Before accessing a data set, your program must issue the OPEN macro to open the data set for processing. Opening a data set causes VSAM to take the following actions:

• Verify that the data set matches the description specified in the ACB or GENCB macro (for example, MACRF=KEY implies that the data set is a key-sequenced data set).
• Construct the internal control blocks that VSAM needs to process your requests for access to the data set.

  To determine which processing options to use, VSAM merges information from the data definition (DD) statement and catalog definition of the data set with information in the access method control block and exit list. The order of precedence follows:

  1. The DD statement AMP parameters
  2. The ACB, EXLST, or GENCB parameters
  3. The catalog entry for the data set

  For example, if both an ACB or GENCB macro and the DD statement have values for buffer space, the values in the DD statement override those in the macro. The catalog entry is the minimum buffer space when it is not specified in the DD statement or macro or when it is less than the amount specified in the data set definition.

• Check for consistency of updates to the prime index and data components if you are opening a key-sequenced data set, an alternate index, or a path. If separate updates occur to data set and its index, VSAM issues a warning message to indicate a time stamp discrepancy.
• An error during OPEN can cause a component that is open for update processing to close improperly, leaving on the open-for-output indicator. When VSAM detects an open-for-output indicator, it issues an implicit VERIFY command and a message that indicates whether the VERIFY command was successful.

  If a subsequent OPEN is issued for update, VSAM turns off the open-for-output indicator at CLOSE. If the data set was open for input, however, VSAM leaves on the open-for-output indicator.

• Check the password your program specified in the ACB PASSWD parameter against the appropriate password (if any) in the catalog definition of the data. The system does not support passwords for system-managed data sets. A password of one level authorizes you to do everything that a password of a lower level authorizes. You must have RACF authorization for the operation. The password requirement depends on the kind of access that is specified in the access method control block:
  • Full access lets you perform all operations (retrieve, update, insert, and delete) on a data set on any associated index or catalog record. The master password lets you delete or alter the catalog entry for the data set or catalog it protects.
  • Control-interval update access requires the control password or RACF control authority. The control lets you use control-interval access to retrieve, update, insert, or delete records in the data set it protects. For information about the use of control-interval access, see Processing Control Intervals.

    Control-interval read access requires only the read password or RACF read authority, that lets you examine control intervals in the data set it protects. The read password or RACF read authority does not let you add, change, or delete records.

  • Update access requires the update password, which lets you retrieve, update, insert, or delete records in the data set it protects.
  • Read access requires the read password, that lets you examine records in the data set it protects. The read password does not permit you to add, change, or delete records.