In AutoCAD versions prior to AutoCAD 2015 you can use the PAUSE symbol argument to acedCommand to wait for user input. In AutoCAD 2015 you need to use acedCommandC.
int nResult = acedCommand( RTSTR, _ACRX_T("_MIRROR"), RTPICKS, sset, RTSTR, _ACRX_T(""), RTSTR, PAUSE, RTSTR, PAUSE, RTSTR, _ACRX_T("_N"), 0);
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Owen Wengerd
ManuSoft