Atomic reads and writes to memory

Question

In Solaris 10 running on x86 64bit processor are reads/writes of an int variable (written in C compiled with gcc) atomic operations? I have read on various forums that memory reads and writes of primitives are atomic but there are people who question this statement.
I know there are atomic_ops provided by Solaris 10 for both user space and kernel but I am only interested in ensuring that an update to an int is atomic?

Answer 1

ps10x# cat fred.c
main() {
  int i;
  i=13;
  i=17;
ps10x# gcc -S fred.c
ps10x# cat fred.s
        .file   "fred.c"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx
        subl    $20, %esp
        movl    $13, -8(%ebp)
        movl    $17, -8(%ebp)
        addl    $20, %esp
        popl    %ecx
        popl    %ebp
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.3.4"So it seems pretty clear that setting a variable is "movl $13, -8(%ebp)"
and while I'm no expert on x86 assembler, I'm fairly sure a single movl instruction is atomic.
However, the real issue is that the compiler might decide to stash a temporary copy of the variable in a register.
And that could throw off the "atomic" nature of things.
I believe that the volatile keyword is supposed to help prevent those kinds of issues.