How to read a value from the Windows registry

Question

Given the key for some registry value  e g  HKEY LOCAL MACHINE blah blah blah foo  how can I    Safely determine that such a key exists  Programmatically  i e  with code  get its value    I have absolutely no intention of writing anything back to the registry  for the duration of my career if I can help it   So we can skip the lecture about every molecule in my body exploding at the speed of light if I write to the registry incorrectly    Prefer answers in C    but mostly just need to know what the special Windows API incantation to get at the value is

User · Answer

const CString REG SW GROUP I WANT    T  SOFTWARE  My Corporation  My Package  Group I want    const CString REG KEY I WANT   T  Key Name     CRegKey regKey  DWORD   dwValue   0   if ERROR SUCCESS    regKey Open HKEY LOCAL MACHINE  REG SW GROUP I WANT       m pobLogger- gt LogError  T  CRegKey  Open failed in Method       regKey Close      goto Function Exit    if  ERROR SUCCESS    regKey QueryValue  dwValue  REG KEY I WANT       m pobLogger- gt LogError  T  CRegKey  QueryValue Failed in Method       regKey Close      goto Function Exit        dwValue has the stuff now - use for further processing

User · Answer

RegQueryValueEx  This gives the value if it exists  and returns an error code ERROR FILE NOT FOUND if the key doesn t exist    I can t tell if my link is working or not  but if you just google for  RegQueryValueEx  the first hit is the msdn documentation

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This console app will list all the values and their data from a registry key for most of the potential registry values  There s some weird ones not often used  If you need to support all of them  expand from this example while referencing this Registry Value Type documentation   Let this be the registry key content you can import from a  reg file format   Windows Registry Editor Version 5 00   HKEY CURRENT USER added subkey   String Value   hello  world    Binary Value  hex 01 01 01 01  Dword value  dword 00001224  QWord val  hex b  24 22 12 00 00 00 00 00  multi-line val  hex 7  4c 00 69 00 6e 00 65 00 20 00 30 00 00 00 4c 00 69 00     6e 00 65 00 20 00 31 00 00 00 4c 00 69 00 6e 00 65 00 20 00 32 00 00 00 00     00  expanded val  hex 2  25 00 55 00 53 00 45 00 52 00 50 00 52 00 4f 00 46 00 49     00 4c 00 45 00 25 00 5c 00 6e 00 65 00 77 00 5f 00 73 00 74 00 75 00 66 00     66 00 00 00   The console app itself    include  lt Windows h gt   include  lt iostream gt   include  lt string gt   include  lt locale gt   include  lt vector gt   include  lt iomanip gt   int wmain         const auto hKey   HKEY CURRENT USER      constexpr auto lpSubKey   TEXT  added  subkey        auto openedKey   HKEY        auto status   RegOpenKeyEx hKey  lpSubKey  0  KEY READ   amp openedKey        if  status    ERROR SUCCESS            auto valueCount   static cast lt DWORD gt  0           auto maxNameLength   static cast lt DWORD gt  0           auto maxValueLength   static cast lt DWORD gt  0           status   RegQueryInfoKey openedKey  NULL  NULL  NULL  NULL  NULL  NULL               amp valueCount   amp maxNameLength   amp maxValueLength  NULL  NULL            if  status    ERROR SUCCESS                DWORD type   0              DWORD index   0              std  vector lt wchar t gt  valueName   std  vector lt wchar t gt  maxNameLength   1               std  vector lt BYTE gt  dataBuffer   std  vector lt BYTE gt  maxValueLength                for  DWORD index   0  index  lt  valueCount  index                      DWORD charCountValueName   static cast lt DWORD gt  valueName size                     DWORD charBytesData   static cast lt DWORD gt  dataBuffer size                     status   RegEnumValue openedKey  index  valueName data     amp charCountValueName                      NULL   amp type  dataBuffer data     amp charBytesData                    if  type    REG SZ                        const auto reg string   reinterpret cast lt wchar t  gt  dataBuffer data                         std  wcout  lt  lt  L Type  REG SZ   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData      lt  lt  reg string  lt  lt  std  endl                                    else if  type    REG EXPAND SZ                        const auto casted   reinterpret cast lt wchar t  gt  dataBuffer data                         TCHAR buffer 32000                       ExpandEnvironmentStrings casted  buffer  32000                       std  wcout  lt  lt  L Type  REG EXPAND SZ   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData     lt  lt  buffer  lt  lt  std  endl                                    else if  type    REG MULTI SZ                        std  vector lt std  wstring gt  lines                      const auto str   reinterpret cast lt wchar t  gt  dataBuffer data                         auto line   str                      lines emplace back line                       for  auto i   0  i  lt  charBytesData   sizeof wchar t  - 1  i                              const auto c   str i                           if  c    0                                line   str   i   1                              const auto new line   reinterpret cast lt wchar t  gt  line                               if  wcsnlen s new line  1024   gt  0                                  lines emplace back new line                                                                       std  wcout  lt  lt  L Type  REG MULTI SZ   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData     lt  lt  std  endl                      for  size t i   0  i  lt  lines size    i                              std  wcout  lt  lt  L  t tLine    lt  lt  i   1  lt  lt  L       lt  lt  lines i   lt  lt  std  endl                                                          if  type    REG DWORD                        const auto dword value   reinterpret cast lt unsigned long  gt  dataBuffer data                         std  wcout  lt  lt  L Type  REG DWORD   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData      lt  lt  std  to wstring  dword value   lt  lt  std  endl                                    else if  type    REG QWORD                        const auto qword value   reinterpret cast lt unsigned long long  gt  dataBuffer data                         std  wcout  lt  lt  L Type  REG DWORD   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData      lt  lt  std  to wstring  qword value   lt  lt  std  endl                                    else if  type    REG BINARY                        std  vector lt uint16 t gt  bins                      for  auto i   0  i  lt  charBytesData  i                              bins push back static cast lt uint16 t gt  dataBuffer i                                               std  wcout  lt  lt  L Type  REG BINARY   lt  lt  std  endl                      std  wcout  lt  lt  L  tName     lt  lt  valueName data    lt  lt  std  endl                      std  wcout  lt  lt  L  tData                        for  size t i   0  i  lt  bins size    i                              std  wcout  lt  lt  L     lt  lt  std  uppercase  lt  lt  std  hex  lt  lt                                std  setw 2   lt  lt  std  setfill L 0    lt  lt  std  to wstring bins i                                              std  wcout  lt  lt  std  endl                                                       RegCloseKey openedKey       return 0      Expected console output    Type  REG SZ         Name  String Value         Data   hello  world  Type  REG BINARY         Name  Binary Value         Data  01 01 01 01 Type  REG DWORD         Name  Dword value         Data   4644 Type  REG DWORD         Name  QWord val         Data   1188388 Type  REG MULTI SZ         Name  multi-line val         Data                  Line 1   Line 0                 Line 2   Line 1                 Line 3   Line 2 Type  REG EXPAND SZ         Name  expanded val         Data  C  Users user name new stuff

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Since Windows   Vista Server 2008  RegGetValue is available  which is a safer function than RegQueryValueEx  No need for RegOpenKeyEx  RegCloseKey or NUL termination checks of string values  REG SZ  REG MULTI SZ  REG EXPAND SZ     include  lt iostream gt   include  lt string gt   include  lt exception gt   include  lt windows h gt        brief                          Returns a value from HKLM as string       exception  std  runtime error  Replace with your error handling     std  wstring GetStringValueFromHKLM const std  wstring amp  regSubKey  const std  wstring amp  regValue        size t bufferSize   0xFFF     If too small  will be resized down below      std  wstring valueBuf     Contiguous buffer since C  11      valueBuf resize bufferSize       auto cbData   static cast lt DWORD gt  bufferSize   sizeof wchar t        auto rc   RegGetValueW          HKEY LOCAL MACHINE          regSubKey c str            regValue c str            RRF RT REG SZ          nullptr          static cast lt void  gt  valueBuf data              amp cbData            while  rc    ERROR MORE DATA                   Get a buffer that is big enough          cbData    sizeof wchar t           if  cbData  gt  static cast lt DWORD gt  bufferSize                         bufferSize   static cast lt size t gt  cbData                     else                       bufferSize    2              cbData   static cast lt DWORD gt  bufferSize   sizeof wchar t                      valueBuf resize bufferSize           rc   RegGetValueW              HKEY LOCAL MACHINE              regSubKey c str                regValue c str                RRF RT REG SZ              nullptr              static cast lt void  gt  valueBuf data                  amp cbData                      if  rc    ERROR SUCCESS                cbData    sizeof wchar t           valueBuf resize static cast lt size t gt  cbData - 1       remove end null character         return valueBuf            else               throw std  runtime error  Windows system error code      std  to string rc             int main         std  wstring regSubKey   ifdef  WIN64    Manually switching between 32bit 64bit for the example  Use dwFlags instead      regSubKey   L SOFTWARE  WOW6432Node  Company Name  Application Name      else     regSubKey   L SOFTWARE  Company Name  Application Name      endif     std  wstring regValue L MyValue        std  wstring valueFromRegistry      try               valueFromRegistry   GetStringValueFromHKLM regSubKey  regValue             catch  std  exception amp  e                std  cerr  lt  lt  e what              std  wcout  lt  lt  valueFromRegistry      Its parameter dwFlags supports flags for type restriction  filling the value buffer with zeros on failure  RRF ZEROONFAILURE  and 32 64bit registry access  RRF SUBKEY WOW6464KEY  RRF SUBKEY WOW6432KEY  for 64bit programs

User · Answer

Typically the register key and value are constants in the program  If so  here is an example how to read a DWORD registry value Computer HKEY LOCAL MACHINE SYSTEM CurrentControlSet Control FileSystem LongPathsEnabled    include  lt windows h gt   DWORD val  DWORD dataSize   sizeof val   if  ERROR SUCCESS    RegGetValueA HKEY LOCAL MACHINE   SYSTEM  CurrentControlSet  Control  FileSystem    LongPathsEnabled   RRF RT DWORD  nullptr   type not required     amp val   amp dataSize       printf  Value is  i n   val        no CloseKey needed because it is a predefined registry key   else     printf  Error reading  n        To adapt for other value types  see https   docs microsoft com en-us windows win32 api winreg nf-winreg-reggetvaluea for complete spec

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include  lt windows h gt   include  lt map gt   include  lt string gt   include  lt stdio h gt   include  lt string h gt   include  lt tr1 stdint h gt   using namespace std   void printerr DWORD dwerror        LPVOID lpMsgBuf      FormatMessage          FORMAT MESSAGE ALLOCATE BUFFER           FORMAT MESSAGE FROM SYSTEM           FORMAT MESSAGE IGNORE INSERTS          NULL          dwerror          MAKELANGID LANG NEUTRAL  SUBLANG DEFAULT      Default language          LPTSTR   amp lpMsgBuf          0          NULL               Process any inserts in lpMsgBuf                    Display the string      if  isOut            fprintf fout    s n   lpMsgBuf         else           printf   s n   lpMsgBuf                Free the buffer      LocalFree lpMsgBuf        bool regreadSZ string amp  hkey  string amp  subkey  string amp  value  string amp  returnvalue  string amp  regValueType        char s 128000       map lt string HKEY gt  keys      keys  HKEY CLASSES ROOT   HKEY CLASSES ROOT      keys  HKEY CURRENT CONFIG   HKEY CURRENT CONFIG    DID NOT SURVIVE      keys  HKEY CURRENT USER   HKEY CURRENT USER      keys  HKEY LOCAL MACHINE   HKEY LOCAL MACHINE      keys  HKEY USERS   HKEY USERS      HKEY mykey       map lt string DWORD gt  valuetypes      valuetypes  REG SZ   REG SZ      valuetypes  REG EXPAND SZ   REG EXPAND SZ      valuetypes  REG MULTI SZ   REG MULTI SZ    probably can t use this       LONG retval RegOpenKeyEx          keys hkey              handle to open key         subkey c str        subkey name         0       reserved         KEY READ     security access mask          amp mykey       handle to open key            if  ERROR SUCCESS    retval   printerr retval   return false       DWORD slen 128000      DWORD valuetype   valuetypes regValueType       retval RegQueryValueEx        mykey                handle to key       value c str        value name       NULL       reserved        LPDWORD   amp valuetype           type buffer        LPBYTE s            data buffer        LPDWORD   amp slen         size of data buffer            switch retval            case ERROR SUCCESS                if  isOut                      fprintf fout  RegQueryValueEx   ERROR SUCCESS succeeded  n                    else                     printf  RegQueryValueEx   ERROR SUCCESS succeeded  n                                break          case ERROR MORE DATA                what do I do now   data buffer is too small              if  isOut                    fprintf fout  RegQueryValueEx   ERROR MORE DATA  need bigger buffer  n                  else                   printf  RegQueryValueEx   ERROR MORE DATA  need bigger buffer  n                              return false          case ERROR FILE NOT FOUND              if  isOut                    fprintf fout  RegQueryValueEx   ERROR FILE NOT FOUND  registry value does not exist  n                  else                   printf  RegQueryValueEx   ERROR FILE NOT FOUND  registry value does not exist  n                              return false          default              if  isOut                    fprintf fout  RegQueryValueEx   unknown error type 0x lx  n   retval                 else                   printf  RegQueryValueEx   unknown error type 0x lx  n   retval                             return false             retval RegCloseKey mykey       if  ERROR SUCCESS    retval   printerr retval   return false        returnvalue   s      return true

User · Answer

The pair RegOpenKey and RegQueryKeyEx will do the trick   If you use MFC CRegKey class is even more easier solution

User · Answer

Here is some pseudo-code to retrieve the following    If a registry key exists What the default value is for that registry key What a string value is What a DWORD value is   Example code   Include the library dependency  Advapi32 lib  HKEY hKey  LONG lRes   RegOpenKeyExW HKEY LOCAL MACHINE  L SOFTWARE  Perl   0  KEY READ   amp hKey   bool bExistsAndSuccess  lRes    ERROR SUCCESS   bool bDoesNotExistsSpecifically  lRes    ERROR FILE NOT FOUND   std  wstring strValueOfBinDir  std  wstring strKeyDefaultValue  GetStringRegKey hKey  L BinDir   strValueOfBinDir  L bad    GetStringRegKey hKey  L    strKeyDefaultValue  L bad     LONG GetDWORDRegKey HKEY hKey  const std  wstring  amp strValueName  DWORD  amp nValue  DWORD nDefaultValue        nValue   nDefaultValue      DWORD dwBufferSize sizeof DWORD        DWORD nResult 0       LONG nError     RegQueryValueExW hKey          strValueName c str            0          NULL          reinterpret cast lt LPBYTE gt   amp nResult            amp dwBufferSize       if  ERROR SUCCESS    nError                nValue   nResult            return nError      LONG GetBoolRegKey HKEY hKey  const std  wstring  amp strValueName  bool  amp bValue  bool bDefaultValue        DWORD nDefValue  bDefaultValue    1   0       DWORD nResult nDefValue       LONG nError   GetDWORDRegKey hKey  strValueName c str    nResult  nDefValue       if  ERROR SUCCESS    nError                bValue    nResult    0    true   false            return nError      LONG GetStringRegKey HKEY hKey  const std  wstring  amp strValueName  std  wstring  amp strValue  const std  wstring  amp strDefaultValue        strValue   strDefaultValue      WCHAR szBuffer 512       DWORD dwBufferSize   sizeof szBuffer       ULONG nError      nError   RegQueryValueExW hKey  strValueName c str    0  NULL   LPBYTE szBuffer   amp dwBufferSize       if  ERROR SUCCESS    nError                strValue   szBuffer            return nError

[c++] How to read a value from the Windows registry

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