Why EnumPrintersA and EnumPrintersW request the same amount of memory?

Question

I call EnumPrintersA/EnumPrintersW functions using node-ffi to get list of local printers accessible from my PC.
You should create the buffer which will be filled with information by EnumPrinters function.
But you do not know the required size of the buffer.
In this case you need to execute EnumPrintersA/EnumPrintersW twice.
During the first call this function calculates the amount of memory for information about printers, during the second call this function fills the buffer with information about printers.
In case of Unicode version of EnumPrinters function, each letter in printers name will be encoded using two characters in Windows.

Why the first call to EnumPrintersW returns the same required amount of memory as the first call to EnumPrintersA?
Unicode strings are twice as long as not-unicode strings, but required buffer size is the same.

var ffi = require('ffi')
var ref = require('ref')
var Struct = require('ref-struct')
var wchar_t = require('ref-wchar')

var int = ref.types.int
var intPtr = ref.refType(ref.types.int)
var wchar_string = wchar_t.string

var getPrintersA =  function getPrinters() {
   var PRINTER_INFO_4A = Struct({
      'pPrinterName' : ref.types.CString,
      'pServerName' : ref.types.CString,
      'Attributes' : int
   });

   var printerInfoPtr = ref.refType(PRINTER_INFO_4A);

   var winspoolLib = new ffi.Library('winspool', {
      'EnumPrintersA': [ int, [ int, ref.types.CString, int, printerInfoPtr, int, intPtr, intPtr ] ]
   });

   var pcbNeeded = ref.alloc(int, 0);
   var pcReturned = ref.alloc(int, 0);

   //Get amount of memory for the buffer with information about printers
   var res = winspoolLib.EnumPrintersA(6, ref.NULL, 4, ref.NULL, 0, pcbNeeded, pcReturned);
   if (res != 0) {
      console.log("Cannot get list of printers. Error during first call to EnumPrintersA. Error: " + res);
      return;
   }

   var bufSize = pcbNeeded.deref();
   var buf = Buffer.alloc(bufSize);

   console.log(bufSize);

   //Fill buf with information about printers
   res = winspoolLib.EnumPrintersA(6, ref.NULL, 4, buf, bufSize, pcbNeeded, pcReturned);
   if (res == 0) {
      console.log("Cannot get list of printers. Eror: " + res);
      return;
   }

   var countOfPrinters = pcReturned.deref();

   var printers = Array(countOfPrinters);
   for (var i = 0; i < countOfPrinters; i++) {
      var pPrinterInfo = ref.get(buf, i*PRINTER_INFO_4A.size, PRINTER_INFO_4A);
      printers[i] = pPrinterInfo.pPrinterName;
   }

   return printers;
};

var getPrintersW =  function getPrinters() {
   var PRINTER_INFO_4W = Struct({
      'pPrinterName' : wchar_string,
      'pServerName' : wchar_string,
      'Attributes' : int
   });

   var printerInfoPtr = ref.refType(PRINTER_INFO_4W);

   var winspoolLib = new ffi.Library('winspool', {
      'EnumPrintersW': [ int, [ int, wchar_string, int, printerInfoPtr, int, intPtr, intPtr ] ]
   });

   var pcbNeeded = ref.alloc(int, 0);
   var pcReturned = ref.alloc(int, 0);

   //Get amount of memory for the buffer with information about printers
   var res = winspoolLib.EnumPrintersW(6, ref.NULL, 4, ref.NULL, 0, pcbNeeded, pcReturned);
   if (res != 0) {
      console.log("Cannot get list of printers. Error during first call to EnumPrintersW. Eror code: " + res);
      return;
   }

   var bufSize = pcbNeeded.deref();
   var buf = Buffer.alloc(bufSize);

   console.log(bufSize);

   //Fill buf with information about printers
   res = winspoolLib.EnumPrintersW(6, ref.NULL, 4, buf, pcbNeeded.deref(), pcbNeeded, pcReturned);
   if (res == 0) {
      console.log("Cannot get list of printers. Eror code: " + res);
      return;
   }

   var countOfPrinters = pcReturned.deref();
   var printers = new Array(countOfPrinters);
   for (var i = 0; i < countOfPrinters; i++) {
      var pPrinterInfo = ref.get(buf, i*PRINTER_INFO_4W.size, PRINTER_INFO_4W);
      printers[i] = pPrinterInfo.pPrinterName;
   }

   return printers;
};

https://msdn.microsoft.com/ru-ru/library/windows/desktop/dd162692(v=vs.85).aspx

BOOL EnumPrinters(
  _In_  DWORD   Flags,
  _In_  LPTSTR  Name,
  _In_  DWORD   Level,
  _Out_ LPBYTE  pPrinterEnum,
  _In_  DWORD   cbBuf,
  _Out_ LPDWORD pcbNeeded,
  _Out_ LPDWORD pcReturned
);

https://msdn.microsoft.com/ru-ru/library/windows/desktop/dd162847(v=vs.85).aspx

typedef struct _PRINTER_INFO_4 {
  LPTSTR pPrinterName;
  LPTSTR pServerName;
  DWORD  Attributes;
} PRINTER_INFO_4, *PPRINTER_INFO_4;

Answer 1

I can confirm that what you found with EnumPrintersA and EnumPrintersW is reproducible. In my machine, they both require 240 bytes.

This got me curious, so I decided to allocate a separate buffer for each function and dump each buffer to a file and opened them with a hex editor. The interesting part of each file is of course the names of the printers.

To keep this short, I'll show you the first 3 names of the printers. The first line is from EnumPrintersA, the second is from EnumPrintersW:

Fax.x...FX DocuPrint C1110 PCL 6..C.1.1.1.0. .P.C.L. .6...Microsoft XPS Document Writer.o.c.u.m.e.n.t. .W.r.i.t.e.r...
F.a.x...F.X. .D.o.c.u.P.r.i.n.t. .C.1.1.1.0. .P.C.L. .6...M.i.c.r.o.s.o.f.t. .X.P.S. .D.o.c.u.m.e.n.t. .W.r.i.t.e.r...

From this result, it appears that EnumPrintersA calls EnumPrintersW for the actual work and then simply converts each string in the buffer to single byte characters and puts the resulting string in the same place. To confirm this, I decided to trace EnumPrintersA code and I found that it definitely calls EnumPrintersW at position winspool.EnumPrintersA + 0xA7. The actual position is likely different in a different Windows version.

This got me even more curious, so I decided to test other functions that have A and W versions. This is what I found:

EnumMonitorsA 280 bytes needed
EnumMonitorsW 280 bytes needed
EnumServicesStatusA 20954 bytes needed
EnumServicesStatusW 20954 bytes needed
EnumPortsA 2176 bytes needed
EnumPortsW 2176 bytes needed
EnumPrintProcessorsA 24 bytes needed
EnumPrintProcessorsW 24 bytes needed

From this result, my conclusion is that EnumPrintersA calls EnumPrintersW for the actual work and converts the string in the buffer and other functions that have A and W versions also do the same thing. This appears to be a common mechanism to avoid duplication of code in expense of larger buffers, maybe because buffers can be deallocated anyway.