Что это такое:

Функции описанные в HLSDK -> cdll_int.h -> структура cl_enginefuncs_s. Структура иниализируется при передаче клиенту функции Initialize (client.dll export). - int (*Initialize)(cl_enginefunc_t *, int);

Полная структура здесь (со всеми недокументированными функциями):

typedef struct cl_enginefuncs_s
{
	// sprite handlers
	HSPRITE						( *pfnSPR_Load )			( const char *szPicName );
	int							( *pfnSPR_Frames )			( HSPRITE hPic );
	int							( *pfnSPR_Height )			( HSPRITE hPic, int frame );
	int							( *pfnSPR_Width )			( HSPRITE hPic, int frame );
	void						( *pfnSPR_Set )				( HSPRITE hPic, int r, int g, int b );
	void						( *pfnSPR_Draw )			( int frame, int x, int y, const wrect_t *prc );
	void						( *pfnSPR_DrawHoles )		( int frame, int x, int y, const wrect_t *prc );
	void						( *pfnSPR_DrawAdditive )	( int frame, int x, int y, const wrect_t *prc );
	void						( *pfnSPR_EnableScissor )	( int x, int y, int width, int height );
	void						( *pfnSPR_DisableScissor )	( void );
	client_sprite_t				*( *pfnSPR_GetList )			( char *psz, int *piCount );

	// screen handlers
	void						( *pfnFillRGBA )			( int x, int y, int width, int height, int r, int g, int b, int a );
	int							( *pfnGetScreenInfo ) 		( SCREENINFO *pscrinfo );
	void						( *pfnSetCrosshair )		( HSPRITE hspr, wrect_t rc, int r, int g, int b );

	// cvar handlers
	struct cvar_s				*( *pfnRegisterVariable )	( char *szName, char *szValue, int flags );
	float						( *pfnGetCvarFloat )		( char *szName );
	char*						( *pfnGetCvarString )		( char *szName );

	// command handlers
	int							( *pfnAddCommand )			( char *cmd_name, void (*function)(void) );
	int							( *pfnHookUserMsg )			( char *szMsgName, pfnUserMsgHook pfn );
	int							( *pfnServerCmd )			( char *szCmdString );
	int							( *pfnClientCmd )			( char *szCmdString );

	void						( *pfnGetPlayerInfo )		( int ent_num, hud_player_info_t *pinfo );

	// sound handlers
	void						( *pfnPlaySoundByName )		( char *szSound, float volume );
	void						( *pfnPlaySoundByIndex )	( int iSound, float volume );

	// vector helpers
	void						( *pfnAngleVectors )		( const float * vecAngles, float * forward, float * right, float * up );

	// text message system
	client_textmessage_t		*( *pfnTextMessageGet )		( const char *pName );
	int							( *pfnDrawCharacter )		( int x, int y, int number, int r, int g, int b );
	int							( *pfnDrawConsoleString )	( int x, int y, char *string );
	void						( *pfnDrawSetTextColor )	( float r, float g, float b );
	void						( *pfnDrawConsoleStringLen )(  const char *string, int *length, int *height );

	void						( *pfnConsolePrint )		( const char *string );
	void						( *pfnCenterPrint )			( const char *string );


// Added for user input processing
	int							( *GetWindowCenterX )		( void );
	int							( *GetWindowCenterY )		( void );
	void						( *GetViewAngles )			( float * );
	void						( *SetViewAngles )			( float * );
	int							( *GetMaxClients )			( void );
	void						( *Cvar_SetValue )			( char *cvar, float value );

	int       					(*Cmd_Argc)					(void);	
	char						*( *Cmd_Argv )				( int arg );
	void						( *Con_Printf )				( char *fmt, ... );
	void						( *Con_DPrintf )			( char *fmt, ... );
	void						( *Con_NPrintf )			( int pos, char *fmt, ... );
	void						( *Con_NXPrintf )			( struct con_nprint_s *info, char *fmt, ... );

	const char					*( *PhysInfo_ValueForKey )	( const char *key );
	const char					*( *ServerInfo_ValueForKey )( const char *key );
	float						( *GetClientMaxspeed )		( void );
	int							( *CheckParm )				( char *parm, char **ppnext );
	void						( *Key_Event )				( int key, int down );
	void						( *GetMousePosition )		( int *mx, int *my );
	int							( *IsNoClipping )			( void );

	struct cl_entity_s			*( *GetLocalPlayer )		( void );
	struct cl_entity_s			*( *GetViewModel )			( void );
	struct cl_entity_s			*( *GetEntityByIndex )		( int idx );

	float						( *GetClientTime )			( void );
	void						( *V_CalcShake )			( void );
	void						( *V_ApplyShake )			( float *origin, float *angles, float factor );

	int							( *PM_PointContents )		( float *point, int *truecontents );
	int							( *PM_WaterEntity )			( float *p );
	struct pmtrace_s			*( *PM_TraceLine )			( float *start, float *end, int flags, int usehull, int ignore_pe );

	struct model_s				*( *CL_LoadModel )			( const char *modelname, int *index );
	int							( *CL_CreateVisibleEntity )	( int type, struct cl_entity_s *ent );

	const struct model_s *		( *GetSpritePointer )		( HSPRITE hSprite );
	void						( *pfnPlaySoundByNameAtLocation )	( char *szSound, float volume, float *origin );

	unsigned short				( *pfnPrecacheEvent )		( int type, const char* psz );
	void						( *pfnPlaybackEvent )		( int flags, const struct edict_s *pInvoker, unsigned short eventindex, float delay, float *origin, float *angles, float fparam1, float fparam2, int iparam1, int iparam2, int bparam1, int bparam2 );
	void						( *pfnWeaponAnim )			( int iAnim, int body );
	float						( *pfnRandomFloat )			( float flLow, float flHigh );
	long						( *pfnRandomLong )			( long lLow, long lHigh );
	void						( *pfnHookEvent )			( char *name, void ( *pfnEvent )( struct event_args_s *args ) );
	int							(*Con_IsVisible)			();
	const char					*( *pfnGetGameDirectory )	( void );
	struct cvar_s				*( *pfnGetCvarPointer )		( const char *szName );
	const char					*( *Key_LookupBinding )		( const char *pBinding );
	const char					*( *pfnGetLevelName )		( void );
	void						( *pfnGetScreenFade )		( struct screenfade_s *fade );
	void						( *pfnSetScreenFade )		( struct screenfade_s *fade );
	void                        *( *VGui_GetPanel )         ( );
	void                         ( *VGui_ViewportPaintBackground ) (int extents[4]);

	byte*						(*COM_LoadFile)				( char *path, int usehunk, int *pLength );
	char*						(*COM_ParseFile)			( char *data, char *token );
	void						(*COM_FreeFile)				( void *buffer );
		
	struct triangleapi_s		*pTriAPI;
	struct efx_api_s			*pEfxAPI;
	struct event_api_s			*pEventAPI;
	struct demo_api_s			*pDemoAPI;
	struct net_api_s			*pNetAPI;
	struct IVoiceTweak_s		*pVoiceTweak;

	// returns 1 if the client is a spectator only (connected to a proxy), 0 otherwise or 2 if in dev_overview mode
	int							( *IsSpectateOnly ) ( void );
	struct model_s				*( *LoadMapSprite )			( const char *filename );

	// file search functions
	void						( *COM_AddAppDirectoryToSearchPath ) ( const char *pszBaseDir, const char *appName );
	int							( *COM_ExpandFilename)				 ( const char *fileName, char *nameOutBuffer, int nameOutBufferSize );

	// User info
	// playerNum is in the range (1, MaxClients)
	// returns NULL if player doesn't exit
	// returns "" if no value is set
	const char					*( *PlayerInfo_ValueForKey )( int playerNum, const char *key );
	void						( *PlayerInfo_SetValueForKey )( const char *key, const char *value );

	// Gets a unique ID for the specified player. This is the same even if you see the player on a different server.
	// iPlayer is an entity index, so client 0 would use iPlayer=1.
	// Returns false if there is no player on the server in the specified slot.
	qboolean					(*GetPlayerUniqueID)(int iPlayer, char playerID[16]);

	// TrackerID access
	int							(*GetTrackerIDForPlayer)(int playerSlot);
	int							(*GetPlayerForTrackerID)(int trackerID);

	// Same as pfnServerCmd, but the message goes in the unreliable stream so it can't clog the net stream
	// (but it might not get there).
	int							( *pfnServerCmdUnreliable )( char *szCmdString );

	void						( *pfnGetMousePos )( struct tagPOINT *ppt );
	void						( *pfnSetMousePos )( int x, int y );
	void						( *pfnSetMouseEnable )( qboolean fEnable );
	struct cvar_s*	( *pfnGetCvarList )( void );
	struct cmd_s* ( *pfnGetCmdList )( void );

	char* ( *pfnGetCvarName )( struct cvar_s* cvar );
	char* ( *pfnGetCmdName )( struct cmd_s* cmd );

	float ( *pfnGetServerTime )( void );
	float ( *pfnGetGravity )( void );
	const struct model_s* ( *pfnPrecacheSprite )( HSPRITE spr );
	void ( *OverrideLightmap )( int override );
	void ( *SetLightmapColor )( float r, float g, float b );
	void ( *SetLightmapDarkness )( float dark );

	//this will always fail with the current engine
	int ( *pfnGetSequenceByName )( int flags, const char* seq );

	void ( *pfnSPR_DrawGeneric )( int frame, int x, int y, const wrect_t *prc, int blendsrc, int blenddst, int unknown1, int unknown2 );

	//this will always fail with engine, don't call
	//it actually has paramenters but i dunno what they do
	void ( *pfnLoadSentence )( void );

	//localizes hud string, uses Legacy font from skin def
	// also supports unicode strings
	int ( *pfnDrawLocalizedHudString )( int x, int y, const char* str, int r, int g, int b );

	//i can't get this to work for some reason, don't use this
	int ( *pfnDrawLocalizedConsoleString )( int x, int y, const char* str );

	//gets keyvalue for local player, useful for querying vgui menus or autohelp
	const char	*(*LocalPlayerInfo_ValueForKey)( const char* key );

	//another vgui2 text drawing function, i dunno how it works
	//it doesn't localize though
	void ( *pfnDrawText_0 )( int x, int y, const char* text, unsigned long font );

	int ( *pfnDrawUnicodeCharacter )( int x, int y, short number, int r, int g, int b, unsigned long hfont );

	//checks sound header of a sound file, determines if its a supported type
	int ( *pfnCheckSoundFile )( const char* path );

	//for condition zero, returns interface from GameUI
	void* ( *GetCareerGameInterface )( void );

	void ( *pfnCvar_Set )( const char* cvar, const char* value );

	//this actually checks for if the CareerGameInterface is found
	//and if a server is being run
	int ( *IsSinglePlayer )( void );

	void ( *pfnPlaySound )( const char* sound, float vol, float pitch );

	void ( *pfnPlayMp3 )( const char* mp3, int flags );

	//get the systems current time as a float
	float ( *Sys_FloatTime )( void );

	void ( *pfnSetArray )( int* array, int size );
	void ( *pfnSetClearArray )( int* array, int size );
	void ( *pfnClearArray )( void );
	void ( *pfnPlaySound2 )( const char* sound, float vol, float pitch );
	
	// Same like pfnFillRGBA - with other mode (substractive)
	int						( *pfnFillRGBA2)				( int x, int y, int w, int h, int r, int g, int b, int a );
	} cl_enginefunc_t;

Получение:

Самое корректное - получать структуру через Initialize. Но если вы не хукаете функции клиента, либо применяете свои хуки после инициализации этой функции, придётся использовать автоматический поиск указателя или статичный оффсет (если используете вполне конкретный патч - в интернете есть куча архивов оффсетов под каждый патч).

Код автоматического поиска указателя (точно не скажу до какого патча работает - возможно надо расширить диапазон поиска):

net_api_s				*pNetAPI;
demo_api_s			*pDemoAPI;
efx_api_s				*pEfxAPI;
triangleapi_s			*pTriAPI;
event_api_s			*pEventAPI;
IVoiceTweak_s			*pVoiceTweak;
cl_enginefuncs_s		*pEngfuncs;

int a;
for(a=0x01D00000;a<0x01D1FFFF;a++)
{
	if(*(PBYTE)(a) == 0x68 && *(PBYTE)(a + 0x05) == 0xE8 &&
	*(PBYTE)(a + 0x0A) == 0x6A && *(PBYTE)(a + 0x0B) == 0x07 &&
	*(PBYTE)(a + 0x0C) == 0x68 && *(PBYTE)(a + 0x11) == 0xFF &&
	*(PBYTE)(a + 0x12) == 0x15 && *(PBYTE)(a + 0x17) == 0x68)
	{
		pEngfuncs = (cl_enginefunc_t*)*(DWORD*)(a+0x0D);
		pNetAPI = pEngfuncs->pNetAPI;
		pEfxAPI = pEngfuncs->pEfxAPI;
		pTriAPI = pEngfuncs->pTriAPI;
		pDemoAPI = pEngfuncs->pDemoAPI; 
		pEventAPI = pEngfuncs->pEventAPI;
		pVoiceTweak = pEngfuncs->pVoiceTweak;
	}
}

;
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