🚀 go-pugleaf

X-Original-Date: 03 Jan 1996 19:38:59 GMT

In article <4c9740$27n@netlab.cs.rpi.edu> Srinivas Vobilisetti
<srv@cs.wayne.edu> writes:

   Nowhere i could find exact meaning of the specifier volatile.

Consider the following piece of code:

int n;
for (int i=0; i<99; i++)
  n=0;

A good optimising compiler should rewrite this as:

int n=0;
int i=100;

{That's right, if you want to delay for 100 micro seconds, the above
is not a portable approach.}

The use of the keyword volatile prevents the compiler from optimising
the reads and writes to a variable.  

volatile int n;
for (int i=0; i<99; i++)
  n=0;

Will cause zero to be written into the variable n 100 times. 
{Note, the compiler may be smart enough to do this without using the
variable i!}.

volatile int n;
for (volatile int i=0; i<99; i++)
  n=0;

Will cause zero to be written into the variable n 100 times.  It will
also cause the variable i to be incremented inbetween writes to n, and
will read i to test it for 99 each time through the loop.

Generally, this is used when certain memory registers have side
effects.  e.g.

int * volatile tty = 0x100;  // 0x100 is the addr of the tty device.

*tty = 'h'; // prints a 'c' on the tty;

void PrintMessage(char *s)
{
  for(; *s; s++) *tty = *s; // prints the string on the tty.
}

You can also use volatile if you want to create stupid delays as in
the first example.

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