If you want to provide the assembler with the values for the system variables &SYSLIB_DSN, &SYSLIB_MEMBER, and &SYSLIB_VOLUME, the user exit must set the return code to 0, the reason code to 4, and place the values in the data set name, member name, and volume serial fields of the exit-specific information block.

If the exit does not supply the data set information, the system variables are set to null, and the library data set details are not shown in the Diagnostic Cross Reference and Assembler Summary section of the listing, nor are they included in the ADATA Library record.