How to prevent gcc optimizing some statements in C?

Question

In order to make a page dirty, I touch the first bytes of the page like this:

pageptr[0] = pageptr[0];

But it seems gcc will eliminate the statement when optimizing. In order to prevent gcc optimizing it, I write the statement like this:

volatile int tmp;
tmp = pageptr[0];
pageptr[0] = tmp;

It seems the trick works, I'd like to know is there any directives or syntax which has the same effect?

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Summary

Dietrich Epp's solution:

((unsigned char volatile *)pageptr)[0] = pageptr[0];

Plow's solution: using function specific option pragmas introduced since gcc 4.4

#pragma GCC push_options
#pragma GCC optimize ("O0")

your code

#pragma GCC pop_options

Answer 1

Use the volatile type qualifier. Example:

((unsigned char volatile *)pageptr)[0] = pageptr[0];

The volatile type qualifier instructs the compiler to be strict about memory stores and loads. It doesn't turn off optimization per se, but it does what you want.

Edit: I had put volatile on the wrong side of the *. Whoops, fixed.

Answer 2

With the gcc option "-O0", it should work. (which mean no optimization)

Answer 3

You can use

#pragma GCC push_options
#pragma GCC optimize ("O0")

your code

#pragma GCC pop_options

to disable optimizations since GCC 4.4.

See the GCC documentation if you need more details.