jacob navia]

Keith Thompson wrote:

jacob navia wrote:

>jayapal wrote:

>>Whenever I use the gets() function, the gnu c compiler gives a
warning that it is dangerous to use gets(). why...?

That function is deprecated and will disappear shortly.
Its usage is not recommended because there is no way to
limit the input that it will receive, and it can overflow
the input buffer.

It's true that gets() has been declared obsolescent and deprecated. This
is reflected in TC3 and in the latest standard draft, n1256.pdf. This
just happened within the last couple of months.

But please don't make the mistake of thinking that it "will disappear
shortly". It has not been removed from the C99 standard. In fact, any
conforming C99 implementation *must* provide gets(), undefined behavior
and all (though any implementation is free to warn about it).

Deprecation means that it will most likely be removed from the *next* C
standard, which is still a number of years away. Consider that the C99
standard is 8 years old, and still has not been fully implemented by the
vast majority of compilers. It will likely be decades, if ever, before
a significant number of implementations conform to a new C20YZ standard.
And even then, compilers will be free to continue to provide it in a
non-conforming mode, perhaps for backward compatibility.

I'm afraid that gets() is going to be around for a very long time. It's
still up to each of us, as programmers, to avoid using it.

jacob, if you really thing gets() will "disappear shortly", I'd be
interested in your reasoning.

Nothing, just hopes that now that is deprecated, people will
stop using it, and it will disappear in a few years.

--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32