C++ 如何从 GetLastError() 返回的错误代码中获取错误信息？

How to get the error message from the error code returned by GetLastError()?

提问by Jamin Grey

After a Windows API call, how can I get the last error message in a textual form?

在 Windows API 调用之后，如何以文本形式获取最后一条错误消息？

GetLastError()returns an integer value, not a text message.

GetLastError()返回整数值，而不是文本消息。

回答by Jamin Grey

//Returns the last Win32 error, in string format. Returns an empty string if there is no error.
std::string GetLastErrorAsString()
{
    //Get the error message, if any.
    DWORD errorMessageID = ::GetLastError();
    if(errorMessageID == 0)
        return std::string(); //No error message has been recorded

    LPSTR messageBuffer = nullptr;
    size_t size = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
                                 NULL, errorMessageID, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&messageBuffer, 0, NULL);

    std::string message(messageBuffer, size);

    //Free the buffer.
    LocalFree(messageBuffer);

    return message;
}

回答by LeviX

Updated (11/2017) to take into consideration some comments.

更新 (11/2017) 以考虑一些评论。

Easy example:

简单的例子：

wchar_t buf[256];
FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
               NULL, GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), 
               buf, (sizeof(buf) / sizeof(wchar_t)), NULL);

回答by bk1e

MSDN has some sample code that demonstrates how to use FormatMessage()and GetLastError()together: Retrieving the Last-Error Code

MSDN 有一些示例代码演示了如何使用FormatMessage()和GetLastError()一起使用：检索上次错误代码

回答by Jonathan Graehl

FormatMessagewill turn GetLastError's integer return into a text message.

FormatMessage会将GetLastError的整数返回转换为文本消息。

回答by Vinay Sajip

In general, you need to use FormatMessageto convert from a Win32 error code to text.

通常，您需要使用FormatMessage将 Win32 错误代码转换为文本。

From the MSDN documentation:

从 MSDN文档：

Formats a message string. The function requires a message definition as input. The message definition can come from a buffer passed into the function. It can come from a message table resource in an already-loaded module. Or the caller can ask the function to search the system's message table resource(s) for the message definition. The function finds the message definition in a message table resource based on a message identifier and a language identifier. The function copies the formatted message text to an output buffer, processing any embedded insert sequences if requested.

格式化消息字符串。该函数需要一个消息定义作为输入。消息定义可以来自传递给函数的缓冲区。它可以来自已加载模块中的消息表资源。或者调用者可以要求函数搜索系统的消息表资源以获取消息定义。该函数根据消息标识符和语言标识符在消息表资源中查找消息定义。该函数将格式化的消息文本复制到输出缓冲区，在需要时处理任何嵌入的插入序列。

The declaration of FormatMessage:

FormatMessage 的声明：

DWORD WINAPI FormatMessage(
  __in      DWORD dwFlags,
  __in_opt  LPCVOID lpSource,
  __in      DWORD dwMessageId, // your error code
  __in      DWORD dwLanguageId,
  __out     LPTSTR lpBuffer,
  __in      DWORD nSize,
  __in_opt  va_list *Arguments
);

回答by IInspectable

GetLastErrorreturns a numerical error code. To obtain a descriptive error message (e.g., to display to a user), you can call FormatMessage:

GetLastError返回数字错误代码。要获取描述性错误消息（例如，向用户显示），您可以调用FormatMessage：

// This functions fills a caller-defined character buffer (pBuffer)
// of max length (cchBufferLength) with the human-readable error message
// for a Win32 error code (dwErrorCode).
// 
// Returns TRUE if successful, or FALSE otherwise.
// If successful, pBuffer is guaranteed to be NUL-terminated.
// On failure, the contents of pBuffer are undefined.
BOOL GetErrorMessage(DWORD dwErrorCode, LPTSTR pBuffer, DWORD cchBufferLength)
{
    if (cchBufferLength == 0)
    {
        return FALSE;
    }

    DWORD cchMsg = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
                                 NULL,  /* (not used with FORMAT_MESSAGE_FROM_SYSTEM) */
                                 dwErrorCode,
                                 MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                                 pBuffer,
                                 cchBufferLength,
                                 NULL);
    return (cchMsg > 0);
}

In C++, you can simplify the interface considerably by using the std::string class:

在 C++ 中，您可以通过使用 std::string 类来大大简化接口：

#include <Windows.h>
#include <system_error>
#include <memory>
#include <string>
typedef std::basic_string<TCHAR> String;

String GetErrorMessage(DWORD dwErrorCode)
{
    LPTSTR psz{ nullptr };
    const DWORD cchMsg = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM
                                         | FORMAT_MESSAGE_IGNORE_INSERTS
                                         | FORMAT_MESSAGE_ALLOCATE_BUFFER,
                                       NULL, // (not used with FORMAT_MESSAGE_FROM_SYSTEM)
                                       dwErrorCode,
                                       MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                                       reinterpret_cast<LPTSTR>(&psz),
                                       0,
                                       NULL);
    if (cchMsg > 0)
    {
        // Assign buffer to smart pointer with custom deleter so that memory gets released
        // in case String's c'tor throws an exception.
        auto deleter = [](void* p) { ::LocalFree(p); };
        std::unique_ptr<TCHAR, decltype(deleter)> ptrBuffer(psz, deleter);
        return String(ptrBuffer.get(), cchMsg);
    }
    else
    {
        auto error_code{ ::GetLastError() };
        throw std::system_error( error_code, std::system_category(),
                                 "Failed to retrieve error message string.");
    }
}

NOTE: These functions also work for HRESULT values. Just change the first parameter from DWORD dwErrorCode to HRESULT hResult. The rest of the code can remain unchanged.

注意：这些函数也适用于 HRESULT 值。只需将第一个参数从 DWORD dwErrorCode 更改为 HRESULT hResult。其余代码可以保持不变。

---

---

这些实现提供了对现有答案的以下改进：

• Complete sample code, not just a reference to the API to call.
• Provides both C and C++ implementations.
• Works for both Unicode and MBCS project settings.
• Takes the error code as an input parameter. This is important, as a thread's last error code is only valid at well defined points. An input parameter allows the caller to follow the documented contract.
• Implements proper exception safety. Unlike all of the other solutions that implicitly use exceptions, this implementation will not leak memory in case an exception is thrown while constructing the return value.
• Proper use of the FORMAT_MESSAGE_IGNORE_INSERTSflag. See The importance of the FORMAT_MESSAGE_IGNORE_INSERTS flagfor more information.
• Proper error handling/error reporting, unlike some of the other answers, that silently ignore errors.

• 完整的示例代码，而不仅仅是对要调用的 API 的引用。
• 提供 C 和 C++ 实现。
• 适用于 Unicode 和 MBCS 项目设置。
• 将错误代码作为输入参数。这很重要，因为线程的最后一个错误代码仅在明确定义的点有效。输入参数允许调用者遵循记录的合同。
• 实施适当的异常安全。与所有其他隐式使用异常的解决方案不同，如果在构造返回值时抛出异常，此实现将不会泄漏内存。
• 正确使用FORMAT_MESSAGE_IGNORE_INSERTS国旗。有关更多信息，请参阅FORMAT_MESSAGE_IGNORE_INSERTS 标志的重要性。
• 与其他一些答案不同，正确的错误处理/错误报告会默默地忽略错误。

---

---

This answer has been incorporated from Stack Overflow Documentation. The following users have contributed to the example: stackptr, Ajay, Cody Gray?, IInspectable.此答案已从 Stack Overflow 文档中合并。以下用户为本示例做出了贡献：stackptr、Ajay、Cody Gray？，可检查。

回答by rboy

If you're using c# you can use this code:

如果您使用的是 c#，则可以使用以下代码：

using System.Runtime.InteropServices;

public static class WinErrors
{
    #region definitions
    [DllImport("kernel32.dll", SetLastError = true)]
    static extern IntPtr LocalFree(IntPtr hMem);

    [DllImport("kernel32.dll", SetLastError = true)]
    static extern int FormatMessage(FormatMessageFlags dwFlags, IntPtr lpSource, uint dwMessageId, uint dwLanguageId, ref IntPtr lpBuffer, uint nSize, IntPtr Arguments);

    [Flags]
    private enum FormatMessageFlags : uint
    {
        FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100,
        FORMAT_MESSAGE_IGNORE_INSERTS = 0x00000200,
        FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000,
        FORMAT_MESSAGE_ARGUMENT_ARRAY = 0x00002000,
        FORMAT_MESSAGE_FROM_HMODULE = 0x00000800,
        FORMAT_MESSAGE_FROM_STRING = 0x00000400,
    }
    #endregion

    /// <summary>
    /// Gets a user friendly string message for a system error code
    /// </summary>
    /// <param name="errorCode">System error code</param>
    /// <returns>Error string</returns>
    public static string GetSystemMessage(int errorCode)
    {
        try
        {
            IntPtr lpMsgBuf = IntPtr.Zero;

            int dwChars = FormatMessage(
                FormatMessageFlags.FORMAT_MESSAGE_ALLOCATE_BUFFER | FormatMessageFlags.FORMAT_MESSAGE_FROM_SYSTEM | FormatMessageFlags.FORMAT_MESSAGE_IGNORE_INSERTS,
                IntPtr.Zero,
                (uint) errorCode,
                0, // Default language
                ref lpMsgBuf,
                0,
                IntPtr.Zero);
            if (dwChars == 0)
            {
                // Handle the error.
                int le = Marshal.GetLastWin32Error();
                return "Unable to get error code string from System - Error " + le.ToString();
            }

            string sRet = Marshal.PtrToStringAnsi(lpMsgBuf);

            // Free the buffer.
            lpMsgBuf = LocalFree(lpMsgBuf);
            return sRet;
        }
        catch (Exception e)
        {
            return "Unable to get error code string from System -> " + e.ToString();
        }
    }
}

回答by victimofleisure

If you need to support MBCS as well as Unicode, Mr.C64's answer is not quite enough. The buffer must be declared TCHAR, and cast to LPTSTR. Note that this code doesn't deal with the annoying newline that Microsoft appends to the error message.

如果需要同时支持MBCS和Unicode，Mr.C64的回答是不够的。缓冲区必须声明为 TCHAR，并转换为 LPTSTR。请注意，此代码不处理 Microsoft 附加到错误消息的烦人的换行符。

CString FormatErrorMessage(DWORD ErrorCode)
{
    TCHAR   *pMsgBuf = NULL;
    DWORD   nMsgLen = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL, ErrorCode, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        reinterpret_cast<LPTSTR>(&pMsgBuf), 0, NULL);
    if (!nMsgLen)
        return _T("FormatMessage fail");
    CString sMsg(pMsgBuf, nMsgLen);
    LocalFree(pMsgBuf);
    return sMsg;
}

Also, for brevity I find the following method useful:

另外，为简洁起见，我发现以下方法很有用：

CString GetLastErrorString()
{
    return FormatErrorMessage(GetLastError());
}

回答by Andriy Kuz

void WinErrorCodeToString(DWORD ErrorCode, string& Message)
{
char* locbuffer = NULL;
DWORD count = FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_ALLOCATE_BUFFER, NULL, ErrorCode,
    0, (LPSTR)&locbuffer, 0, nullptr);
if (locbuffer)
{
    if (count)
    {
        int c;
        int back = 0;
        //
        // strip any trailing "\r\n"s and replace by a single "\n"
        //
        while (((c = *CharPrevA(locbuffer, locbuffer + count)) == '\r') ||
            (c == '\n')) {
            count--;
            back++;
        }

        if (back) {
            locbuffer[count++] = '\n';
            locbuffer[count] = '
#include <Windows.h>

/***
 * returns 0 if there was enough space, size of buffer in bytes needed
 * to fit the result, if there wasn't enough space. -1 on error.
 */
__declspec(dllexport)
int GetErrorMessageA(DWORD dwErrorCode, LPSTR lpResult, DWORD dwBytes)
{    
    LPSTR tmp;
    DWORD result_len;

    result_len = FormatMessageA (
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ALLOCATE_BUFFER,
        NULL,
        dwErrorCode,
        LANG_SYSTEM_DEFAULT,
        (LPSTR)&tmp,
        0,
        NULL
    );        

    if (result_len == 0) {
        return -1;
    }

    // FormatMessage's return is 1 character too short.
    ++result_len;

    strncpy(lpResult, tmp, dwBytes);

    lpResult[dwBytes - 1] = 0;
    LocalFree((HLOCAL)tmp);

    if (result_len <= dwBytes) {
        return 0;
    } else {
        return result_len;
    }
}

/***
 * returns 0 if there was enough space, size of buffer in bytes needed
 * to fit the result, if there wasn't enough space. -1 on error.
 */
__declspec(dllexport)
int GetErrorMessageW(DWORD dwErrorCode, LPWSTR lpResult, DWORD dwBytes)
{   
    LPWSTR tmp;
    DWORD nchars;
    DWORD result_bytes;

    nchars = dwBytes >> 1;

    result_bytes = 2 * FormatMessageW (
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ALLOCATE_BUFFER,
        NULL,
        dwErrorCode,
        LANG_SYSTEM_DEFAULT,
        (LPWSTR)&tmp,
        0,
        NULL
    );    

    if (result_bytes == 0) {
        return -1;
    } 

    // FormatMessage's return is 1 character too short.
    result_bytes += 2;

    wcsncpy(lpResult, tmp, nchars);
    lpResult[nchars - 1] = 0;
    LocalFree((HLOCAL)tmp);

    if (result_bytes <= dwBytes) {
        return 0;
    } else {
        return result_bytes * 2;
    }
}
';
        }

        Message = "Error: ";
        Message += locbuffer;
    }
    LocalFree(locbuffer);
}
else
{
    Message = "Unknown error code: " + to_string(ErrorCode);
}
}

回答by Dmitry

i'll leave this here since i will need to use it later. It's a source for a small binary compatible tool that will work equally well in assembly, C and C++.

我将把它留在这里，因为我以后需要使用它。它是一个小型二进制兼容工具的来源，该工具在汇编、C 和 C++ 中同样有效。

GetErrorMessageLib.c (compiled to GetErrorMessageLib.dll)

GetErrorMessageLib.c（编译为 GetErrorMessageLib.dll）

#ifndef GetErrorMessage_H 
#define GetErrorMessage_H 
#include <Windows.h>    

/***
 * returns 0 if there was enough space, size of buffer in bytes needed
 * to fit the result, if there wasn't enough space. -1 on error.
 */
static inline int GetErrorMessageA(DWORD dwErrorCode, LPSTR lpResult, DWORD dwBytes)
{    
    LPSTR tmp;
    DWORD result_len;

    result_len = FormatMessageA (
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ALLOCATE_BUFFER,
        NULL,
        dwErrorCode,
        LANG_SYSTEM_DEFAULT,
        (LPSTR)&tmp,
        0,
        NULL
    );        

    if (result_len == 0) {
        return -1;
    }

    // FormatMessage's return is 1 character too short.
    ++result_len;

    strncpy(lpResult, tmp, dwBytes);

    lpResult[dwBytes - 1] = 0;
    LocalFree((HLOCAL)tmp);

    if (result_len <= dwBytes) {
        return 0;
    } else {
        return result_len;
    }
}

/***
 * returns 0 if there was enough space, size of buffer in bytes needed
 * to fit the result, if there wasn't enough space. -1 on error.
 */
static inline int GetErrorMessageW(DWORD dwErrorCode, LPWSTR lpResult, DWORD dwBytes)
{   
    LPWSTR tmp;
    DWORD nchars;
    DWORD result_bytes;

    nchars = dwBytes >> 1;

    result_bytes = 2 * FormatMessageW (
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ALLOCATE_BUFFER,
        NULL,
        dwErrorCode,
        LANG_SYSTEM_DEFAULT,
        (LPWSTR)&tmp,
        0,
        NULL
    );    

    if (result_bytes == 0) {
        return -1;
    } 

    // FormatMessage's return is 1 character too short.
    result_bytes += 2;

    wcsncpy(lpResult, tmp, nchars);
    lpResult[nchars - 1] = 0;
    LocalFree((HLOCAL)tmp);

    if (result_bytes <= dwBytes) {
        return 0;
    } else {
        return result_bytes * 2;
    }
}

#endif /* GetErrorMessage_H */

inline version(GetErrorMessage.h):

内联版本（GetErrorMessage.h）：

#include <Windows.h>
#include <Winbase.h>
#include <assert.h>
#include <stdio.h>

int main(int argc, char **argv)
{   
    int (*GetErrorMessageA)(DWORD, LPSTR, DWORD);
    int (*GetErrorMessageW)(DWORD, LPWSTR, DWORD);
    char result1[260];
    wchar_t result2[260];

    assert(LoadLibraryA("GetErrorMessageLib.dll"));

    GetErrorMessageA = (int (*)(DWORD, LPSTR, DWORD))GetProcAddress (
        GetModuleHandle("GetErrorMessageLib.dll"),
        "GetErrorMessageA"
    );        
    GetErrorMessageW = (int (*)(DWORD, LPWSTR, DWORD))GetProcAddress (
        GetModuleHandle("GetErrorMessageLib.dll"),
        "GetErrorMessageW"
    );        

    GetErrorMessageA(33, result1, sizeof(result1));
    GetErrorMessageW(33, result2, sizeof(result2));

    puts(result1);
    _putws(result2);

    return 0;
}

dynamic usecase(assumed that error code is valid, otherwise a -1 check is needed):

动态用例（假设错误代码有效，否则需要-1检查）：

#include <stdio.h>
#include "GetErrorMessage.h"
#include <stdio.h>

int main(int argc, char **argv)
{
    char result1[260];
    wchar_t result2[260];

    GetErrorMessageA(33, result1, sizeof(result1));
    puts(result1);

    GetErrorMessageW(33, result2, sizeof(result2));
    _putws(result2);

    return 0;
}

regular use case(assumes error code is valid, otherwise -1 return check is needed):

常规用例（假设错误代码有效，否则需要 -1 返回检查）：

    .global _WinMain@16

    .section .text
_WinMain@16:
    // eax = LoadLibraryA("GetErrorMessageLib.dll")
    push $sz0
    call _LoadLibraryA@4 // stdcall, no cleanup needed

    // eax = GetProcAddress(eax, "GetErrorMessageW")
    push $sz1
    push %eax
    call _GetProcAddress@8 // stdcall, no cleanup needed

    // (*eax)(errorCode, szErrorMessage)
    push 0
    push $szErrorMessage
    push errorCode       
    call *%eax // cdecl, cleanup needed
    add , %esp

    push $szErrorMessage
    call __putws // cdecl, cleanup needed
    add , %esp

    ret 

    .section .rodata
sz0: .asciz "GetErrorMessageLib.dll"    
sz1: .asciz "GetErrorMessageW"
errorCode: .long 33

    .section .data
szErrorMessage: .space 200

example using with assembly gnu as in MinGW32(again, assumed that error code is valid, otherwise -1 check is needed).

与 MinGW32 中的程序集 gnu 一起使用的示例（再次假设错误代码有效，否则需要 -1 检查）。

##代码##

result: The process cannot access the file because another process has locked a portion of the file.

结果： The process cannot access the file because another process has locked a portion of the file.