Confusion of Malloc and Sizeof

Hi Group,

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

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);
  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?


Thanks

Mark
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