How to Read Fields in a Table from the LCK

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for Windows, version 2.6a

SUMMARY

This article explains how to read the contents of a field in a table using the Library Construction Kit (LCK).

MORE INFORMATION

When you write a C program using the Library Construction Kit, you may sometimes need to know the value of a field in a table. To find that value, follow these steps:

1. Call the _NameTableIndex() function. It returns a name table index that corresponds to the field name.

2. Call the _FindVar() function. It uses the name table index to fill a Locator structure with information about the field.

3. Call the _Load() function. It uses the information in the Locator structure to place values for the field in a Value structure.

4. Refer to the Value structure. It contains the contents of the field initialized in Step 3. For more information on the nature of a Locator and Value structure, please see the "Data Structures" section in the Microsoft FoxPro Library Construction Kit documentation.

Example

The following code sample illustrates the concepts explained above. This code can be compiled into an FLL for use with FoxPro for Windows, a PLB for use with FoxPro for MS-DOS, or an MLB for use with FoxPro for Macintosh. This code creates a function called ReadDb.

The syntax for the ReadDb() function is:

   ReadDb(<expN>,<expC>)

    - expN is the work area in which the table is located. If a work area
      of 0 is specified, ReadDb() will search all works areas to find the
      specified field.

    - expC is the field whose value this function should display.

   = ReadDb(1,"flda")

Code Sample

#include <pro_ext.h>

void FAR ReadDb(ParamBlk FAR *parm)

  char FAR * fldname ;    // name of field
  char FAR * fldtxt ;     // contents of field
  NTI nti;                // for _NameTableIndex
  Locator loc;            // a locator structure
  Value val ;             // a value structure

  // Increase the size of the handle to the character string
  // containing the field name so it can be null terminated.

  if (!_SetHandSize(parm->p[1].val.ev_handle, parm->p[1].val.ev_length +

  {
    _Error(182); // "Insufficient memory"
  }

  // Lock the handle to the character string containing
  // the name of the field.

  _HLock(parm->p[1].val.ev_handle) ;

  // Establish a pointer to the character string containing
  // the name of the field.

  fldname = (char FAR *) _HandToPtr(parm->p[1].val.ev_handle) ;

  // Null terminate the character string containing the
  // name of the field.

  fldname[parm->p[1].val.ev_length] = '\0' ;

  // Get nti value for the field.

  nti = _NameTableIndex(fldname) ;

  // Unlock the handle to the character string containing
  // the name of the field.

  _HUnLock(parm->p[1].val.ev_handle) ;

  if (nti == -1)
  {
    // This should not happen if a valid field name was passed.

    _PutStr("\nNameTableIndex Failure") ;
  }
  else
  {
    // Fill locator structure with information about the field:

    // Note that the second parameter to the _FindVar() function
    // is the first parameter that was passed to this function.
    // The first parameter to this function is a numeric value
    // specified the work area in which the field is located.

    if (! _FindVar(nti, (int) parm->p[0].val.ev_long, &loc))
    {

      // This should not happen if a valid field name was passed.

      _PutStr("\nFindVar Failure") ;

    }
    else
    {

      // Fill a value structure with field values.

      _Load(&loc,&val) ;

      // This code is written to support fields that are of
      // type character only.

      if (val.ev_type == 'C')
      {

        // Create a null-terminated string which contains the
        // text in the specified field.

        if (!_SetHandSize(val.ev_handle, val.ev_length + 1))
        {
          _Error(182); // "Insufficient memory"
        }

        _HLock(val.ev_handle) ;
        fldtxt = (char FAR *) _HandToPtr(val.ev_handle) ;
        fldtxt[val.ev_length] = '\0' ;

        // Display the values on the screen.


        _PutStr("\n") ;
        _PutStr(fldtxt) ;

        // Unlock the handle to the text.

        _HUnLock(val.ev_handle) ;
      }
      else
      {
        _PutStr("\nThe specified field is not of type Character.") ;

      }
    }
  }

FoxInfo myFoxInfo[] = {

     {"READDB", (FPFI) ReadDb, 2, "I,C"},

FoxTable _FoxTable = {

     (FoxTable FAR *) 0, sizeof(myFoxInfo)/sizeof(FoxInfo), myFoxInfo