strstr crashes on NULL

[linq936@hotmail.com]

Hi,
I have the following code:

#include <stdio.h>

int main(void){
char* str1 = "abc";
char* str2 = '\0';
if ( strstr(str1, str2) == NULL ){
printf("yes\n") ;
}
else{
printf("no\n");
}

return 0;
}

It crashes on strstr() call, so is it because none of strstr()
parameter can be NULL?

[Walter Roberson]

Re: strstr crashes on NULL

In article <1184197966.126 134.202820@n2g2 000hse.googlegr oups.com>,
<linq936@hotmai l.comwrote:

I have the following code:

>#include <stdio.h>

>int main(void){
char* str1 = "abc";
char* str2 = '\0';
if ( strstr(str1, str2) == NULL ){
printf("yes\n") ;
}
else{
printf("no\n");
}
>
return 0;
>}

It crashes on strstr() call, so is it because none of strstr()
>parameter can be NULL?

Right. Your char* str2 = '\0' is setting str2 to a NULL pointer
instead of to an empty string, which would be "" or "\0";
'\0' happens to be an integral constant with value 0 and so is
a valid NULL pointer initializer (if strange). In C89 at least,
strstr() is not defined for the case of a NULL pointer
(but is defined for the case of a pointer to an empty string.)

--
I was very young in those days, but I was also rather dim.
-- Christopher Priest

[pete]

Re: strstr crashes on NULL

linq936@hotmail .com wrote:

>
Hi,
I have the following code:
>
#include <stdio.h>

You're missing:
#include <string.h>

>
int main(void){
char* str1 = "abc";
char* str2 = '\0';

That's a bad way to write
char* str2 = NULL;

If you change that to
char* str2 = "";
then you can output "no".

if ( strstr(str1, str2) == NULL ){
printf("yes\n") ;
}
else{
printf("no\n");
}
>
return 0;
}
>
It crashes on strstr() call, so is it because none of strstr()
parameter can be NULL?

Yes.

--
pete

[Keith Thompson]

Re: strstr crashes on NULL

linq936@hotmail .com writes:

I have the following code:
>
#include <stdio.h>
>
int main(void){
char* str1 = "abc";
char* str2 = '\0';
if ( strstr(str1, str2) == NULL ){
printf("yes\n") ;
}
else{
printf("no\n");
}
>
return 0;
}
>
It crashes on strstr() call, so is it because none of strstr()
parameter can be NULL?

Yes. You may also be confused about the various meanings of "null".

In your declaration

char *str2 = '\0';

you're not setting str2 to point to an empty string; you're setting it
to a null pointer, which doesn't point to anything. If you wanted
str2 to point to a empty string, you could have declared:

char *str2 = "";

Normally the character constant '\0' is used to refer to a null
character, the character whose value is zero (sometimes called NUL
with one 'L'). This is the character used to mark the end of a
string.

But as it happens, the character constant '\0' is exactly equivalent
to the integer constant 0, and in this context it indicates a null
pointer, not a null character. Using '\0' in this context isn't
exactly wrong (the compiler is perfectly happy with it), but it's
misleading. If you want a null pointer, it's better to use the NULL
macro.

So, you can either declare:

char *str1 = "abc";
char *str2 = NULL;

(but then passing str2 to strstr invokes undefined behavior), or you
can declare:

char *str1 = "abc";
char *str2 = "";

In that case, strstr(str1, str2) will return the value of str1.

I suggest you take a look at sections 4, 5, and 6 of the comp.lang.c
FAQ, <http://www.c-faq.com/>.

You also need to add a '#include <string.h>' to your program, since
you're calling the strstr function. It may happen to work without it,
but the #include directive is *not* optional. (A note to my fellow
pedants: yes, he could declare strstr himself, but that would be
silly.)

--
Keith Thompson (The_Other_Keit h) kst-u@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"

[linq936@hotmail.com]

Re: strstr crashes on NULL

On Jul 11, 6:33 pm, Keith Thompson <k...@mib.orgwr ote:

linq...@hotmail .com writes:

I have the following code:

>

#include <stdio.h>

>

int main(void){
char* str1 = "abc";
char* str2 = '\0';
if ( strstr(str1, str2) == NULL ){
printf("yes\n") ;
}
else{
printf("no\n");
}

>

return 0;
}

>

It crashes on strstr() call, so is it because none of strstr()
parameter can be NULL?

>
Yes. You may also be confused about the various meanings of "null".
>
In your declaration
>
char *str2 = '\0';
>
you're not setting str2 to point to an empty string; you're setting it
to a null pointer, which doesn't point to anything. If you wanted
str2 to point to a empty string, you could have declared:
>
char *str2 = "";
>
Normally the character constant '\0' is used to refer to a null
character, the character whose value is zero (sometimes called NUL
with one 'L'). This is the character used to mark the end of a
string.
>
But as it happens, the character constant '\0' is exactly equivalent
to the integer constant 0, and in this context it indicates a null
pointer, not a null character. Using '\0' in this context isn't
exactly wrong (the compiler is perfectly happy with it), but it's
misleading. If you want a null pointer, it's better to use the NULL
macro.
>
So, you can either declare:
>
char *str1 = "abc";
char *str2 = NULL;
>
(but then passing str2 to strstr invokes undefined behavior), or you
can declare:
>
char *str1 = "abc";
char *str2 = "";
>
In that case, strstr(str1, str2) will return the value of str1.
>
I suggest you take a look at sections 4, 5, and 6 of the comp.lang.c
FAQ, <http://www.c-faq.com/>.
>
You also need to add a '#include <string.h>' to your program, since
you're calling the strstr function. It may happen to work without it,
but the #include directive is *not* optional. (A note to my fellow
pedants: yes, he could declare strstr himself, but that would be
silly.)
>
--
Keith Thompson (The_Other_Keit h) k...@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"

Thanks.

My intention is that I use '\0' to initialize a string so that later I
can know its state. How about this code:

#include <string.h>
#include <stdio.h>

int main(void){
char str1[10], str2[10];

memset( str1, '\0', 10);
memset( str2, '\0', 10);

strcpy(str1, "abc");
if ( strstr(str1, str2) == NULL ){
printf("yes");
}
else{
printf("no");
}

return 0;
}

This code works fine on the same machine with same compiler, of
course I see "no". I tried commenting out string.h include, it still
works.

So does this mean it is really an un-expected behavior?

[Richard Heathfield]

Re: strstr crashes on NULL

linq936@hotmail .com said:

<snip>

My intention is that I use '\0' to initialize a string so that later I
can know its state.

An empty string is very different to a null pointer.

const char *nullpointer = '\0';
const char *emptystring = "\0";

or, equivalently: const char *emptystring = "";

How about this code:
>
#include <string.h>
#include <stdio.h>
>
int main(void){
char str1[10], str2[10];
>
memset( str1, '\0', 10);
memset( str2, '\0', 10);

That's fine, but this works just as well:

char str1[10] = "", str2[10] = "";

strcpy(str1, "abc");
if ( strstr(str1, str2) == NULL ){

Yes, that's fine now.

This code works fine on the same machine with same compiler, of
course I see "no".

Yes - strstr is defined to return str1 if str2 is an empty string.

I tried commenting out string.h include, it still
works.

It is true that the program may give the appearance of continuing to
work when you chop off important bits, in much the same way that a used
car continues to work (for a while) with sand in the carburettor.

So does this mean it is really an un-expected behavior?

The code you posted this time is fine. Perfectly well-defined.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Keith Thompson]

Re: strstr crashes on NULL

santosh <santosh.k83@gm ail.comwrites:

linq...@hotmail .com wrote:

[...]

> strcpy(str1, "abc");
> if ( strstr(str1, str2) == NULL ){
> printf("yes");

>
Unless you terminate the string supplied to printf with a newline
character, '\n', or call fflush(stdout), your output is not guaranteed
to appear immediately

[...]

Actually, it's not guaranteed to appear *at all* unless the last
character you write to stdout is a new-line. It's
implementation-defined whether a text stream must end in a new-line.
The standard doesn't say what happens if a text stream does require a
trailing new-line and you don't provide one, so the behavior is
undefined; you might get no output at all, or demons might fly out of
your nose.

This is another of those things that will *probably* work just the way
you expect it to (the program will print "yes" or "no" without a
new-line), but if you write it correctly in the first place, you don't
have to worry about it.

--
Keith Thompson (The_Other_Keit h) kst-u@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"