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March 1st, 2012, 12:43 AM
#1
Accessing a limited struct's fields
Hello guys,
I'm an experienced coder in many languages, but not really in C/C++ even if I have now some experience, but I'm still lacking some habits.
I'm currently working on the ioquake3 engine if some of you know about it.
The ioquake3 engine is separated into 2 different main threads at runtime: the gamecode and the engine. Both are communicating but not all informations, and my problem resides here.
In the gamecode, there's a struct called gentity_t which contains a lot of fields:
Code:
typedef struct gentity_s gentity_t;
struct gentity_s {
entityState_t s; // communicated by server to clients
entityShared_t r; // shared by both the server system and game
// DO NOT MODIFY ANYTHING ABOVE THIS, THE SERVER
// EXPECTS THE FIELDS IN THAT ORDER!
//================================
struct gclient_s *client; // NULL if not a client
qboolean inuse;
char *classname; // set in QuakeEd
int spawnflags; // set in QuakeEd
qboolean neverFree; // if true, FreeEntity will only unlink
// bodyque uses this
int flags; // FL_* variables
char *model;
char *model2;
int freetime; // level.time when the object was freed
int eventTime; // events will be cleared EVENT_VALID_MSEC after set
qboolean freeAfterEvent;
qboolean unlinkAfterEvent;
qboolean physicsObject; // if true, it can be pushed by movers and fall off edges
// all game items are physicsObjects,
float physicsBounce; // 1.0 = continuous bounce, 0.0 = no bounce
int clipmask; // brushes with this content value will be collided against
// when moving. items and corpses do not collide against
// players, for instance
// movers
moverState_t moverState;
int soundPos1;
int sound1to2;
int sound2to1;
int soundPos2;
int soundLoop;
gentity_t *parent;
gentity_t *nextTrain;
gentity_t *prevTrain;
vec3_t pos1, pos2;
char *message;
int timestamp; // body queue sinking, etc
float angle; // set in editor, -1 = up, -2 = down
char *target;
char *targetname;
char *team;
char *targetShaderName;
char *targetShaderNewName;
gentity_t *target_ent;
float speed;
vec3_t movedir;
int nextthink;
void (*think)(gentity_t *self);
void (*reached)(gentity_t *self); // movers call this when hitting endpoint
void (*blocked)(gentity_t *self, gentity_t *other);
void (*touch)(gentity_t *self, gentity_t *other, trace_t *trace);
void (*use)(gentity_t *self, gentity_t *other, gentity_t *activator);
void (*pain)(gentity_t *self, gentity_t *attacker, int damage);
void (*die)(gentity_t *self, gentity_t *inflictor, gentity_t *attacker, int damage, int mod);
int pain_debounce_time;
int fly_sound_debounce_time; // wind tunnel
int last_move_time;
int health;
qboolean takedamage;
int damage;
int splashDamage; // quad will increase this without increasing radius
int splashRadius;
int methodOfDeath;
int splashMethodOfDeath;
int count;
gentity_t *chain;
gentity_t *enemy;
gentity_t *activator;
gentity_t *teamchain; // next entity in team
gentity_t *teammaster; // master of the team
int kamikazeTime;
int kamikazeShockTime;
int watertype;
int waterlevel;
int noise_index;
// timing variables
float wait;
float random;
gitem_t *item; // for bonus items
};
This whole entity is passed to the engine at runtime, but only the first two fields are declared for the engine:
Code:
// the server looks at a sharedEntity, which is the start of the game's gentity_t structure
typedef struct {
entityState_t s; // communicated by server to clients
entityShared_t r; // shared by both the server system and game
} sharedEntity_t;
My problem is that I need to access the health field of gentity_t from the engine. Technically, this is possible, but the health field is not declared in sharedEntity (which is the same memory address than gentity_t in the gamecode), so this is not straightforward.
I am looking for an elegant way to do this, and my constraint is that I must not edit the gamecode, only the engine.
The solutions I've thought:
- Just copy the whole gentity_t fields into sharedEntity_t. This would work I think but would be redundant, and I would like to avoid copying this huge set of fields.
- Include the two headers files declaring the gentity_t and sharedEntity_t structs, and create a Getter and a Setter functions that would cast a gentity_t over a sharedEntity_t and return/set a field. The problem is that I can't simply include them because they are both including some common headers files and this produce a recursive include error (and I can't modify the files to add a check, these are normally in the gamecode).
- Directly access the health field using a clever memory pointer, but I don't even know if that's possible given the huge number of fields prior health with many different types?
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I humbly ask for any help or idea on how to do this.
Thank you for reading me,
Stephen L.
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March 1st, 2012, 08:20 AM
#2
Re: Accessing a limited struct's fields
Originally Posted by lrq3000
Directly access the health field using a clever memory pointer, but I don't even know if that's possible given the huge number of fields prior health with many different types?
To get a pointer to a particular field, try the offsetof() macro.
http://www.cplusplus.com/reference/c...ddef/offsetof/
Regards,
Paul McKenzie
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March 1st, 2012, 12:06 PM
#3
Re: Accessing a limited struct's fields
I'd just cast the pointer if you are sure it is the correct type.
Originally Posted by Paul McKenzie
Would offsetof be "smarter/better" than pointer casting? Honest question.
Is your question related to IO?
Read this C++ FAQ article at parashift by Marshall Cline. In particular points 1-6.
It will explain how to correctly deal with IO, how to validate input, and why you shouldn't count on "while(!in.eof())". And it always makes for excellent reading.
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