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However, as demonstrated in \[[Eide and Regehr|AA. C References#Eide and Regehr]\], all tested compilers generated some percentage of incorrect compiled code with regards to {{volatile}} accesses.&nbsp; Therefore, it is necessary to know how your compiler behaves when the standard {{volatile}} behavior is required.&nbsp; There is also a workaround that eliminates some or all of these errors \[[Eide and Regehr|AA. C References#Eide and Regehr]\].

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Noncompliant Code Example

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As demonstrated in \[[Eide and Regehr\|AA. C References#Eide and Regehr]\], the following code example compiles incorrectly using GCC version 4.3.0 for IA32 and the {{\-Os}} optimization flag:

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bgColor	#ffcccc

const volatile int x;
volatile int y;
void foo(void) {
    for(y = 0; y < 10; y++) {
        int z = x;
    }
}

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Because the variable {{x}} is {{volatile}}\-qualified, it should be accessed ten times in this program.&nbsp; However, as shown in the compiled object code, it is only accessed once due to a loop-hoisting optimization \[[Eide and Regehr|AA. C References#Eide and Regehr]\]:

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bgColor	#ffcccc

foo:
    movl $0, y
    movl x, %eax
    jmp .L2
.L3:
    movl y, %eax
    incl %eax
    movl %eax, y
.L2:
    movl y, %eax
    cmpl $10, %eax
    jg .L3
    ret

Should x represent a hardware register or some other memory-mapped device that has side effects when accessed, the previous miscompiled code example may produce unexpected behavior.

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Compliant Solution

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Eide and Regehr tested a workaround by wrapping {{volatile}} accesses with function calls.&nbsp; They describe it with the intuition that "we can replace an action that compilers empirically get wrong by a different action â--- a function call â--- that compilers can get right" \[[Eide and Regehr|AA. C References#Eide and Regehr]\].  For example, the workaround considerfor the followingnoncompliant code example thatwould could be miscompiled:

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volatile x; x = x;

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To work around some of the {{volatile}}-access compiler bugs, wrap both accesses to {{x}} in function calls \[[Eide and Regehr]\]:

bgColor	#ccccff

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int vol_read_int(volatile int *vp) {
    return *vp;
}
volatile int *vol_id_int(volatile int *vp) {
    return vp;
}

const volatile int x;
xvolatile =int xy;
void foo(void) {
    for(*vol_id_int(&xy) = 0; vol_read_int(&y) < 10; *vol_id_int(&y) = vol_read_int(&y) + 1) {
        int z = vol_read_int(&x);
    }
}

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The workarounds proposed in \[[Eide and Regehr|AA. C References#Eide and Regehr]\] fix many of the {{volatile}}\-access bugs in the tested compilers.&nbsp; However, compilers are always changing so critical sections of code should be compiled as if for deployment and the compiled object code should be inspected for the correct behavior.

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\[[Eide and Regehr|AA. C References#Eide and Regehr]\] "Volatiles Areare Miscompiledmiscompiled, and Whatwhat to Dodo about Itit"
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.3, "Type qualifiers"

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