This function has no parameters.

    cg_local.h says:
    // force a screen update, only used during gamestate load
    void trap_UpdateScreen( void );
    
    the Trap_UpdateScreen itself appears to be:
    cg_syscalls.c
    void trap_UpdateScreen( void ) {
    syscall( CG_UPDATESCREEN );
    }
    

This is also called by:

    UI_Atoms.C
    
    void UI_UpdateScreen( void ) {
    trap_UpdateScreen();
    }
    

it is also called by:

    ui_local.h
    void trap_R_DrawStretchPic( float x, float y, float w, float h, float s1, float t1, float s2, float t2, qhandle_t hShader );
    void trap_UpdateScreen( void );
    int trap_CM_LerpTag( orientation_t *tag, clipHandle_t mod, int startFrame, int endFrame, float frac, const char *tagName );
    

The locations of the calls and the remarks placed around it imply that this trap is ONLY for updating the screen during connection to local or remote host.
(eg: Loading....shotgun ; Loading.... Gauntlet ect... during the load of a map.)

This function has no return value.

Waxy_Th_Chicken