Unexpected AfxThrowFileException in CStdioFile::Read()

Question

Unexpected AfxThrowFileException in CStdioFile::Read()

Praveena S Kumar 20

I am using CStdioFile::Read in an MFC application, and I noticed that an exception is thrown in the following condition:

if ((nRead = (UINT)fread(lpBuf, sizeof(BYTE), nCount, m_pStream)) == 0 && !feof(m_pStream))         AfxThrowFileException(CFileException::genericException, _doserrno, m_strFileName);

I understand that nRead == 0 means no bytes were read, but since feof(m_pStream) is false, it suggests that EOF has not been reached.

Also, before calling CStdioFile::Read, we have checked that the file is open using CStdioFile::Open function too.

Below is the sample code :

static bool ReadIniFile(
    const CString& cstrIniFilePath,   
    std::map<CString, CString>& IniMap
)
{
    bool bRet = false;
    CStdioFile cFile;
    UINT TextMode = CFile::modeRead | CFile::typeText | CFile::shareDenyNone;
 
    IniMap.clear();
 
    if (cFile.Open(cstrIniFilePath, CFile::modeRead | CFile::typeBinary | CFile::shareDenyNone))   
    {
        BYTE cBuf[16];
        if ((cFile.Read(cBuf, 2) == 2) && (cBuf[0] == 0xFF) && (cBuf[1] == 0xFE)) // Crash point
        {
            TextMode = CFile::modeRead | CFile::typeUnicode | CFile::shareDenyNone;
        }
        cFile.Close();
 
        if(cFile.Open(cstrIniFilePath, TextMode))
        {
            if ((TextMode & CFile::typeUnicode) != 0)
            {
                cFile.Seek(2, CFile::begin);
            }
 
            CString strLine;
            CString strSection;
            CString strKey;
            CString strValue;
            while(cFile.ReadString(strLine))
            {
                strLine.Trim();
                int len = strLine.GetLength();
                if(len > 2)
                {
                    if(strLine[0] == '[')
                    {
                        int n = strLine.Find(']');
                        if(n >= 2)
                        {
                            strSection = strLine.Mid(1, (n - 1));
                        }
                    }
                    else if(!strSection.IsEmpty())
                    {
                        int n = strLine.Find('=');
                        if((n > 0) && ((n + 1) < len))
                        {
                            strKey = strLine.Left(n);
                            strKey.TrimRight();
                            strValue = strLine.Mid(n + 1, len - n);
                            strValue.TrimLeft();
 
                            CString strMapKey = strSection + _T("&") + strKey;
                            strMapKey.MakeUpper();
                            strValue.Replace(_T("\\n"), _T("\n"));
                            auto itr = IniMap.find(strMapKey);
                            if(itr == IniMap.end())
                            {
                                IniMap[strMapKey] = strValue;
                            }
                        }
                    }
                }
            }
            cFile.Close();
            bRet = true;
        }
    }
    return bRet;
}

Here file seems valid and not corrupted.

Question:

What are the possible reasons for this exception to be triggered?

Could this be due to file permissions, locking, or disk errors?
Are there any specific scenarios where fread fails but feof remains false?
How should I handle this exception properly to differentiate between different failure scenarios?

RLWA32 47,611 Reputation points

2025-02-03T11:54:07.39+00:00

I suggest you share a sample that reproduces the problem so we don't have to guess. For example, I can cause that exception by opening a file with CFile::modeWrite and then trying to read from it.
David Lowndes 2,590 Reputation points MVP

2025-02-03T12:15:59.7433333+00:00

Is nCount 0?
RLWA32 47,611 Reputation points

2025-02-03T15:22:26.09+00:00

@David Lowndes, CStdioFile::Read does an early exit if nCount = 0;
Minxin Yu 13,011 Reputation points Microsoft External Staff

2025-02-04T08:53:13.6+00:00

I cannot reproduce the problem using the sample above. Have you tried new ini files or on other computers?
RLWA32 47,611 Reputation points

2025-02-04T09:19:47.75+00:00

I could not reproduce the problem when testing the posted code against a well-formed .ini file.

However, if another process is running that has locked a range of bytes in the file the exception will be thrown. In this case the system error code will be 33.

A DEBUG build of the code should cause MFC to write an informative message to the VS debugger's Output pane.
RLWA32 47,611 Reputation points

2025-02-06T12:47:27.02+00:00

@Praveena S Kumar Anything new? Were you able to get some information from MFC's exception object about the cause of the problem?
Praveena S Kumar 20 Reputation points

2025-02-26T16:29:39.6033333+00:00
@RLWA32 Actually myself also not able to reproduce the issue in my application.

I am checking possible conditions for reproducing the issue.

I have shared the call stack of issue occurred thread in the issue time.

Below is the situation under which the issue was occurred,

The above given function ReadIniFile() is to read a file (say FileA). It is exposed by a dll.

Two separate processes access the file (FileA) using the same exposed dll function parallely.

So, my doubt is like, whether there is any chance for exception to be thrown in above situation.
RLWA32 47,611 Reputation points

2025-02-26T17:06:51.02+00:00

I have shared the call stack of issue occurred thread in the issue time.

For me, this is too far removed from the MFC source code that was posted to help diagnose the problem.

Ratheesh V A 0

Hi all,

I am updating the questions instead of Praveena S Kumar, from my account.

Kindly help to resolve the issue.

CallStack_IssueThreads

The above call stack is got during Windbg dump analysis.

I have also shared the test app which I have created for reproducing this issue.

Test App details:

In the test app, I have created two threads and they access(file read) the below functions parallelly for file reading.

Is this method of file access from two threads or process creates any chance for exception (as mentioned above).

Test app code:

CRITICAL_SECTION g_sCsFactory; **Globals

std::map<CString, CString> g_IniMap;

CString csLastFilePath_;

FirstFunction: At this stage, calling thread entered CriticalSection and then calls Second Function.

void ReadUtils::GetPrivateProfileStringUser( const CString& cstrSection, const CString& cstrKey, const CString& cstrIniFilePath, CString& cstrItem, bool& bIsFound)
{
    EnterCriticalSection(&g_sCsFactory);
    bool bMapEnable = false;
    csLastFilePath_ = _T("");
    if(cstrIniFilePath == csLastFilePath_)
    {
        bMapEnable = true;
    }
    else
    {
        csLastFilePath_ = cstrIniFilePath;
        bMapEnable = true;
        GetPrivateProfileStringValue(cstrIniFilePath, g_IniMap);
    }
    cstrItem.Empty();
    if (bMapEnable)
    {
        CString strMapKey = cstrSection + _T("&") + cstrKey;
        strMapKey.Trim();
        strMapKey.MakeUpper();
        auto itr = g_IniMap.find(strMapKey);
        if (itr != g_IniMap.end())
        {
            cstrItem = itr->second;
            bIsFound = true;
        }
    }
    LeaveCriticalSection(&g_sCsFactory);
}

SecondFunction: At this stage, calling thread acquires lock using Mutex(CMyMutex object) for file reading.

bool ReadUtils::GetPrivateProfileStringValue( const CString& cstrIniFilePath, std::map<CString, CString>& IniMap )
{
    CMyMutex::GetInstance()->Lock();
    bool bReadStatus = false;
    CStdioFile cFile;
    UINT TextMode = CFile::modeRead | CFile::typeText | CFile::shareDenyNone;
    IniMap.clear();
    // check unicode
    if (cFile.Open(cstrIniFilePath, CFile::modeRead | CFile::typeBinary | CFile::shareDenyNone))
    {
        BYTE cBuf[16];
        if ((cFile.Read(cBuf, 2) == 2) && (cBuf[0] == 0xFF) && (cBuf[1] == 0xFE))
        {
            TextMode = CFile::modeRead | CFile::typeUnicode | CFile::shareDenyNone;
        }
        cFile.Close();
        if (cFile.Open(cstrIniFilePath, TextMode))
        {
            // skip unicode binary
            if ((TextMode & CFile::typeUnicode) != 0)
            {
                cFile.Seek(2, CFile::begin);
            }
            CString strLine;
            CString strSection;
            CString strKey;
            CString strValue;
            while (cFile.ReadString(strLine))
            {
                strLine.Trim();
                int len = strLine.GetLength();
                if (len > 2)
                {
                    if (strLine[0] == '[')
                    {
                        int n = strLine.Find(']');
                        if (n >= 2)
                        {
                            strSection = strLine.Mid(1, (n - 1));
                        }
                    }
                    else if (!strSection.IsEmpty())
                    {
                        int n = strLine.Find('=');
                        if ((n > 0) && ((n + 1) < len))
                        {
                            strKey = strLine.Left(n);
                            strKey.TrimRight();
                            strValue = strLine.Mid(n + 1, len - n);
                            strValue.TrimLeft();
                            CString strMapKey = strSection + _T("&") + strKey;
                            strMapKey.MakeUpper();
                            strValue.Replace(_T("\\n"), _T("\n"));
                            auto itr = IniMap.find(strMapKey);
                            if (itr == IniMap.end())
                            {
                                 IniMap[strMapKey] = strValue;
                                bReadStatus = true;
                            }
                        }
                    }
                }
            }
            cFile.Close();
        }
    }
    CMyMutex::GetInstance()->UnLock();
    return bReadStatus;
}

I have exposed the above two functions in a dll.

Then I created two process/threads to call this functions in a loop like below.

I have created this test app, to simulate the call stack which I have shared above.

for (; index < count; index++)
    {
        ReadUtils::GetPrivateProfileStringUser(_T("ValueSection"), _T("Value3"), csfilepath, keyValue, bIsFound);
        (bIsFound) ? readSuccessCount++ : readSuccessCount;
    }

1 answer

Your answer

RLWA32 47,611 Reputation points

2025-02-03T11:54:07.39+00:00

I suggest you share a sample that reproduces the problem so we don't have to guess. For example, I can cause that exception by opening a file with CFile::modeWrite and then trying to read from it.
David Lowndes 2,590 Reputation points MVP

2025-02-03T12:15:59.7433333+00:00

Is nCount 0?
RLWA32 47,611 Reputation points

2025-02-03T15:22:26.09+00:00

@David Lowndes, CStdioFile::Read does an early exit if nCount = 0;
Minxin Yu 13,011 Reputation points Microsoft External Staff

2025-02-04T08:53:13.6+00:00

I cannot reproduce the problem using the sample above. Have you tried new ini files or on other computers?
RLWA32 47,611 Reputation points

2025-02-04T09:19:47.75+00:00

I could not reproduce the problem when testing the posted code against a well-formed .ini file.

However, if another process is running that has locked a range of bytes in the file the exception will be thrown. In this case the system error code will be 33.

A DEBUG build of the code should cause MFC to write an informative message to the VS debugger's Output pane.
RLWA32 47,611 Reputation points

2025-02-06T12:47:27.02+00:00

@Praveena S Kumar Anything new? Were you able to get some information from MFC's exception object about the cause of the problem?
Praveena S Kumar 20 Reputation points

2025-02-26T16:29:39.6033333+00:00

@RLWA32 Actually myself also not able to reproduce the issue in my application.

I am checking possible conditions for reproducing the issue.

I have shared the call stack of issue occurred thread in the issue time.

Below is the situation under which the issue was occurred,

The above given function ReadIniFile() is to read a file (say FileA). It is exposed by a dll.

Two separate processes access the file (FileA) using the same exposed dll function parallely.

So, my doubt is like, whether there is any chance for exception to be thrown in above situation.
RLWA32 47,611 Reputation points

2025-02-26T17:06:51.02+00:00

I have shared the call stack of issue occurred thread in the issue time.

For me, this is too far removed from the MFC source code that was posted to help diagnose the problem.

Answer 1

Ratheesh V A 0

@RLWA32 , @David Lowndes Can you please check is this informative for issue scenario.

RLWA32 47,611 Reputation points

2025-03-07T12:07:07.9333333+00:00

Please provide a complete sample that can be built and run on our own systems to reproduce the problem. In doing so you can remove any code that has no relevance to the issue.

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Unexpected AfxThrowFileException in CStdioFile::Read()

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