Question

I just had Apple’s C/C++ compiler initialize a float to a non-zero value (approx “-0.1”).

That was a big surprise – and only happened occasionally (but 100% repeatably, if you ran through the same function calls / args beforehand). It took a long time to track down (using assertions).

I’d thought floats were zero-initialized. Googling suggests that I was thinking of C++ (which of course is much more precise about this stuff – c.f. SO: What are primitive types default-initialized to in C++? ).

But maybe Apple’s excuse here is that their compiler was running in C mode … so: what about C? What should happen, and (more importantly) what’s typical?

(OF COURSE I should have initialized it manually – I normally do – but in this one case I failed. I didn’t expect it to blow up, though!)

(Google is proving worse than useless for any discussion of this – their current search refuses to show “C” without “C++”. Keeps deciding I’m too stupid, and ignoring even my input even when running in advanced mode)

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Here’s the actual source example where it happened. At first I thought there might be a problem with definitions of MAX and ABS (maybe MAX(ABS,ABS) doesnt always do what you’d expect?) … but digging with assertions and debugger, I eventually found it was the missing initialization – that float was getting init’d to non-zero value VERY occasionally):

float crossedVectorX = ... // generates a float
float crossedVectorY = ... // generates a float

float infitesimal; // no manual init
float smallPositiveFloat = 2.0 / MAX( ABS(crossedVectorX), ABS(crossedVectorY));

// NB: confirmed with debugger + assertions that smallPositiveFloat was always positive

infitesimal += smallPositiveFloat;

NSAssert( infitesimal >= 0.0, @"This is sometimes NOT TRUE" );

Answer 1

Only objects with static storage duration are initialized to 0 if there is no explicit initializer.

#include <stdio.h>

float f;         // initialized to 0, file scope variables have static storage
static float g;  // initialized to 0

int main(void)
{
    float h;  // not initialized to 0, automatic storage duration
    static float i;  // initialized to 0

    return 0;
}

Objects with automatic storage duration (like h in the example above) that are not explicitly initialized have an indeterminate value. Reading their value is undefined behavior.

EDIT: for the sake of completeness, since C11 objects with thread storage duration are also initialized to 0 if there is no explicit initializer.