lundi 2 février 2015

Initializing complex structure, GCC warns: initialized field with side-effects overwritten


I'm coding a backgammon position evaluation system and I'm trying to use designated initalizers, compound literals and a variadic macro to create a keyword argument function like described here , and in Ben Klemens' excellent book 21st Century C.



// from board.h
typedef struct _board_t board_t;

// from evaluator.h (evaluator is an "interface" in a primitive object system.)
#define EVALUATOR(v) (evaluator_t*)(v)

enum { OUTPUT_WIN, OUTPUT_WINGAMMON, OUTPUT_WINBACKGAMMON,
OUTPUT_LOSEGAMMON, OUTPUT_LOSEBACKGAMMON, N_OUTPUT };

typedef struct _evaluator_t evaluator_t;

// from composite_evaluator.h
#define N_MAX_EVALUATORS_IN_COMPOSITE 10

typedef evaluator_t * (*new_func)( void *d );

typedef struct {
int n_eval;
int (*classifier)( const board_t *b );
struct {
new_func construct;
void *args;
} config[N_MAX_EVALUATORS_IN_COMPOSITE];
} composite_evaluator_config_t;

// some evaluator implementations (just decl for _new)
typedef struct _overevaluator_t overevaluator_t;
typedef struct _onesidebearoff_t onesidebearoff_t;
typedef struct _neuralnet_evaluator_t neuralnet_evaluator_t;
overevaluator_t *overevaluator_new (void *args);
onesidebearoff_t *onesidebearoff_new (void *args);
neuralnet_evaluator_t *neuralnet_evaluator_new (void *args);

// and a classifing function decl.
int classify_position( const board_t *b );

// Example from main program
#include <stdio.h>
int main()
{
// This is of course coded in a variadic macro, but the macro is left out for simplicity of the example.
composite_evaluator_config_t myeval = {
.n_eval = 5,
.classifier = classify_position,
.config = {
[0] = { .construct = (new_func)overevaluator_new },
[1] = { .construct = (new_func)onesidebearoff_new },
[2] = { .construct = (new_func)neuralnet_evaluator_new, .args = &((char*[]){"race.weights", "race", "race"}) },
[3] = { .construct = (new_func)neuralnet_evaluator_new, .args = &((char*[]){"contact.weights", "contact", "contact"}) },
[4] = { .construct = (new_func)neuralnet_evaluator_new, .args = &((char*[]){"crashed.weights", "crashed", "contact"}) },
// The following line is added by a __VA_ARGS__, hence overriding the [3] above.
[3] = { .construct = (new_func)neuralnet_evaluator_new, .args = &((char*[]){"td1228.weights", "contact", "contact"}) },
}
};

char **args = (char**) myeval.config[3].args;
printf("Neural network weights read from '%s'.\n", args[0]);

return 0;
}


I believe this is legal C99 (?), however when I compile with GCC (4.9.2 20141224) I get the following warning:



$ gcc -Wall -Wextra -Wno-override-init -c evaltest.c
evaltest.c: In function ‘main’:
evaltest.c:63:13: warning: initialized field with side-effects overwritten
[3] = { .construct = (new_func)neuralnet_evaluator_new, .args = &((char*[]){"td1228.weights", "contact", "contact"}) },
^
evaltest.c:63:13: warning: (near initialization for ‘myeval.config[3]’)


However, when I compile with clang, like this:



clang -Wall -Wextra -Wno-initializer-overrides -c evaltest.c


I get absolutely no warnings at all! However both compilers seems to build the intended code.


So, clang and I think this is OK code. Can someone please explain why GCC send me this warning? Is the warning valid? Can I suppress it in any way? Or is it a GCC bug?





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