Hello,
here I have a strange problem with a real simple strtok example.
The program is as follows:
### BEGIN STRTOK ###
#include <string.h>
#include <stdio.h>
int main()
{
char *input1 = "Hello, World!";
char *tok;
tok = strtok(input1, " ");
if(tok) printf("%s\n", tok);
tok = strtok(NULL, " ");
if(tok) printf("%s\n", tok);
return(0);
}
### END STRTOK ###
Now, when I run it from the command line, I get a bus error:
### BEGIN COMMAND LINE OUTPUT ###
gcc -ggdb -Wall -o strtok strtok.c
./strtok
Bus error (core dumped)
Exit 138
### END COMMAND LINE OUTPUT ###
When I run it step by step in GDB, the program terminates normally:
### BEGIN DEBUGGER OUTPUT ###
gdb ./strtok
GNU gdb 6.1.1 [FreeBSD]
[snip]GDB copyright and bla bla[/snip]
(gdb) break main
Breakpoint 1 at 0x8048570: file strtok.c, line 6.
(gdb) run
Starting program: /home/piter/strtok
Breakpoint 1, main () at strtok.c:6
6 char *input1 = "Hello, World!";
(gdb) next
10 tok = strtok(input1, " ");
(gdb)
11 if(tok) printf("%s\n", tok);
(gdb)
Hello,
13 tok = strtok(NULL, " ");
(gdb)
14 if(tok) printf("%s\n", tok);
(gdb)
World!
16 return(0);
(gdb)
18 }
(gdb)
0x08048485 in _start ()
(gdb)
Single stepping until exit from function _start,
which has no line number information.
Program exited normally.
(gdb)
### END DEBUGGER OUTPUT ###
Is there something I'm missing wrt C and/or strtok, or it's rather a
problem related to my environment (in which case I'll be happy to post
in the right newsgroup) ?
Thanx in advance
--
Pietro Cerutti
PGP Public Key ID: http://gahr.ch/pgp
May 16 '07
29  2588 
In article <40************ *************** @news.hispeed.c h>,
Pietro Cerutti <ga**@gahr.chwr ote:
>But if a string literal is - by definition - not modifiable, then how
can it happen that GDB actually modifies it using strtok?
It's not modifiable in that you're not allowed to modify it. It's not
required that the implementation signal an error when you do it. It's
a constraint on you, not on the system.
My guess as to why you don't see an error with GDB is that the
debugger needs the text segment to be writable, so that it can set
breakpoints.
-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.
Pietro Cerutti <ga**@gahr.chwr ites:
Ian Collins wrote:
>Pietro Cerutti wrote:
>>Pietro Cerutti wrote:
char *input1 = "Hello, World!";
just in case, I know that the string to be tokenized shouldn't be a
constant, but rather an array of chars.
So, it should be declared as
char input1[14] = "Hello, World!";
The thing I don't understand is: why does it works in GDB?
Luck?
Ya, maybe.
The point is:
I understand what UB means, so WW3 could start now and I'd know why...
But if a string literal is - by definition - not modifiable, then how
can it happen that GDB actually modifies it using strtok?
I think you don't *quite* understand what UB means.
The actual definition (C99 3.4.3) is:
behavior, upon use of a nonportable or erroneous program construct
or of erroneous data, for which this International Standard
imposes no requirements
and C99 6.4.5p6 says:
[...] If the program attempts to modify such an array, the
behavior is undefined.
For example, consider this program:
#include <stdio.h>
int main(void)
{
char *s = "Hello, world";
s[0] = 'J'; /* attempt to modify a string literal */
puts(s);
return 0;
}
One of the infinitely many possibly results is that the string literal
is actually modified, and the program prints "Jello, world".
The standard doesn't say that string literals are not modifiable. It
says that attempting to modify a string literal invokes undefined
behavior.
--
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"
Keith Thompson wrote:
The standard doesn't say that string literals are not modifiable. It
says that attempting to modify a string literal invokes undefined
behavior.
Got it. Thanks!
--
Pietro Cerutti
PGP Public Key: http://gahr.ch/pgp
Pietro Cerutti wrote:
Chris Dollin wrote:
>You're not allowed to write into a string literal: that gets you
undefined behaviour.
An implementation may just write into the string.
Uh? So you mean that a string literal isn't unmodifiable by definition?
Yes, that's what I (well, the C standard) says.
Specifically, it says that if you attempt to write into a string literal,
/the effect is undefined/. Anything can happen. C washes it's hands of
your code. It cares not. Mind the gap. Do as you will.
An implementation may implement this freedom by changing the content of
the literal, if that's convenient.
Hence: don't go writing into string literals. Even though it /might/
get you a date, it probably won't, and I am assured that nasal demons
are not fun to have.
--
"I'm still here and I'm holding the answers" - Karnataka, /Love and Affection/
Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN
Richard Tobin wrote:
My guess as to why you don't see an error with GDB is that the
debugger needs the text segment to be writable, so that it can set
breakpoints.
GDB on Debian/GNU Linux gives an error when I try to modify it.
On FreeBSD it doesn't, that's why I'm asking right now the FreeBSD
people whether the behavior is wanted or erroneous.
Thanx
>
-- Richard
--
Pietro Cerutti
PGP Public Key: http://gahr.ch/pgp
Chris Dollin wrote:
Pietro Cerutti wrote:
>Chris Dollin wrote:
>>You're not allowed to write into a string literal: that gets you
undefined behaviour.
An implementation may just write into the string.
Uh? So you mean that a string literal isn't unmodifiable by definition?
Yes, that's what I (well, the C standard) says.
Specifically, it says that if you attempt to write into a string literal,
/the effect is undefined/. Anything can happen. C washes it's hands of
your code. It cares not. Mind the gap. Do as you will.
An implementation may implement this freedom by changing the content of
the literal, if that's convenient.
Hence: don't go writing into string literals. Even though it /might/
get you a date, it probably won't, and I am assured that nasal demons
are not fun to have.
Clear. Thanks to you too.
--
Pietro Cerutti
PGP Public Key: http://gahr.ch/pgp
Pietro Cerutti said:
Richard Tobin wrote:
>My guess as to why you don't see an error with GDB is that the
debugger needs the text segment to be writable, so that it can set
breakpoints.
GDB on Debian/GNU Linux gives an error when I try to modify it.
That's an acceptable outcome of undefined behaviour.
On FreeBSD it doesn't,
So's that.
that's why I'm asking right now the FreeBSD
people whether the behavior is wanted or erroneous.
It is neither Debian nor FreeBSD, but rather your program, that is
erroneous.
--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999 http://www.cpax.org.uk
email: rjh at the above domain, - www.
In article <Qq************ *************** ***@bt.com>,
Richard Heathfield <rj*@see.sig.in validwrote:
>GDB on Debian/GNU Linux gives an error when I try to modify it.
That's an acceptable outcome of undefined behaviour.
>On FreeBSD it doesn't,
So's that.
that's why I'm asking right now the FreeBSD
>people whether the behavior is wanted or erroneous.
It is neither Debian nor FreeBSD, but rather your program, that is
erroneous.
I think he meant "erroneous" in the sense of a mistake, rather than
a violation of the C standard.
It certainly seems desirable to have programs behave the same way
under the debugger as without it, so it would be good if the FreeBSD
version could be changed. Meanwhile, we at least have a clue that if
a segmentation fault goes away in the debugger then the cause may well
be attempted modification of literal strings.
-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.
"Keith Thompson" <ks***@mib.orgw rote in message news:ln******** ****@nuthaus.mi b.org...
Pietro Cerutti <ga**@gahr.chwr ites:
>Ian Collins wrote:
>>Pietro Cerutti wrote:
Pietro Cerutti wrote:
char *input1 = "Hello, World!";
just in case, I know that the string to be tokenized shouldn't be a
constant, but rather an array of chars.
So, it should be declared as
char input1[14] = "Hello, World!";
The thing I don't understand is: why does it works in GDB?
Luck?
Ya, maybe.
The point is:
I understand what UB means, so WW3 could start now and I'd know why...
But if a string literal is - by definition - not modifiable, then how
can it happen that GDB actually modifies it using strtok?
I think you don't *quite* understand what UB means.
The actual definition (C99 3.4.3) is:
behavior, upon use of a nonportable or erroneous program construct
or of erroneous data, for which this International Standard
imposes no requirements
and C99 6.4.5p6 says:
[...] If the program attempts to modify such an array, the
behavior is undefined.
For example, consider this program:
#include <stdio.h>
int main(void)
{
char *s = "Hello, world";
s[0] = 'J'; /* attempt to modify a string literal */
puts(s);
return 0;
}
One of the infinitely many possibly results is that the string literal
is actually modified, and the program prints "Jello, world".
The standard doesn't say that string literals are not modifiable. It
says that attempting to modify a string literal invokes undefined
behavior.
<OT>
Yes, _but_: from the point of view of gdb users and maintainers, they
may still consider it a gdb bug if, on a single platform, _any_ program
executes differently under gdb than it does when run normally. After all, the
underlying problem -- writing into r/o storage -- could be triggered from
an assembler program. And gdb doesn't have the same standards-contract
relationship with anything that a C implementation does.
It is, however, a separate issue from the fact that the program invokes UB.
</OT>
Richard Tobin wrote:
Pietro Cerutti <ga**@gahr.chwr ote:
>But if a string literal is - by definition - not modifiable, then
how can it happen that GDB actually modifies it using strtok?
It's not modifiable in that you're not allowed to modify it. It's
not required that the implementation signal an error when you do
it. It's a constraint on you, not on the system.
My guess as to why you don't see an error with GDB is that the
debugger needs the text segment to be writable, so that it can set
breakpoints.
To get an error with gcc, add "-Wwrite-strings" to the command. No
quote chars used.
--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
<http://kadaitcha.cx/vista/dogsbreakfast/index.html>
cbfalconer at maineline dot net
--
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