Linux const char type

[Michaël Boland]

I have a fonction to trim a string :

char *trimstr(char *pbuf1)
{
char *pbuf2 , *pbuf3;

pbuf2 = pbuf1 ;
pbuf3 = pbuf1 ;

while(*pbuf1 != '\0') { if(*pbuf1 != ' ') { pbuf2 = pbuf1; pbuf2++; }
pbuf1++; }
while(*pbuf2 != '\0') { *pbuf2 = '\0'; pbuf2++; }
return(pbuf3) ;
}

main(int argc, char *argv[], char *env[])
{
trimstr(" tata ");

return 0;
}

It's ok with Unix Digital.
The result with Linux is Segmentation fault.
I think the problem is " tata " is a const *char and C Linux don't
allow to change a constant.
Is it possible in the function trimstr to know if the argument is a
*char or a const *char?
Or anything else solution?

Regards.

Michaël

[Richard Bos]

mi************@ advalvas.be (=?ISO-8859-1?Q?Micha=EBl_B oland?=) wrote:

I have a fonction to trim a string :

char *trimstr(char *pbuf1)
{
char *pbuf2 , *pbuf3;

pbuf2 = pbuf1 ;
pbuf3 = pbuf1 ;

while(*pbuf1 != '\0') { if(*pbuf1 != ' ') { pbuf2 = pbuf1; pbuf2++; }
pbuf1++; }
while(*pbuf2 != '\0') { *pbuf2 = '\0'; pbuf2++; }
return(pbuf3) ;
}
That's a funny way to write a trimming function, but let that be...
main(int argc, char *argv[], char *env[])
This is not a valid declaration of main(). Main is either

int main(void)

or

int main(int argc, char **argv)

or anything equivalent; it cannot take a third argument in ISO C. In
C99, implicit int is out as well, but you've probably got a C89
compiler, so that's OK.
{
trimstr(" tata ");

return 0;
}

It's ok with Unix Digital.
Only by accident.
The result with Linux is Segmentation fault.
I think the problem is " tata " is a const *char
Not quite; it's a non-modifiable array of char. For historical and
convenience reasons, it isn't actually const. And in this case, the
convenience just happens to be inconvenient :-(
and C Linux don't allow to change a constant.
Correct. And rightly so.
Is it possible in the function trimstr to know if the argument is a
*char or a const *char?

No. That is, yes; if it _had_ been a const char *, you would have had a
compile-time diagnostic. But since string literals aren't actually
const-qualified, there is no way to tell. Just don't pass string
literals to functions which modify them; it invokes undefined behaviour,
and the effects may range from accidentally working, through seeming to
work but doing nothing, causing a segfault, and in theory mailing lurid
proposals to Elio Di Rupo. So just don't do that.
Oh, btw: it's a char *, not a * char.

Richard

[Dan Pop]

In <77************ **************@ posting.google. com> mi************@ advalvas.be (=?ISO-8859-1?Q?Micha=EBl_B oland?=) writes:

I have a fonction to trim a string :

char *trimstr(char *pbuf1)
{
char *pbuf2 , *pbuf3;

pbuf2 = pbuf1 ;
pbuf3 = pbuf1 ;

while(*pbuf1 != '\0') { if(*pbuf1 != ' ') { pbuf2 = pbuf1; pbuf2++; }
pbuf1++; }
while(*pbuf2 != '\0') { *pbuf2 = '\0'; pbuf2++; }
return(pbuf3 ) ;
}

main(int argc, char *argv[], char *env[])
{
trimstr(" tata ");

return 0;
}

It's ok with Unix Digital.
The result with Linux is Segmentation fault.
I think the problem is " tata " is a const *char and C Linux don't
allow to change a constant.
Is it possible in the function trimstr to know if the argument is a
*char or a const *char?
Or anything else solution?

Never pass the address of a string literal to *any* function that doesn't
promise (one way or another) not to use it for writing purposes.

Therefore, the solution to your problem is not inside trimstr() but
outside it.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Jack Klein]

On 25 Jul 2003 02:08:52 -0700, mi************@ advalvas.be (Michaël
Boland) wrote in comp.lang.c:

I have a fonction to trim a string :

char *trimstr(char *pbuf1)
{
char *pbuf2 , *pbuf3;

pbuf2 = pbuf1 ;
pbuf3 = pbuf1 ;

while(*pbuf1 != '\0') { if(*pbuf1 != ' ') { pbuf2 = pbuf1; pbuf2++; } ^^^^^^^^^^^^^
ITYM *pbuf2 = *pbuf1;
pbuf1++; }
while(*pbuf2 != '\0') { *pbuf2 = '\0'; pbuf2++; }
return(pbuf3) ;
}

main(int argc, char *argv[], char *env[])
{
trimstr(" tata ");

return 0;
}

It's ok with Unix Digital.
The result with Linux is Segmentation fault.
I think the problem is " tata " is a const *char and C Linux don't
allow to change a constant.
Is it possible in the function trimstr to know if the argument is a
*char or a const *char?
Or anything else solution?

Regards.

Michaël

--
Jack Klein
Home: http://JK-Technology.Com
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