OPEN

The assembler calls the exit with a request type of 1 (OPEN) at the start of the assembly. This call is the first call to the exit.

The exit sets the return code in the Exit Parameter List to one of these values:

0: Instructs the assembler to open the library data set, and supply the macro and copy code library input records to the exit in later PROCESS calls.
Note: A reason code of 4 indicates that the exit expects to receive END OF MEMBER calls.
A reason code of 16 indicates a REINIT call is required.
4: Indicates that the exit supplies the macro and copy code library records to the assembler in later READ calls. 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 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. The assembler also shows this information in the Diagnostic Cross Reference and Assembler Summary section of the listing, and includes it in the associated data file Library record.
Note: A reason code of 4 indicates that the exit expects to receive END OF MEMBER calls.
A reason code of 16 indicates a REINIT call is required.
8: Indicates that both the assembler and user exit supply the macro and copy code library records. On return from the exit, the assembler opens the library data set. When a macro or copy member is required, the assembler calls the exit with a FIND request. If the member is found by the exit, the exit supplies the records in later READ calls. If the exit cannot find the member, the assembler attempts to find the member in the library data set. If the assembler finds the member, the records are passed to the exit in later PROCESS calls.
Note: A reason code of 4 indicates that the exit expects to receive END OF MEMBER calls.
A reason code of 16 indicates a REINIT call is required.
16: Instructs the assembler to open the library data set, and make no further calls to the exit.

If you provide a character string in the str2 suboption of the EXIT assembler option, the buffer pointer field points to the character string, and buffer length contains the length of the character string. The buffer length is set to zero if there is no character string.