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Pointer dereferencing fun!

Started by Darkeye Nov 19, 2001 at 5:25 PM 6 replies 350+ views

Original Post

Nov 19, 2001 at 5:25 PM

Ok, I lied in the subject, this isn''t fun at all. Basically, I need to pass a string to a function and allow that function to modify the string. At the moment, this is what I have: char *category; memset( &category, 0, sizeof( category ) ); That is the string I need to pass and modify in the other function. get_submitted_question( index, submit_filename, &category, etc etc. That is the call to the function that needs to modify the string. (The function takes a char** as the param) Then, in that function, whenever I try to modify the string by dereferencing it, for example: strcpy((*category), "Category"); The program crashes with: Unhandled exception at 0x004033eb in Quizbot.exe: 0xC0000005: Access violation writing location 0x00000000 Any ideas would be welcome.

thuned

Nov 19, 2001 at 5:32 PM

char *category isn''t really a string. its a pointer to a char (which could be a string, but wouldn''t work here).
try changing it to
char category[80];

Red Ant

Nov 19, 2001 at 5:36 PM

You have to make sure that the memory allocated to category is large enough to actually contain "Category" plus the terminating ''\0''.

Pactuul

Nov 19, 2001 at 7:33 PM

char *category;
memset( &category, 0, sizeof( category ) );

well all you have is a pointer
and when you call sizeof (category) this returns the size of the pointer which is like 4 bytes ( i believe), but your using &category, which is partially correct, but your zeroing out only first 4 bytes.

you need to do this

char* category
category = new char[MAXIUM_SIZE_EVER];
memset(category, 0, sizeof (category));

//then
strcpy(category, "Category");

this should work now
Pactuul

Null and Void

163

Null and Void

Nov 19, 2001 at 7:38 PM

Pactuul, you have an error too. Change that to:

  
char *category;
category = new char[SIZE_WANTED];
memset(category, 0, SIZE_WANTED);

And just for clarification, do this later (when you''re done using the string, you HAVE to do this):

  
delete [] category;

[Resist Windows XP''s Invasive Production Activation Technology!]

Fruny

203

Fruny

Nov 20, 2001 at 9:47 AM

Access violation writing location 0x00000000

That''s a NULL pointer you''re trying to write on.

Mayrel

Nov 20, 2001 at 12:06 PM

Assuming he''s using C++, Null and Void.

Much the same is:

  
char *category;
category = malloc(sizeof(char) * size_wanted);
  or, in c++
category = new char[size_wanted];
memset(category, 0, size_wanted);

You don''t need to change the strcpy line - it should be correct as it is (because category is a char** in get_submitted_question).

  
strcpy((*category), "Category");

All your bases belong to us (I know. It''s irony.)

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