malloc for structs and arrays

Hey guys,

First of all I know there have been previous threads like this and that there's also a faq item about this.
But I am unable to apply any of those examples to my problem, simply because I haven't done this before and I dont understand the instructions I've read. Therefore I hope you can help me with this specific problem.

Here's my problem:
I have a LINE_COUNT defined as 10000:

Code:

#define LINE_COUNT                    10000

And I have some structs and arrays defined that use this LINE_COUNT value:

Code:

   char buf[BUFSIZ], id_file[256], id_file_array[LINE_COUNT][30], id_array[LINE_COUNT][30];

   struct CAN_line
   {
      int cnt;
      int time;
      int identifier;
      int datalength;
      int db[8];
   };
   struct CAN_line linenr[LINE_COUNT];

Now when I increase the LINE_COUNT to say 11000 the program hangs. I figured it would have to do with memory available and came across the malloc function as a possible solution for this.
My question is, how should I use malloc in this situation?
Do I request memory for every array and the struct that use this LINE_COUNT? Or maybe I can simply request an amount of space when I launch the program?
Anyway, I have tried the following line but I haven't got a clue if it's right or not and when it is I wouldn't even know how to check if it actually works...

Code:

q = (char *) malloc(2 * sizeof(struct CAN_line));
...
free(q);

Sorry for the noob level of this question It's just so not clear for me how this works.

Any help would be greatly appreciated!

René

malloc will return a NULL pointer if the size of the block to be allocated is zero, or if there was insufficient memory

Originally Posted by rkooij

My question is, how should I use malloc in this situation?
Do I request memory for every array and the struct that use this LINE_COUNT?

Yes. Something a bit like this.

Code:

#include <stdio.h>
#include <stdlib.h>

#define LINE_COUNT 10000

struct CAN_line
{
   int cnt;
   int time;
   int identifier;
   int datalength;
   int db[8];
};

int main()
{
   char buf[BUFSIZ], id_file[256];
   char **id_file_array;
   char **id_array;
   struct CAN_line *linenr;
   int i;

   id_file_array = malloc(LINE_COUNT * sizeof *id_file_array);
   if ( id_file_array != NULL )
   {
      for ( i = 0; i < LINE_COUNT; ++i )
      {
         id_file_array[i] = malloc(30 * sizeof *id_file_array[i]);
         if ( id_file_array[i] == NULL )
         {
            /* ADD CLEANUP */
            return 0;
         }
      }
   }

   id_array = malloc(LINE_COUNT * sizeof *id_array);
   if ( id_array != NULL )
   {
      for ( i = 0; i < LINE_COUNT; ++i )
      {
         id_array[i] = malloc(30 * sizeof *id_array[i]);
         if ( id_array[i] == NULL )
         {
            /* ADD CLEANUP */
            return 0;
         }
      }
   }

   linenr = malloc(LINE_COUNT * sizeof *linenr);
   if ( linenr == NULL )
   {
      /* ADD CLEANUP */
      return 0;
   }

   /* ... */

   for ( i = 0; i < LINE_COUNT; ++i )
   {
      free(id_file_array[i]);
      free(id_array[i]);
   }
   free(id_file_array);
   free(id_array);
   free(linenr);
   return 0;
}

Originally Posted by rkooij

Or maybe I can simply request an amount of space when I launch the program?

Possibly. Are these variable declared within a function? Could they instead be global?

Thank you very much for the example. I have tried it but I get an error message:

invalid conversion from `void*' to `char**'

For example on this line:

Code:

   id_file_array = malloc(LINE_COUNT * sizeof *id_file_array);

Have I done something wrong?

Thanks again, René

Looks like you try to compile this c-code as c++.
In C a void* is compatible with any pointertype.
Kurt

Originally Posted by ZuK

Looks like you try to compile this c-code as c++.
In C a void* is compatible with any pointertype.
Kurt

Ok, I changed to a C file but now I get looooads of other errors... is 'bool' as a definition not C compatible? What can I do about that?

syntax error before "exit_flag"

Code:

bool exit_flag=FALSE, error_flag=FALSE, file_flag=FALSE, capture_flag=FALSE;

I use Dev-C++ 4.9.9.2 by the way...

Thanks!

no there is no bool in c.
if you want to compile your code as c++ you have to use new instead of malloc ( the preferred way ) or cast the return of malloc to the right type.
but if it's just the bool that makes troubles you could simply put a

Code:

typedef int bool;

in your code
Kurt

Originally Posted by ZuK

no there is no bool in c.
if you want to compile your code as c++ you have to use new instead of malloc ( the preferred way ) or cast the return of malloc to the right type.
but if it's just the bool that makes troubles you could simply put a

Code:

typedef int bool;

in your code
Kurt

or if you have a c99 compatible compiler

#include <stdbool.h>


int main(void)
{
bool b = false;

return 0;
}

Dave_sinkula thanks so much for the example and the rest of you guys for the tips regarding boolean. I used Laserve's tip to include stdbool.h and it's all working perfectly fine now!
Have on question left though... in the example I see the comment line:

Code:

/* ADD CLEANUP */

What exactly is meant with that line? I dont have a clue what to add...

Thanks again, this was the only thing left for me to create a stable base for my program.

For every address returned by malloc :
In order to free memory use the function free().

like

Code:

char  *p;

if ( ! ( p = malloc(20)) )
   {
   printf("malloc error\n");
   return;
   }

/* do things with p */


free(p); /* Let the program reuse this memory space
                 for other malloc calls */

Originally Posted by dude543

For every address returned by malloc :
In order to free memory use the function free().

like

Code:

char  *p;

if ( ! ( p = malloc(20)) )
   {
   printf("malloc error\n");
   return;
   }

/* do things with p */


free(p); /* Let the program reuse this memory space
                 for other malloc calls */

Hehe, errmm sorry... is that an answer to the question in my last post?

Yes. Remember to /*add cleanup*/ by free()-ing all your memory when you're done.

Originally Posted by jafet

Yes. Remember to /*add cleanup*/ by free()-ing all your memory when you're done.

Sorry guys, I still don't understand what to add. The memory request works perfectly as it is now, and I use free() at the end of the function to clear the memory space. I can see that it works in the task manager.
What do you mean with cleanup? Is there something I forget or is the above sufficient for the right functionality of my program?

Edit: hmmmm, do you mean I need to free the memory there as well...? So when "if ( id_file_array[i] == NULL )" is true, that means the memory allocation failed?

Sorry, for the noobness

You've added the free() already? Well... pat yourself on the back, that's whap "cleanup" means

Originally Posted by jafet

You've added the free() already? Well... pat yourself on the back, that's whap "cleanup" means

Hahahaha! Yay!
I haven't added free() on that line but at the end of the function, my guess according to the above is that I have to put it at the /* Add Cleanup */ line as well
Thanks for the help