Re: Confusion of Malloc and Sizeof

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 can't.

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

You will have to pass the size of the buffer as a parameter to do_read(),
or store the size of the buffer in some global variable. (Using global
variables
is generally strongly discouraged for a number of reasons though.)

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

sizeof works at compile time. It cannot be used to tell how much memory
has been dynamically allocated.
In this case 'sizeof(*buffer)' will return the size of the element
that 'buffer' is pointing to: the size of one char in this case
(which is always equal to 1 anyway.)

>  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?

You will have to keep track of it yourself.
There is no other ****table way of doing it.


The compiler and/or runtime library and/or OS will normally also keep
track
of this information somehow so it knows how much memory is to be freed
when
you call free(), but how it does that varies between different compilers
and
systems, and is usually not do***ented anyway.


-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013@[EMAIL PROTECTED]
 
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