Problem with Array of char*

error1408

Hi, I have a function in a class that has the following sig:

int strtoken(char *str, char *separator, char *token[])

for the *token I do this before:

char *token[256];
for(int i = 0; i < 256; i++)
{
token[i] = '\0';
}

I call the function but at this line it crashes:

token[0] = strtok(str, separator);

Error on writing etc. (German Error, so I don't know exactly what it's called in english) Can someone help me?

Sarath C

You can simply initialize the array as follows. char *token[256] = { 0 }; strtok return a char* (array of character) not a single char. See the example on using strtok taken from msdn. Read the documentation before using it.

#include #include char string[] = "A string\tof ,,tokens\nand some more tokens";
char seps[] = " ,\t\n";
char *token;

int main( void )
{
	printf( "Tokens:\\n" );

	// Establish string and get the first token:
	token = strtok( string, seps ); // C4996
	// Note: strtok is deprecated; consider using strtok\_s instead
	while( token != NULL )
	{
		// While there are tokens in "string"
		printf( " %s\\n", token );

		// Get next token: 
		token = strtok( NULL, seps ); // C4996
	}
}

-Sarath. "Great hopes make everything great possible" - Benjamin Franklin

My blog - Sharing My Thoughts

_Superman_

It's very difficult to guess without any error codes or messages. All I can think of is that you passed a constant string for the str parameter because strtok will modify the str parameter.

«_Superman_»

CPallini

IMHO there is no apparent reason for the crash, could you please elaborate a bit (i.e. what are str and separator values? Could you please post

strtoken

code? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

error1408

Ok look, here is the full function (that I did NOT write myself) which should work correct, but I think >>I<< do something NOT correct. Its a tokenizer for char-strings

int strtoken(char *str, char *separator, char *token[])
{
int i = 0;

	token\[0\] = strtok(str, separator); //This line crashes

	while ( token\[i\] ) 
	{
		i++;
		token\[i\] = strtok(NULL, separator);
	}
	return ( i );
}

And I call it that way atm:

		char \*token\[256\]; //I think that is an array of char\*, isn't it?
		for(int i = 0; i < 256; i++)
		{
			token\[i\] = '\\0';
		}
		Helper::instance()->strtoken(name, ".", token);

name is a char* e.g. "system.fullscreen"

CPallini

OK, as already suggested by «_Superman_», probably your code is crashing because you're passing a string literal to the function (strtok cannot access for writing a constant string). for instance

char * name = "system.fullscreen";
token[0] = strtok(name, "."); // exception here

would crash the application, while

char name[] = "system.fullscreen";
token[0] = strtok(name, ".");

should work fine. :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Arman S

You missed to tell us how your 'name' variable is declared/defined :). Except for it, other stuff seems OK to me (in the sense that they should not provoke a crash).

-- Arman

error1408

Your right, I get a char* name as a parameter and use it to call the function. Thats the cause of the crash. But how do I solve this problem? I have this char *name and it has to be tokenized. How can I do that as strtok crashes with it? Btw. thanks for your help, I would have never thought of THAT ^^

CPallini

well, if you really need to use that fucntion, then you may assign the array the way I shown in my previous post, or do someting like:

char * name;
name = _strdup("system.fullscreen");
if ( name )
{
strtoken(name, ".", token);
free(name); // don't forget to free memory
}

:)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

error1408

But if I call it with that char* it crashes! I have a function

doSomething(char *name)
{
[...]
strtoken(name, ".", token); //crashes
}

That works but it does not help me:

doSomething(char *name)
{
[...]
char foo[] = "hello.world";
strtoken(foo, ".", token); //does NOT crash
}

But how can I use strtoken with char *name? Can I make a char foo[] out of char *name?

CPallini

if you really need to pass a constant string to doSomething, the you may write (as already suggested)

doSomething(char * name)
{
//...
char * name_writable_copy = _strdup( name );
if ( name_writable_copy )
{
strtoken(name_writable_copy, ".", token);
free( name_writable_copy );
}
else
{
// handle (unusual) allocation error
}
}

The above code shouldn't crash. :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

error1408

Ok thx. I will try it in the evening today and post my results.

CPallini

Well, good luck! :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

David Crow

error1408 wrote:

Helper::instance()->strtoken(name, ".", token);

How about something like:

char *temp = new char[strlen(name) + 1];
strcpy(temp, name);
Helper::instance()->strtoken(temp, ".", token);

"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

error1408

DavidCrow wrote:

How about something like: char *temp = new char[strlen(name) + 1]; strcpy(temp, name); Helper::instance()->strtoken(temp, ".", token);

Ok this works! But I don't understand why...could someone explain it to me? Why does the param char * not work but the char * thats created here? Btw. I have to DELETE the char *temp afterwards, right?

David Crow

error1408 wrote:

Why does the param char * not work but the char * thats created here?

Work through this:

void main( void )
{
char *abc = "First";
char xyz[] = "Last";

abc\[0\] = '1';
xyz\[0\] = '2';

}

error1408 wrote:

Btw. I have to DELETE the char *temp afterwards, right?

Correct (since it points to heap memory).

"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

error1408

Ok I think I got it. So is it right, that in my case I can not know if the char *name that I get is a literal or not so to be sure it's not I have to create a temp array?

David Crow

error1408 wrote:

...I can not know if the char *name that I get is a literal or not so to be sure it's not I have to create a temp array?

You'll notice in my example that both variables pointed to a string literal, yet only one of them could be changed. Therein lies the difference between a char* vs. char[]. See here for more.

"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

ilostmyid2

i agree with the pallini's solution and also david gave a good description. string literals r located in a const part of memory which is not writable. this is why arrays don't cause crash. they're located in heap or stack based on whether u allocate them or use local variables which r both writable parts of memory. there's an API function to determine whether a block of memory or a string is const named AfxIsValidAddress. it may also test whether the block is writable. but if i were u, i would use _strdup anyway, and would pass the arg as const for the caller to make sure that the original version of the passed string is not altered.