FIELD: file access on operating system layer. SUBSTANCE: computer system whose operating system periodically requests open files stored in memory device functions to lock up file opening request identifying first file to be opened on operating system layer, to separate first-file identifier from file opening request, to take decision on whether first-file identifier should be replaced by identifier of substituting file to be opened instead of first file; if identifier is to be replaced, first-file identifier is replaced in request by substituting-file identifier; then file opening request is conveyed to operating system for execution. EFFECT: improved reliability of application program licensing. 14 cl, 4 dwg
