Question

I am trying to compile this piece of code found below.
However, I got an error saying “Initialization value must be constant expression.”on every line of iteminfo_fields, weaponinfo_fields and projectileinfo_fields definitions, except at the last line of each “{NULL, 0, 0}”
I am guessing it is from ITEMINFO_OFS(), WEAPON_OFS(), PROJECTILE_OFS().

I am trying to understand these three lines:

#define WEAPON_OFS(x) (size_t)&(((weaponinfo_t *)0)->x)
#define PROJECTILE_OFS(x) (size_t)&(((projectileinfo_t *)0)->x)
#define ITEMINFO_OFS(x) (size_t)&(((iteminfo_t *)0)->x)

But, I really cannot understand how they work…???
I need help dissecting them. Can anyone read them and rewrite them in a simpler form?

#define WEAPON_OFS(x) (size_t)&(((weaponinfo_t *)0)->x)
#define PROJECTILE_OFS(x) (size_t)&(((projectileinfo_t *)0)->x)
#define ITEMINFO_OFS(x) (size_t)&(((iteminfo_t *)0)->x)

fielddef_t iteminfo_fields[] =
{
{"name", ITEMINFO_OFS(name), FT_STRING},
{"model", ITEMINFO_OFS(model), FT_STRING},
{"modelindex", ITEMINFO_OFS(modelindex), FT_INT},
{"type", ITEMINFO_OFS(type), FT_INT},
{"index", ITEMINFO_OFS(index), FT_INT},
{"respawntime", ITEMINFO_OFS(respawntime), FT_FLOAT},
{"mins", ITEMINFO_OFS(mins), FT_FLOAT|FT_ARRAY, 3},
{"maxs", ITEMINFO_OFS(maxs), FT_FLOAT|FT_ARRAY, 3},
{NULL, 0, 0}
};
//weapon definition
static fielddef_t weaponinfo_fields[] =
{
{"number", WEAPON_OFS(number), FT_INT}, //weapon number
{"name", WEAPON_OFS(name),FT_STRING},   //name of the weapon
{"level", WEAPON_OFS(level), FT_INT},
{"model", WEAPON_OFS(model), T_STRING}, //model of the weapon
{"weaponindex", WEAPON_OFS(weaponindex), FT_INT},//index of weapon in inventory
{"flags", WEAPON_OFS(flags), FT_INT},//special flags
{"projectile", WEAPON_OFS(projectile), FT_STRING},//projectile used by the weapon
{"numprojectiles", WEAPON_OFS(numprojectiles), FT_INT}, //number of projectiles
{"hspread", WEAPON_OFS(hspread), FT_FLOAT},//horizontal spread of projectiles (degrees from middle)
{"vspread", WEAPON_OFS(vspread), FT_FLOAT},//vertical spread of projectiles (degrees from middle)
{"speed", WEAPON_OFS(speed), FT_FLOAT}, //speed of the projectile (0 = instant hit)
{"acceleration", WEAPON_OFS(acceleration), FT_FLOAT},//"acceleration" * time (in seconds) + "speed" = projectile speed
{"recoil", WEAPON_OFS(recoil), FT_FLOAT|FT_ARRAY, 3},//amount of recoil the player gets from the weapon
{"offset", WEAPON_OFS(offset), FT_FLOAT|FT_ARRAY, 3},//projectile start offset relative to eye and view angles
{"angleoffset", WEAPON_OFS(angleoffset), FT_FLOAT|FT_ARRAY, 3},//offset of the shoot angles relative to the view angles
{"extrazvelocity", WEAPON_OFS(extrazvelocity), FT_FLOAT},//extra z velocity the projectile gets
{"ammoamount", WEAPON_OFS(ammoamount), FT_INT}, //ammo amount used per shot
{"ammoindex", WEAPON_OFS(ammoindex), FT_INT},   //index of ammo in inventory
{"activate", WEAPON_OFS(activate), FT_FLOAT},   //time it takes to select the weapon
{"reload", WEAPON_OFS(reload), FT_FLOAT},   //time it takes to reload the weapon
{"spinup", WEAPON_OFS(spinup), FT_FLOAT},   //time it takes before first shot
{"spindown", WEAPON_OFS(spindown), FT_FLOAT},   //time it takes before weapon stops firing
{NULL, 0, 0, 0}
};

//projectile definition
static fielddef_t projectileinfo_fields[] =
{
{"name", PROJECTILE_OFS(name), FT_STRING},//name of the projectile
{"model", WEAPON_OFS(model), FT_STRING},//model of the projectile
{"flags", PROJECTILE_OFS(flags), FT_INT},//special flags
{"gravity", PROJECTILE_OFS(gravity), FT_FLOAT}, //amount of gravity applied to the projectile [0,1]
{"damage", PROJECTILE_OFS(damage), FT_INT},//damage of the projectile
{"radius", PROJECTILE_OFS(radius), FT_FLOAT},//radius of damage
{"visdamage", PROJECTILE_OFS(visdamage), FT_INT},//damage of the projectile to visible entities
{"damagetype", PROJECTILE_OFS(damagetype), FT_INT},//type of damage (combination of the DAMAGETYPE_? flags)
{"healthinc", PROJECTILE_OFS(healthinc), FT_INT},//health increase the owner gets
{"push", PROJECTILE_OFS(push), FT_FLOAT},//amount a player is pushed away from the projectile impact
{"detonation", PROJECTILE_OFS(detonation), FT_FLOAT},//time before projectile explodes after fire pressed
{"bounce", PROJECTILE_OFS(bounce), FT_FLOAT},   //amount the projectile bounces
{"bouncefric", PROJECTILE_OFS(bouncefric), FT_FLOAT}, //amount the bounce decreases per bounce
{"bouncestop", PROJECTILE_OFS(bouncestop), FT_FLOAT},//minimum bounce value before bouncing stops
//recurive projectile definition??
{NULL, 0, 0, 0}
};

Note:
My situation is a bit complicated at the moment. However,
I do not know the name of the compiler. The environment is Linux tho.
It is not my code, but I need to have it compile.

Answer 1

The macros attempt to extract the offset of a named member from the start of the struct containing that member.

Instead of WEAPON_OFS(name) use offsetof(weaponinfo_t,name) – and similar for the other macros.

EDIT: What the macros actually do.

#define WEAPON_OFS(x) (size_t)&(((weaponinfo_t *)0)->x)

This macro “pretends” there is a weaponinfo_t struct at address 0, and takes the address of a specific member of this weaponinfo_t struct. This yields the offset from the structs location (0) to the member variables location (0 or higher) as a pointer. The pointer is then cast to an integral, which still is the distance from the start of the struct to the start of the member variable – the offset of the member variable in the struct.

Apparently your compiler is unable to resolve this compiletime (which is fully possible to do though) – so you get an error instead of the constant value needed for your initializers.