C / C++ / MFC

5-Apr-05 5:26

hiho@ll

i know this site is for windows programming, but i just wanted to know if my C-Code could have a problem, and this website is very well visited i think

for those who also use linux:
i'm coding on a suse linux box with kernel 2.4.21-226-default running gcc v 3.3.1

the problem is a function named strtok
which tokenizes my string using a delimiter
char test[]="hallo hallo2";
z.B. first call strtok(test," "); // returns hallo
seconde call strtok(NULL," "); // returns hallo2

this code works!:
char str[] ="This is a sample string,just testing.";
char * pch;
printf ("Splitting string \"%s\" in tokens:\n",str);
pch = strtok (str," ");
while (pch != NULL)
{
printf ("%s\n",pch);
pch = strtok (NULL, " ,.");
}

this code doesn't work! (i get a segmentation fault):
char *str ="This is a sample string,just testing."; // just changed char[]
char * pch;
printf ("Splitting string \"%s\" in tokens:\n",str);
pch = strtok (str," ");
while (pch != NULL)
{
printf ("%s\n",pch);
pch = strtok (NULL, " ,.");
}

could there a problem with the code itself?
what's the difference between pointers and arrays?
i thought there is no difference!

thx@ll

David Crow5-Apr-05 5:58

5-Apr-05 5:58

strtok() actually modifies the source string. That's why str[] works and str* does not.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

ThinkingPrometheus5-Apr-05 6:06

5-Apr-05 6:06

?? 1. is there a difference?
char *str="hello";
str[0]='z';
--> str = "zello";

char str[]="hello";
str[0]='z';
--> str = "zello";

so what's the difference
and function header looks like this:
char *strtok(char *s, const char *delim);

2. str* doesn't work and str[] works! and not the other way, like u said

thx

David Crow5-Apr-05 6:19

5-Apr-05 6:19

ThinkingPrometheus wrote:
1. is there a difference?

Yes.

http://www.eskimo.com/~scs/C-faq/s6.html

ThinkingPrometheus wrote:
and function header looks like this:
char *strtok(char *s, const char *delim);

Since the first parameter is not const, that indicates it can/will be modified.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

ThinkingPrometheus5-Apr-05 6:29

5-Apr-05 6:29

you wrote: strtok() actually modifies the source string. That's why str[] works and str* does not

look at my example prog!!!!
it works the other way!
str* doesn't work and str[] works! why?
thx

David Crow5-Apr-05 6:38

5-Apr-05 6:38

ThinkingPrometheus wrote:
you wrote: strtok() actually modifies the source string. That's why str[] works and str* does not

look at my example prog!!!!
it works the other way!

I think you need to re-read this thread. Your code indicates that char[] works but char* does not. Now you state that it works the other way.

The strtok() function works as it is intended to. Read its documentation. I provided you a link that explains the difference between a pointer and an array. Please read it.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

ThinkingPrometheus5-Apr-05 6:42

5-Apr-05 6:42

i read the doc of strtok and your link

but it confuses me more than before

much more i'm asking why it doesn't work
cause strtok wants a pointer
i give it a pointer it makes segmentation fault
i give it an array it works!!!???

shouldn't it work if i give it a pointer and it wants a pointer!?
*confused*

Michael Dunn5-Apr-05 7:00

Michael Dunn

5-Apr-05 7:00

Because when you write:

char* s = "hello";

the compiler/linker may allocate read-only memory for that string, and set s to the address of that memory. Thus when you try and modify that string, it segfaults.

--Mike--
Visual C++ MVP Cool | :cool:

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David Crow5-Apr-05 7:03

5-Apr-05 7:03

ThinkingPrometheus wrote:
much more i'm asking why it doesn't work

Because it is not supposed to work. The statement char *p = "Hello" says that p is a pointer that points to a spot in memory that holds the string constant. Trying to change the contents of p[1] is illegal because is refers to a constant spot in memory. That spot in memory has no name.

The statement char p[] = "Hello" says that 6 bytes of memory is allocated for the string constant and that spot in memory is referred to as p.

In that link that I provided, read sections 6.2 and 6.4 very carefully. The difference is explained very well, and why they can't always be interchanged.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

ThinkingPrometheus5-Apr-05 7:19

property sheets apply event

5-Apr-05 7:19

WOW

got it
that was the problem

thx to both of you

laiju5-Apr-05 4:07

laiju

5-Apr-05 4:07

I would like to know if I can override any function such as OnApply() or OnOk() for a class derived from CPropertySheet.
The idea is i want to save some default settings property sheet whenever the user clicks apply /ok in a property sheet.

laiju

Re: property sheets apply event

David Crow5-Apr-05 4:11