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ftell() not consistent in C11 #8360

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14d63ab
ftell() have a different behavior in Windows 11 than before.
damorinCan Mar 21, 2026
1876e70
Applied comments.
damorinCan Apr 11, 2026
89e02be
Major issue:
damorinCan Apr 18, 2026
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20 changes: 20 additions & 0 deletions lib/checkio.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,7 @@ namespace {
// CVE ID used:
static const CWE CWE119(119U); // Improper Restriction of Operations within the Bounds of a Memory Buffer
static const CWE CWE398(398U); // Indicator of Poor Code Quality
static const CWE CWE474(474U); // Use of Function with Inconsistent Implementations
static const CWE CWE664(664U); // Improper Control of a Resource Through its Lifetime
static const CWE CWE685(685U); // Function Call With Incorrect Number of Arguments
static const CWE CWE686(686U); // Function Call With Incorrect Argument Type
Expand Down Expand Up @@ -116,6 +117,8 @@ namespace {
nonneg int op_indent{};
enum class AppendMode : std::uint8_t { UNKNOWN_AM, APPEND, APPEND_EX };
AppendMode append_mode = AppendMode::UNKNOWN_AM;
enum class ReadMode : std::uint8_t { READ_TEXT, READ_BIN };
ReadMode read_mode = ReadMode::READ_BIN;
std::string filename;
explicit Filepointer(OpenMode mode_ = OpenMode::UNKNOWN_OM)
: mode(mode_) {}
Expand Down Expand Up @@ -188,6 +191,7 @@ void CheckIO::checkFileUsage()
}
} else if (Token::Match(tok, "%name% (") && tok->previous() && (!tok->previous()->isName() || Token::Match(tok->previous(), "return|throw"))) {
std::string mode;
bool isftell = false;
const Token* fileTok = nullptr;
const Token* fileNameTok = nullptr;
Filepointer::Operation operation = Filepointer::Operation::NONE;
Expand Down Expand Up @@ -249,6 +253,9 @@ void CheckIO::checkFileUsage()
fileTok = tok->tokAt(2);
if ((tok->str() == "ungetc" || tok->str() == "ungetwc") && fileTok)
fileTok = fileTok->nextArgument();
else if (tok->str() == "ftell") {
isftell = true;
}
operation = Filepointer::Operation::UNIMPORTANT;
} else if (!Token::Match(tok, "if|for|while|catch|switch") && !mSettings->library.isFunctionConst(tok->str(), true)) {
const Token* const end2 = tok->linkAt(1);
Expand Down Expand Up @@ -304,10 +311,15 @@ void CheckIO::checkFileUsage()
f.append_mode = Filepointer::AppendMode::APPEND_EX;
else
f.append_mode = Filepointer::AppendMode::APPEND;
}
else if (mode.find('r') != std::string::npos &&
mode.find('t') != std::string::npos) {
f.read_mode = Filepointer::ReadMode::READ_TEXT;
} else
f.append_mode = Filepointer::AppendMode::UNKNOWN_AM;
f.mode_indent = indent;
break;

case Filepointer::Operation::POSITIONING:
if (f.mode == OpenMode::CLOSED)
useClosedFileError(tok);
Expand Down Expand Up @@ -340,6 +352,8 @@ void CheckIO::checkFileUsage()
case Filepointer::Operation::UNIMPORTANT:
if (f.mode == OpenMode::CLOSED)
useClosedFileError(tok);
if (isftell && windows && f.read_mode == Filepointer::ReadMode::READ_TEXT && printPortability)
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@danmar danmar Apr 9, 2026

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the error message says something about the C11 standard. Nothing about windows.

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@damorinCan damorinCan Apr 11, 2026 •
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I will add a comment about this reference to this Microsoft page:

https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/ftell-ftelli64?view=msvc-170

An update for the error message with this, would it be OK ?

According to Microsoft, the value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return-line feed translation. See also 7.21.9.4 in C11 standard.".

I will remove the reference to _wfopen(), it's not part of the C++ standard.

ftellFileError(tok);
break;
case Filepointer::Operation::UNKNOWN_OP:
f.mode = OpenMode::UNKNOWN_OM;
Expand Down Expand Up @@ -398,6 +412,12 @@ void CheckIO::seekOnAppendedFileError(const Token *tok)
"seekOnAppendedFile", "Repositioning operation performed on a file opened in append mode has no effect.", CWE398, Certainty::normal);
}

void CheckIO::ftellFileError(const Token *tok)
{
reportError(tok, Severity::portability,
"ftellTextModeFile", "For a text stream, its file position indicator contains unspecified information. See Section 7.21.9.4p2 of the C11 standard", CWE474, Certainty::normal);
}

void CheckIO::incompatibleFileOpenError(const Token *tok, const std::string &filename)
{
reportError(tok, Severity::warning,
Expand Down
1 change: 1 addition & 0 deletions lib/checkio.h
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Expand Up @@ -106,6 +106,7 @@ class CPPCHECKLIB CheckIO : public Check {
void writeReadOnlyFileError(const Token *tok);
void useClosedFileError(const Token *tok);
void seekOnAppendedFileError(const Token *tok);
void ftellFileError(const Token *tok);
void incompatibleFileOpenError(const Token *tok, const std::string &filename);
void invalidScanfError(const Token *tok);
void wrongPrintfScanfArgumentsError(const Token* tok,
Expand Down
16 changes: 16 additions & 0 deletions test/testio.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,7 @@ class TestIO : public TestFixture {
TEST_CASE(fileIOwithoutPositioning);
TEST_CASE(seekOnAppendedFile);
TEST_CASE(fflushOnInputStream);
TEST_CASE(ftellCompatibility);
TEST_CASE(incompatibleFileOpen);

TEST_CASE(testScanf1); // Scanf without field limiters
Expand Down Expand Up @@ -704,6 +705,21 @@ class TestIO : public TestFixture {
ASSERT_EQUALS("", errout_str()); // #6566
}

void ftellCompatibility() {

check("void foo() {\n"
" FILE *f = fopen(\"\", \"rt\");\n"
" if (f)\n"
" {\n"
" fseek(f, 0, SEEK_END);\n"
" (void)ftell(f);\n"
" fclose(f);\n"
" }\n"
"}\n", dinit(CheckOptions, $.platform = Platform::Type::Win32A, $.portability = true));
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@danmar danmar Apr 9, 2026

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test what happens on unix platform . test what happens when C99 is used - that works as expected right?

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@damorinCan damorinCan Apr 11, 2026 •
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Selecting Win32A is too limiting, it should apply to all platforms for portability.

ASSERT_EQUALS("[test.cpp:6:16]: (portability) For a text stream, its file position indicator contains unspecified information. See Section 7.21.9.4p2 of the C11 standard [ftellTextModeFile]\n", errout_str());
}


void fflushOnInputStream() {
check("void foo()\n"
"{\n"
Expand Down
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