Open processing

When the IAM data set is opened for the file load, IAM reads the attributes, parameters and overrides that were specified when the file was defined, and checks for current overrides. Any IAM overrides provided for the file load will supersede the values specified when the file was defined. If there are any errors detected by the override processing, the file load will fail. This is done to prevent a data set from being improperly loaded with incorrect attributes, which can result in subsequent problems. Using the merged information set, all the file attributes are validated and established. Conflicts and errors within the merged attribute set will cause the OPEN command to fail with a return code of 8, and an appropriate error code. As most of the errors are caught during the define process, file attribute errors are very rare during the open process. If one occurs, an error message is displayed in the Job Log, indicating the reason for the error. After the Open process successfully completes, the program can begin writing records into the data set.