What is volatile? - comp.lang.c

Message from discussion What is volatile?

Alex Vinokur

View profile

More options Mar 17 2004, 2:52 am

Newsgroups: comp.lang.c

From: "Alex Vinokur" <ale...@big.foot.com>

Date: Wed, 17 Mar 2004 09:52:56 +0200

Local: Wed, Mar 17 2004 2:52 am

Subject: Re: What is volatile?

  Reply to author
  |
  Forward
  | Print
  | View thread
  | Show original
  | Report this message
  | Find messages by this author
  

"Anil Akkur" <anilak...@yahoo.co.in> wrote in message news:d46b1147.0403162202.74cbd777@posting.google.com...
> Hi All,
> Please explain me exact meaning of "volatile" qualifier? Where exactly we use it?
> Please explain me with a code snippet.
> Thanks in advance.

--------------------------------------------------
Compiler : GNU gcc version 3.3.1 (cygming special)
--------------------------------------------------

====== C code : BEGIN ======
------ File foo1.cpp -------
int main(int argc, char** argv)
{
int i;
i = argv[1][0];
i = argv[1][1];
return i;

}

----------------------------

------ File foo2.cpp -------
int main(int argc, char** argv)
{
volatile int i;
i = argv[1][0];
i = argv[1][1];
return i;

}

----------------------------

====== C code : END ========

====== Compilation (No optimization): BEGIN ========

$ gcc -save-temps foo1.c

$ gcc -save-temps foo2.c

$ ls foo*.s
foo1.s foo2.s // Files containing assembler code

$ diff foo1.s foo2.s
1c1
< .file "foo1.c"
---

> .file "foo2.c"

====== Compilation (No optimization): END ==========

====== Compilation (Optimization O1): BEGIN ========

$ gcc -O1 -save-temps foo1.c

$ gcc -O1 -save-temps foo2.c

$ ls foo*.s
foo1.s foo2.s // Files containing assembler code

$ diff foo1.s foo2.s
1c1
< .file "foo1.c"
---

> .file "foo2.c"

15,16c15,20
< movl 4(%eax), %eax
< movsbl 1(%eax),%eax
---

> movl 4(%eax), %edx
> movsbl (%edx),%eax
> movl %eax, -4(%ebp)
> movsbl 1(%edx),%eax
> movl %eax, -4(%ebp)
> movl -4(%ebp), %eax

====== Compilation (Optimization O1): END ==========

Conclusion.

No optimization : Assembler code for foo1.c and foo2.c is identical.
Optimization O1 : Assembler code for foo1.c and foo2.c is different.

--
Alex Vinokur
mailto:ale...@connect.to
http://mathforum.org/library/view/10978.html

Reply to author Forward