CODEWITHSUNDEEP
winapigo
Jacob Marble
Jacob Marble

Reputation: 30122

Use structs with Golang syscall on Windows?

The EnumPrinters Win32 function takes and argument _Out_ LPBYTE pPrinterEnum, a pointer to an allocated buffer. In C, it works like this:

DWORD cbNeeded, nPrinters;
EnumPrinters(PRINTER_ENUM_LOCAL, NULL, 5, NULL, 0, &cbNeeded, &nPrinters);

BYTE *pPrnInfo = malloc(cbNeeded);
EnumPrinters(PRINTER_ENUM_LOCAL, NULL, 5, pPrnInfo, cbNeeded, &cbNeeded, &nPrinters);

PRINTER_INFO_5 *pPrinterInfo = (PRINTER_INFO_5 *) pPrnInfo;
for (int i=0; i < nPrinters; i++) {
  printf("pPrinterName: %s\n", pPrinterInfo[i].pPrinterName);
}

How is the same accomplished in Go, using syscall instead of cgo? So far, this much compiles, but I don't know how to cast the resulting byte slice to an array of structs (without using cgo).

type PrinterInfo5 struct {
    pPrinterName             *uint16
    pPortName                *uint16
    attributes               uint32
    deviceNotSelectedTimeout uint32
    transmissionRetryTimeout uint32
}

...

dll := syscall.MustLoadDLL("winspool.drv")
f := dll.MustFindProc("EnumPrintersW")

var cbNeeded, nPrinters uint32
fmt.Println(cbNeeded, nPrinters)
f.Call(PRINTER_ENUM_LOCAL, 0, 5, 0, 0, uintptr(unsafe.Pointer(&cbNeeded)), uintptr(unsafe.Pointer(&nPrinters)))
fmt.Println(cbNeeded, nPrinters)

var pPrnInfo []byte = make([]byte, cbNeeded)
f.Call(PRINTER_ENUM_LOCAL, 0, 5, uintptr(unsafe.Pointer(&pPrnInfo)), uintptr(cbNeeded), uintptr(unsafe.Pointer(&cbNeeded)), uintptr(unsafe.Pointer(&nPrinters)))

I have tried this, which prints one iteration successfully, then fails with fatal error: heapBitsBulkBarrier: unaligned arguments:

hdr := reflect.SliceHeader{
  Data: uintptr(unsafe.Pointer(&pPrnInfo)),
  Len:  int(nPrinters),
  Cap:  int(nPrinters),
}
s := *(*[]PrinterInfo5)(unsafe.Pointer(&hdr))
for _, t := range s {
  fmt.Println(t)
}

Upvotes: 2

Views: 2274

Answers (1)

andlabs
andlabs

Reputation: 11588

uintptr(unsafe.Pointer(&pPrnInfo))

in both places in the code above is wrong; it gives you a pointer to the slice header, not to the actual backing array. You want this instead:

uintptr(unsafe.Pointer(&pPrnInfo[0]))

(Since the backing array is contiguous, a pointer to the first element of the backing array is the same as a pointer to the backing array itself.)

Upvotes: 4

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