How To Pass Structures to a DLL

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This article was previously published under Q181289

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SUMMARY

Many DLLs require that the calling program pass a structure. FoxPro can pass by reference a string of the same length as the required structure. Depending on the datatype, some variables need to be converted to the proper format. For instance, DWORD is a low-high format, so any 4-byte values need to be converted to a string in the DWORD format. DLLs may also return values that need to be converted from a string in a low-high format to an integer value.

MORE INFORMATION

The Win32 API function GetSystemInfo requires that a pointer to a structure be passed to the function. The following example demonstrates how to make the call then convert the elements of the returned string using the StrToLong function. The LongToStr function to do the reverse is also included.

Copy this code into a program file (.prg) and run the program. This program uses GetSystemInfo to return information about a computer's processor and memory.

The code is listed below:

   #DEFINE CR CHR(13)
   #DEFINE PROCESSOR_INTEL_386 386
   #DEFINE PROCESSOR_INTEL_486 486
   #DEFINE PROCESSOR_INTEL_PENTIUM 586
   #DEFINE PROCESSOR_MIPS_R4000 4000
   #DEFINE PROCESSOR_ALPHA_21064 21064

   DECLARE GetSystemInfo IN win32api STRING @SystemInfo
   DECLARE GlobalMemoryStatus IN win32api STRING @MemStat
   SystemInfo = SPACE(36)
   m.stru = LongToStr(32) + REPLICATE(CHR(0), 28)
   =GetSystemInfo(@SystemInfo)
   =GlobalMemoryStatus(@m.stru)
   m.oem_id = StrToLong(LEFT(SystemInfo, 4))
   m.page_size = StrToLong(SUBSTR(SystemInfo, 5, 4))
   m.minappaddr = StrToLong(SUBSTR(SystemInfo, 9, 4))
   m.maxappaddr = StrToLong(SUBSTR(SystemInfo, 13, 4))
   m.processormask = StrToLong(SUBSTR(SystemInfo, 17, 4))
   m.num_processors = StrToLong(SUBSTR(SystemInfo, 21, 4))
   m.processortype = StrToLong(SUBSTR(SystemInfo, 25, 4))
   m.processorname = ""
   m.memoryload = StrToLong(SUBSTR(m.stru, 5, 4))
   m.totalphys = StrToLong(SUBSTR(m.stru, 9, 4))
   m.availphys = StrToLong(SUBSTR(m.stru, 13, 4))
   m.totalpagefile = StrToLong(SUBSTR(m.stru, 17, 4))
   m.availpagefile = StrToLong(SUBSTR(m.stru, 21, 4))
   m.totalvirtual = StrToLong(SUBSTR(m.stru, 25, 4))
   m.availvirtual = StrToLong(SUBSTR(m.stru, 29, 4))
   DO CASE
   CASE m.processortype = PROCESSOR_INTEL_386
      m.processorname = "INTEL 386"
   CASE m.processortype = PROCESSOR_INTEL_486
      m.processorname = "INTEL 486"
   CASE m.processortype = PROCESSOR_INTEL_PENTIUM
      m.processorname = "INTEL Pentium"
   CASE m.processortype = PROCESSOR_MIPS_R4000
      m.processorname = "MIPS R4000"
   CASE m.processortype = PROCESSOR_ALPHA_21064
      m.processorname = "ALPHA 21064"
   ENDCASE
   m.allocationgranularity = StrToLong(SUBSTR(SystemInfo, 29, 4))
   m.reserved = StrToLong(SUBSTR(SystemInfo, 33, 4))
   =MESSAGEBOX("OEM ID:" + STR(m.oem_id) + CR + ;
    "Page Size: " + STR(m.page_size) + CR + ;
    "Minimum application address: " + STR(m.minappaddr) + CR +;
    "Maximum application address: " + STR(m.maxappaddr) + CR + ;
    "Processor mask: " + STR(m.processormask) + CR + ;
    "Number of processors: " + STR(m.num_processors) + CR + ;
    "Allocation granularity: " + STR(m.allocationgranularity) + CR + ;
    "Percent memory in use: " + STR(m.memoryload) + CR + ;
    "Bytes of physical memory present: " + STR(m.totalphys) + CR + ;
    "Bytes available physical memory: " + STR(m.availphys) + CR + ;
    "Bytes of paging file: " + STR(m.totalpagefile) + CR + ;
    "Bytes available paging file: " + STR(m.availpagefile) + CR + ;
    "Total virtual memory: " + STR(m.totalvirtual) + CR + ;
    "Available bytes virtual memory: " + STR(m.availvirtual),0+64+0)

   *-- The following function converts a long integer to an ASCII
   *-- character representation of the passed value in low-high format.
   ******************
   FUNCTION LongToStr
   ******************
   * Passed : 32-bit non-negative numeric value (lnLongval)
   * Returns : ascii character representation of passed value in low-high
   * format (lcRetstr)
   * Example :
   *   m.long = "999999"
   *   m.longstr = long2str(m.long)

   PARAMETERS lnLongval

   PRIVATE i, lcRetstr

   lcRetstr = ""
   FOR i = 24 TO 0 STEP -8
      lcRetstr = CHR(INT(lnLongval/(2^i))) + lcRetstr
      lnLongval = MOD(lnLongval, (2^i))
   NEXT
   RETURN lcRetstr

   *-- The following function converts a string in low-high format to a
   *-- long integer.
   ******************
   FUNCTION StrToLong
   ******************
   * Passed:  4-byte character string (lcLongstr) in low-high ASCII format
   * Returns:  long integer value
   * Example:
   * m.longstr = "1111"
   * m.longval = str2long(m.longstr)

   PARAMETERS lcLongstr

   PRIVATE i, lnRetval

   lnRetval = 0
   FOR i = 0 TO 24 STEP 8
      lnRetval = lnRetval + (ASC(lcLongstr) * (2^i))
      lcLongstr = RIGHT(lcLongstr, LEN(lcLongstr) - 1)
   NEXT

   RETURN lnRetval