Works OK for me, as long as Ive got #include <acknex.h> at the top
and have inserted level_load(NULL); into the start of main,
otherwise its all your code.

The below code is the ENTIRE *.c file I am testing with, if its different
to yours, post the COMPLETE file of your demo code.

Code:
#include <acknex.h>
#include <default.c>
//
#define MODE_STAND 0
#define MODE_WALK 1

typedef struct
{
	var health;
	var shield;
	var count;
	var state;
	var pathnode;
	var lin_speed;
	var rot_speed;
	var anim_speed;
	var vision_range;
	var weapon_type;
	var weapon;
	var weapon_range;
	var ammo;
	ENTITY* model;
} entity3d; 

function func(entity3d* ent);


function main()
{
	level_load(NULL);
	entity3d enemy;
	enemy.model = ent_create ("mymodel.mdl", vector(0,0, 25), NULL);
	func(enemy);
}


function func(entity3d* ent)
{
	VECTOR v1;
	ent.state = MODE_WALK;
	v1.x = ent.model.x; 			// no Error and crash! for me
	vec_set (v1.x, ent.model.x); 		// no Error and crash! for me
	ent.model.y -= 1; 			// no Error and crash! for me
}


"There is no fate but what WE make." - CEO Cyberdyne Systems Corp.
A8.30.5 Commercial