Re: Confusion of Malloc and Sizeof

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

By remembering the number and passing it somehow or putting it in
a global variable.  There are more imaginative ways than using a
global variable or passing it as a second function argument.

Some people use arrays of integers (assuming that's what they want
to store) and store the length in a[0].

Another approach is to have a structure containing a pointer to the
array and the length allocated (and perhaps the number of elements
CURRENTLY in use as well as the number allocated), and pass around
pointers to that structure.  This permits routines that use it to
be able to determine that the array needs to grow, and to let
subsequent users know it has grown.


>For instance in my example how do I determine the number of characters
>available to write into?

Given this interface, the only safe assumption is to assume that
numCharAvailable = 1   .  Hopefully you will never pass in the
result of malloc(0).

>#include <stdio.h>
>#include <stdlib.h>
>
>void do_read (char *buffer)
>{
>  int numCharAvailable ;
>  numCharAvailable = sizeof (*buffer);
>  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?

Then remember it when you allocated it.
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