CHAIN (Random Retrieval from a File)

Free-Form Syntax CHAIN{(ENHMR)} search-arg name {data-structure}
Code Factor 1 Factor 2 Result Field Indicators
CHAIN (E N) search-arg name (file or record format) data-structure NR ER _

The CHAIN operation retrieves a record from a full procedural file, sets a record identifying indicator on (if specified on the input specifications), and places the data from the record into the input fields.

The search argument, search-arg, must be the key or relative record number used to retrieve the record. If access is by key, search-arg can be a a single key in the form of a field name, a named constant, a figurative constant, or a literal.

If the file is an externally-described file, search-arg can also be a composite key in the form of a KLIST name, a list of values, or %KDS. For keys specified using a KLIST, key fields must have the same CCSID as the key in the file. For an example of %KDS, see the example at the end of %KDS (Search Arguments in Data Structure). If access is by relative record number, search-arg must be an integer literal or a numeric field with zero decimal positions.

The name operand specifies the file or record format name that is to be read. A record format name is valid with an externally described file. If a file name is specified in name and access is by key, the CHAIN operation retrieves the first record that matches the search argument.

If name is a record format name and access is by key, the CHAIN operation retrieves the first record of the specified record type whose key matches the search argument. If no record is found of the specified record type that matches the search argument, a no-record-found condition exists.

If the data-structure operand is specified, the record is read directly into the data structure. If name refers to a program-described file, the data structure can be any data structure of the same length as the file's declared record length. If name refers to an externally-described file or a record format from an externally described file, the data structure must be a data structure defined with EXTNAME(...:*INPUT or *ALL) or LIKEREC(...:*INPUT or *ALL). See File Operations for information on how to define the data structure and how data is transferred between the file and the data structure.

For a WORKSTN file, the CHAIN operation retrieves a subfile record.

For a multiple device file, you must specify a record format in the name operand. Data is read from the program device identified by the field name specified in the DEVID(fieldname) keyword in the file specifications for the device file. If the keyword is not specified, data is read from the device for the last successful input operation to the file.

If the file is specified as an input DISK file, all records are read without locks and so no operation extender can be specified. If the file is specified as update, all records are locked if the N operation extender is not specified.

If you are reading from an update disk file, you can specify an N operation extender to indicate that no lock should be placed on the record when it is read (e.g. CHAIN (N)). See the Rational Development Studio for i: ILE RPG Programmer's Guide for more information.

You can specify an indicator in positions 71-72 that is set on if no record in the file matches the search argument. This information can also be obtained from the %FOUND built-in function, which returns '0' if no record is found, and '1' if a record is found.

To handle CHAIN exceptions (file status codes greater than 1000), either the operation code extender 'E' or an error indicator ER can be specified, but not both. For more information on error handling, see File Exception/Errors.

Positions 75 and 76 must be blank.

When the CHAIN operation is successful, the file specified in name is positioned such that a subsequent read operation retrieves the record logically following or preceding the retrieved record. When the CHAIN operation is not completed successfully (for example, an error occurs or no record is found), the file specified in name must be repositioned (for example, by a CHAIN or SETLL operation) before a subsequent read operation can be done on that file.

If an update (on the calculation or output specifications) is done on the file specified in name immediately after a successful CHAIN operation to that file, the last record retrieved is updated.

See Database Null Value Support for information on handling records with null-capable fields and keys.

For more information, see File Operations.

Note: Operation code extenders H, M, and R are allowed only when the search argument is a list or is %KDS().
Figure 1. CHAIN Operation with a File Name
 *  The CHAIN operation retrieves the first record from the file,
 *  FILEX, that has a key field with the same value as the search
 *  argument KEY (factor 1).

  //  If a record with a key value equal to the search argument is
  //  not found, %FOUND returns '0' and the EXSR operation is
  //  processed. If a record is found with a key value equal
  //  to the search argument, the program continues with
  //  the calculations after the EXSR operation.
     IF  NOT %FOUND;
        EXSR  Not_Found;
Figure 2. CHAIN Operation Using a List of Key Fields
FCUSTFILE   IF   E           K DISK
         // Specify the search keys directly in a list
         chain ('abc' : 'AB') custrec;
         // Expressions can be used in the list of keys
         chain (%xlate(custname : LO : UP) : companyCode + partCode)
Figure 3. CHAIN Operation Using a Data Structure with an Externally-Described File
FCUSTFILE   IF   E           K DISK
D custRecDs       ds                  likerec(custRec)

         // Read the record directly into the data structure
         chain ('abc' : 'AB') custRec custRecDs;
         // Use the data structure fields
         if (custRecDs.code = *BLANKS);
            custRecDs.code = getCompanyCode (custRecDs);
            update custRec custRecDs;