READLOB

READLOB

ALL
(operand1)

[IN] [FILE] view-name
  [PASSWORD=operand2]
  [CIPHER=operand3]
  [[WITH] ISN [=] operand4]
  [[STARTING] [AT] OFFSET [=] operand5]
  statement
END-READLOB (structured mode only)
LOOP (reporting mode only)

This document covers the following topics:

For explanations of the symbols used in the syntax diagram, see Syntax Symbols.

Related Statements: READ | FIND | GET | UPDATELOB

Belongs to Function Group: Database Access and Update

Function

The READLOB statement is used on a single record, where the defined LOB field (Large OBject field) is read in fixed length segments during the loop processing. It is only applicable to read this LOB field.

At loop beginning, the offset inside the LOB field is set from where to get the first data. On the next loop iteration, the segment following the last segment is returned. If the LOB data end is reached, the loop terminates.

This statement causes a processing loop to be initiated. See also Loop Processing in the Programming Guide.

Restrictions

The READLOB statement can only be used for access to Adabas databases.

Syntax Description

Operand Definition Table:

Operand Possible Structure Possible Formats Referencing Permitted Dynamic Definition
operand1 C S           N P I B *               yes no
operand2 C S       A                         yes no
operand3 C S           N                     yes no
operand4 C S           N P I B *               yes no
operand5 C S           N P I B *               yes no

* Format B of operand1, operand4 and operand5 may be used with a length of less than or equal to 4.

Syntax Element Description:

Syntax Element Description
operand1
Number of LOB Segments to Be Read:

The number of loop executions to be performed may be limited by specifying operand1 (enclosed in parentheses, immediately after the keyword READLOB) - either as a numeric constant (0 - 4294967295) or as the name of a numeric variable.

Example: 

READLOB (5) IN FILE VIEW01 ...
#CNT := 10
READLOB (#CNT) IN FILE VIEW01 ...

For this statement, the specified limit has priority over a limit set with a LIMIT statement. If a smaller limit is set with the profile or session parameter LT, the LT limit applies.

Note:
operand1 is evaluated when the READLOB is started. If the value of operand1 is modified within the READLOB loop, this does not affect the number of loop iterations.

ALL
ALL Option:

To emphasize that the LOB data is to be read until its end, you can optionally specify the keyword ALL.

The ALL option is used by default if operand1 and ALL are omitted.

view-name
View Name:

As view-name, you specify the name of a view, which must have been defined either within a DEFINE DATA statement or outside the program in a global or local data area.

  • The view has to contain just a single-valued LOB field, additional fields are not allowed.

  • If the LOB is a MU or PE field, a unique occurrence must be specified; a range notation is not allowed.

  • The LOB field must be defined in the DDM with a fixed (non-dynamic) length (which represents the segment length of the LOB field).

PASSWORD=operand2
CIPHER=operand3

PASSWORD and CIPHER Clauses:

The PASSWORD clause is used to provide a password when retrieving data from a file which is password-protected.

The CIPHER clause is used to provide a cipher key when retrieving data from a file which is enciphered.

See the statements FIND and PASSW for further information.

WITH ISN=operand4
WITH ISN Option:

This option is used to specify the ISN of the record which is accessed by the READLOB statement. During the complete loop execution, only this record is fetched.

operand4 must be provided either in the form of a numeric constant or as a user-defined variable, or via the Natural system variable *ISN. The field is not modified by the READLOB execution.

Note:
operand4 is evaluated when the READLOB is started. If the value of operand4 is modified within the READLOB loop, this does not affect the record being fetched.

If this option is omitted, the *ISN field of the last active database statement is used by default.

STARTING AT OFFSET=operand5
STARTING AT OFFSET Clause:

Provides the start offset within the LOB field, where the first segment read begins. The first byte of the LOB field is offset zero (0).

operand5 must be provided either in the form of a numeric constant or as a user-defined variable, without precision digits. The field is not modified by the READLOB execution.

If this clause is omitted, start offset (0) is assumed, which causes the LOB field to be read from the beginning.

See also *NUMBER during the processing described in System Variables Available with READLOB.

END-READLOB
End of READLOB Statement:

In structured mode, the Natural reserved keyword END-READLOB must be used to end the READLOB statement.

In reporting mode, the Natural statement LOOP is used to end the READLOB statement.

LOOP

System Variables Available with READLOB

The Natural system variables *ISN, *COUNTER and *NUMBER are provided with the READLOB statement.

The format/length of these system variables is P10. This format/length cannot be changed.

The purpose of the Natural system variables, when used with the READLOB statement, is explained below:

System Variable Explanation
*ISN Contains the Adabas ISN of the record currently being processed. Since a READLOB statement always makes access to the same record, the *ISN value returned is the same over all loop iterations.
*COUNTER Contains the number of times the processing loop has been passed through.
*NUMBER Before the call: It specifies the byte offset in the LOB field from which the segment is to be read. Value zero (0) represents the leftmost byte in the LOB field.

This does not apply for the first loop iteration. In this case the read offset is determined by the STARTING AT OFFSET clause.

After the call: If data was found (that is, the offset was less than the LOB field length), it receives the offset plus the segment length. This may lead to a *NUMBER value which is higher than the length of the entire LOB field.

If no data was found (that is, the offset was higher or equal to the LOB field length), the value of *NUMBER remains unchanged.

If a consecutive read over a LOB field is requested, the *NUMBER value must not be modified within the READLOB, as it contains the offset to exactly continue with the next segment in the subsequent loop iteration. However, if a continuation at another place within the LOB field is desired (re-position), you may change the *NUMBER value to this offset. If *NUMBER is reset, this leads to the next segment coming from the LOB start. If *NUMBER is incremented by (n), this number of bytes will be skipped in the LOB field processing.

Functional Considerations

  • The READLOB statement always reads the record without hold. In order to guarantee stability on the LOB data (that is, to prevent an update by other users) while the READLOB browses over the LOB field, the record can be set into hold with the database statement providing the ISN; either

    • into an exclusive hold, due to an UPDATE or DELETE referring to an outer READ or FIND statement or

    • into a shared hold with an explicit IN SHARED HOLD option applied in a READ or FIND statement. If the additional subparameter MODE=Q is used, the record is automatically released from hold if the next record is fetched in the read sequence.

  • Since the READLOB statement always reads the record without hold, an UPDATE, DELETE or GET SAME statement must not refer to a READLOB statement.

Examples

Example 1 - Get Record Number from READ Loop

DEFINE DATA LOCAL
1 VIEW01 VIEW OF ..
  2 NAME 
  2 L@LOBFIELD
1 VIEW02 VIEW OF ..
  2 LOBFIELD_SEGMENT                /* LOB field defined in DDM with (A1000).
END-DEFINE
*
READ VIEW01 BY NAME = 'SMITH'       /* Outer statement reads all demanded record 
                                    /* fields, except the LOB field.
     IN SHARED HOLD MODE=Q          /* Set record into shared hold to enforce LOB 
                                    /* data stability during READLOB.
  DISPLAY NAME 'Total-length LOB-field' L@LOBFIELD 
  READLOB VIEW02                    /* Record number used from active record of 
                                    /* READ statement.
                                    /* LOB is read in segments with length 1000. 
      STARTING AT OFFSET = 2000     /* Start to read the LOB field at byte 2000.
    WRITE 'Loop counter:' *COUNTER  10X ' Next offset:' *NUMBER
    PRINT VIEW02.LOBFIELD_SEGMENT  
  END-READLOB
END-READ   
END

Example 2 - Get Record Number by User-defined Value

DEFINE DATA LOCAL
1 #ISN  (I4)
1 #CNT  (I4)
1 #OFF  (I4)
1 VIEW02 VIEW OF ..
  2 LOBFIELD_SEGMENT            /* LOB field defined in DDM with (A1000).
END-DEFINE
*
INPUT (AD=T)
  / ' Read record (ISN):' #ISN
  / 'Number of segments:' #CNT
  / '   Start at offset:' #OFF
* 
READLOB (#CNT) VIEW02           /* Read max. (#CNT) segments with length 1000.
    WITH ISN = #ISN             /* Record number provided by user.
                                /* Record is not in hold.
    STARTING AT OFFSET = #OFF   /* Start to read the LOB field at byte (#OFF).
  WRITE 'Loop counter:' *COUNTER  10X ' Next offset:' *NUMBER
  PRINT VIEW02.LOBFIELD_SEGMENT  
END-READLOB
END

Example 3 - Get Record Number from READ Loop (with Exclusive Hold) 

DEFINE DATA LOCAL
1 VIEW01 VIEW OF ..
  2 NAME 
1 VIEW02 VIEW OF ..
  2 LOBFIELD_SEGMENT           /* LOB field defined in DDM with (A1000).
END-DEFINE
*
R1. READ VIEW01 BY NAME = 'SMITH'/* Outer statement reads all demanded  
                                 /* record fields, except the LOB field.
  DISPLAY NAME 
  READLOB VIEW02            /* Record number from active record of READ.
                            /* LOB is read in segments with length 1000. 
      STARTING AT OFFSET = 2000 /* Start to read LOB field at byte 2000. 
    WRITE 'Loop counter:' *COUNTER 10X ' Next offset:' *NUMBER
    PRINT VIEW02.LOBFIELD_SEGMENT  
  END-READLOB
  ...
  UPDATE (R1.)            /* Set record into exclusive hold that 
                          /* enforces LOB data stability during READLOB.
  END OF TRANSACTION
END-READ   
END