GlobalAlloc instead of other allocation method

Question

While going through code on MSDN for finding the owner of the given file object, the program uses GlobalAlloc() to allocate the memory to AcctName. Since I'm new to using the Win32 API, my curiosity made me enquired why we are using GlobalAlloc() instead of other allocation methods. If other better allocation methods exist, how to use them in the given code?

#include <stdio.h>
#include <windows.h>
#include <tchar.h>
#include "accctrl.h"
#include "aclapi.h"
#pragma comment(lib, "advapi32.lib")

int main(void)
{
DWORD dwRtnCode = 0;
PSID pSidOwner = NULL;
BOOL bRtnBool = TRUE;
LPTSTR AcctName = NULL;
LPTSTR DomainName = NULL;
DWORD dwAcctName = 1, dwDomainName = 1;
SID_NAME_USE eUse = SidTypeUnknown;
HANDLE hFile;
PSECURITY_DESCRIPTOR pSD = NULL;


// Get the handle of the file object.
hFile = CreateFile(
                  TEXT("myfile.txt"),
                  GENERIC_READ,
                  FILE_SHARE_READ,
                  NULL,
                  OPEN_EXISTING,
                  FILE_ATTRIBUTE_NORMAL,
                  NULL);

// Check GetLastError for CreateFile error code.
if (hFile == INVALID_HANDLE_VALUE) {
          DWORD dwErrorCode = 0;

          dwErrorCode = GetLastError();
          _tprintf(TEXT("CreateFile error = %d\n"), dwErrorCode);
          return -1;
}



// Get the owner SID of the file.
dwRtnCode = GetSecurityInfo(
                  hFile,
                  SE_FILE_OBJECT,
                  OWNER_SECURITY_INFORMATION,
                  &pSidOwner,
                  NULL,
                  NULL,
                  NULL,
                  &pSD);

// Check GetLastError for GetSecurityInfo error condition.
if (dwRtnCode != ERROR_SUCCESS) {
          DWORD dwErrorCode = 0;

          dwErrorCode = GetLastError();
          _tprintf(TEXT("GetSecurityInfo error = %d\n"), dwErrorCode);
          return -1;
}

// First call to LookupAccountSid to get the buffer sizes.
bRtnBool = LookupAccountSid(
                  NULL,           // local computer
                  pSidOwner,
                  AcctName,
                  (LPDWORD)&dwAcctName,
                  DomainName,
                  (LPDWORD)&dwDomainName,
                  &eUse);

// Reallocate memory for the buffers.
AcctName = (LPTSTR)GlobalAlloc(
          GMEM_FIXED,
          dwAcctName);

// Check GetLastError for GlobalAlloc error condition.
if (AcctName == NULL) {
          DWORD dwErrorCode = 0;

          dwErrorCode = GetLastError();
          _tprintf(TEXT("GlobalAlloc error = %d\n"), dwErrorCode);
          return -1;
}

    DomainName = (LPTSTR)GlobalAlloc(
           GMEM_FIXED,
           dwDomainName);

    // Check GetLastError for GlobalAlloc error condition.
    if (DomainName == NULL) {
          DWORD dwErrorCode = 0;

          dwErrorCode = GetLastError();
          _tprintf(TEXT("GlobalAlloc error = %d\n"), dwErrorCode);
          return -1;

    }

    // Second call to LookupAccountSid to get the account name.
    bRtnBool = LookupAccountSid(
          NULL,                   // name of local or remote computer
          pSidOwner,              // security identifier
          AcctName,               // account name buffer
          (LPDWORD)&dwAcctName,   // size of account name buffer 
          DomainName,             // domain name
          (LPDWORD)&dwDomainName, // size of domain name buffer
          &eUse);                 // SID type

    // Check GetLastError for LookupAccountSid error condition.
    if (bRtnBool == FALSE) {
          DWORD dwErrorCode = 0;

          dwErrorCode = GetLastError();

          if (dwErrorCode == ERROR_NONE_MAPPED)
              _tprintf(TEXT
                  ("Account owner not found for specified SID.\n"));
          else 
              _tprintf(TEXT("Error in LookupAccountSid.\n"));
          return -1;

    } else if (bRtnBool == TRUE) 

        // Print the account name.
        _tprintf(TEXT("Account owner = %s\n"), AcctName);

    return 0;
}

Answer 1

Thanks for everyone. I have just modified my code by using malloc.

    AccountBuff = (LPSTR)malloc(AccountBufflength * sizeof(LPSTR));
    DomainBuff = (LPSTR)malloc(DomainBufflength * sizeof(LPSTR));

Answer 2

See this Microsoft documentation page for an overview of the different memory allocation functions that currently exist in Microsoft Windows.

Basically, GlobalAlloc is a remnant of 16-bit Windows. In 32-bit and 64-bit Windows, there is no longer a distinction between GlobalAlloc and LocalAlloc, as modern operating systems use a flat memory model. If you are interested in what significance these functions had in 16-bit Windows, you can read this article by Microsoft blogger Raymond Chen.

As stated in the offical documentation of the function GlobalAlloc, this function should generally not be used in modern 32-bit or 64-bit applications, except in situations where the documentation specifically tells you to do so.

Since the official documentation on the function LookupAccountSid does not specify that any of the passed buffers must have been allocated with GlobalAlloc, there is no reason to use this function. It would be more appropriate to for example use the function HeapAlloc, as this function has lower overhead than GlobalAlloc. Alternatively, you can use normal C++ memory allocations (for example new or std::vector), which, depending on the implementation, probably calls the function HeapAlloc indirectly.