Re: Confusion of Malloc and Sizeof

On Fri, 28 Mar 2008 19:20:27 -0500 (CDT), shofu_au@[EMAIL PROTECTED]
 wrote:

>Hi Group,
>
>How do you determine the number of elements in a dynamically allocated
>variable, without passing the number or having a global variable.

You don't.  These are the two solutions popular available

>
>For instance in my example how do I determine the number of characters
>available to write into?
>
>#include <stdio.h>
>#include <stdlib.h>
>
>void do_read (char *buffer)
>{
>  int numCharAvailable ;
>  numCharAvailable = sizeof (*buffer);

Since this always evaluates to 1, it obviously doesn't do what you
want.

>  printf ( "Number of elements available %d\n", numCharAvailable);
>}
>
>int main (int argc, char **argv)
>{
>  char *readBuffer;
>  readBuffer = (char *) malloc (15);
>  do_read (readBuffer);
>}
>
>I wish to use the readBuffer pointer in ASYNC callbacks and I would
>like to know the number of elements dynamically allocated?

In the special case where the buffer has had values stored it in, you
have the additional option of placing a sentinel value immediately
after the last "real" value.  do_read could then step through the
buffer looking for the sentinel (similar to the way strlen works).

If you are allocating space for an array (as opposed to a structure),
you could allocate one extra element (or in the case or a char array,
sizeof(int) extra elements) and store the size in the first element
(element 0) and use elements 1 through N for your array.


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