Bug in ScardListReaders when called with NULL context?

Zoltan Kelemen 21 Reputation points

I would like to ask if someone can confirm if the following is a bug in Windows.

When SCardListReaders is called with a NULL context (which is allowed by the API), and the reader list is empty in the registry, the function returns without zero-terminating the buffer. The bug is present from Windows 7 at least, and only in the ANSI variant (SCardListReadersA). The Unicode version works fine.

To trigger the problem, remove all subkeys in: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\Calais\Readers

The example program below shows the problem. When run with an empty reader list in the registry, it reports "Missing zero terminator in reader list".

#include <windows.h>
#include <winscard.h>
#include <stdio.h>
#include <stdlib.h>

#pragma comment(lib, "winscard.lib")

int main(void)
    char *buf;
    DWORD bufSize = 0;
    char *p;
    DWORD r;

    if ((r = SCardListReadersA(0, NULL, NULL, &bufSize)) != SCARD_S_SUCCESS) {
        printf("Failed to get reader list size: %08x\n", r);
        return 1;

    if (!bufSize) {
        printf("Invalid reader list size\n");
        return 1;

    buf = malloc(bufSize);
    memset(buf, 0xff, bufSize);

    if ((r = SCardListReadersA(0, NULL, buf, &bufSize)) != SCARD_S_SUCCESS) {
        printf("Failed to get reader list: %08x\n", r);
        return 1;

    if (buf[bufSize - 1]) {
        printf("Missing zero terminator in reader list\n");
        return 1;

    if (!buf[0]) {
        printf("No readers found\n");
        return 0;

    printf("Reader list:\n");
    for (p = buf; *p != '\0'; p += strlen(p) + 1)
        printf("%s\n", p);

    return 0;
Windows 10
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Windows API - Win32
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Accepted answer
  1. David Lowndes 4,711 Reputation points

    Yes, it's still the same for me running the latest Windows 10 insider version.
    The Unicode version returns the null while the ANSI version doesn't.
    You should report it on the feedback hole, but given its history I suspect it won't change and all you can do is work around it.

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