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POHow do I create #define macro with a ternary operator with one operand that does nothing?
 

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  1. POHow do I create #define macro with a ternary operator with one operand that does nothing?
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    <p>I am reading "No Bugs!" by David Thielen and chapter 3 discusses a way to determine if a given code path has been hit. It suggests having a macro that check an argument, and, if it is true, executes an assembly instruction that generates a specific interrupt (0x3) to create a debugger breakpoint. If the argument to the macro is false, it simply does nothing.</p> <p>The macro, then, look like this:</p> <pre><code>#ifdef DEBUG #define Trap(t) ( (t) ? __asm__("int $0x3") : ) #endif </code></pre> <p>This code, however, causes a compile error for gcc:</p> <pre><code>int_test.c:16:35: error: expected expression before ‘__asm__’ </code></pre> <p>I learned from <a href="https://stackoverflow.com/a/6118305/3047531">here</a> that because gcc's asm is a block statement and not an expression, I must use <a href="http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Statement-Exprs.html" rel="nofollow noreferrer">statement expressions</a> in order to use the <strong>asm</strong> in this way. So, now it becomes:</p> <pre><code>#define Trap(t) ( (t) ? ({ __asm__("int $0x3"); }) : ) </code></pre> <p>The compiler still complains:</p> <pre><code>int_test.c:16:64: error: expected expression before ‘)’ token </code></pre> <p>Okay, so now I have to do this?</p> <pre><code>#define Trap(t) ( (t) ? ({ __asm__("int $0x3"); }) : ({ ; }) ) </code></pre> <p>That seems really dumb. Can't I just have the preprocessor insert nothing if <code>t</code> is false without using this annoying syntax?</p> <p>Note: I have left out a few other similar macros for simplicity and I have adapted the syntax from the book to work for gcc (such as replacing the book's _asm with <strong>asm</strong> as well as using AT&amp;T syntax and surrounding the assmembly in "")</p>
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    1. USmaster_latch
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    1. COIf you only need side effects and not values, why use the ternary operator instead of an if statement?
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      1. USJim Lewis
    2. CO+1 to using `if` instead of ternary operator. **HOWEVER** I have to ask, why in the world are you using assembly instructions to trigger breakpoints instead of just using a real debugger and setting breakpoints at places of interest? This seems like a very, very primitive, unnecessary, low level and downright *ancient* debugging method...
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      1. POHow do I create #define macro with a ternary operator with one operand that does nothing?
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    3. COTraditionally this is done through the comma operator. E.g. cond && (function_with_side_effects(), 1) though I'm not certain whether this will work for GCC inline assembly statements, so you may need to place the actual breakpoint in a separate function. Any particular reason you don't want to make the whole thing an (inline) function?
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