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Fix #1473: warn when fclose() is used as a while loop condition #8444

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Fix #1473: warn when fclose() is used as a while loop condition #8444

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5fa3789
Fix #1473: warn when fclose() is used as a while loop condition
francois-berder Apr 12, 2026
969864c
fixup! Fix #1473: warn when fclose() is used as a while loop condition
francois-berder Apr 27, 2026
a81456c
fixup! fixup! Fix #1473: warn when fclose() is used as a while loop c…
francois-berder Apr 27, 2026
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29 changes: 29 additions & 0 deletions lib/checkio.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -245,6 +245,35 @@ void CheckIO::checkFileUsage()
} else if (tok->str() == "fclose") {
fileTok = tok->tokAt(2);
operation = Filepointer::Operation::CLOSE;

// #1473 Check if fclose is in a while loop condition
if (fileTok) {
const Token* tmp = tok->astParent();
const Token* loopTok = nullptr;
while (tmp) {
if (Token::simpleMatch(tmp->previous(), "while (")) {
loopTok = tmp->previous();
break;
}
tmp = tmp->astParent();
}
if (loopTok) {
const Token* bodyEnd = nullptr;
const Token* bodyStart = nullptr;

if (Token::simpleMatch(loopTok->previous(), "}")) { // Handle do-while loops
bodyEnd = loopTok->previous();
bodyStart = bodyEnd->link();
} else {
bodyStart = loopTok->linkAt(1)->next();
bodyEnd = bodyStart->link();
}

// Do not trigger a warning if the loop always exits or if the file is opened again in the loop.
if (!isReturnScope(bodyEnd, mSettings->library) && Token::findmatch(bodyStart, "%var% = fopen|freopen", bodyEnd, fileTok->varId()) == nullptr)
useClosedFileError(tok);
}
}
} else if (whitelist.find(tok->str()) != whitelist.end()) {
fileTok = tok->tokAt(2);
if ((tok->str() == "ungetc" || tok->str() == "ungetwc") && fileTok)
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24 changes: 23 additions & 1 deletion test/testio.cpp
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Expand Up @@ -544,7 +544,29 @@ class TestIO : public TestFixture {
" FILE *a = fopen(\"aa\", \"r\");\n"
" while (fclose(a)) {}\n"
"}");
TODO_ASSERT_EQUALS("[test.cpp:3:5]: (error) Used file that is not opened. [useClosedFile]\n", "", errout_str());
ASSERT_EQUALS("[test.cpp:3:12]: (error) Used file that is not opened. [useClosedFile]\n", errout_str());
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@danmar danmar Apr 16, 2026

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add a test that ensures there are no false positives if files are opened in the loop.
I think it's really weird to use a fclose call as a loop condition anyway..

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@francois-berder francois-berder Apr 27, 2026

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@danmar I added two more checks that should handle a few more FP. I noticed that it does not handle do-while and for loops correctly.

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@francois-berder francois-berder Apr 27, 2026

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I have now extended the check to handle do-while loops in my latest commit.


check("void foo() {\n"
" FILE *a = fopen(\"aa\", \"r\");\n"
" while (fclose(a)) {\n"
" break;\n"
" }\n"
"}");
ASSERT_EQUALS("", errout_str());

check("void foo() {\n"
" FILE *a = fopen(\"aa\", \"r\");\n"
" while (fclose(a)) {\n"
" a = fopen(\"aa\", \"r\");\n"
" }\n"
"}");
ASSERT_EQUALS("", errout_str());

check("void foo() {\n"
" FILE *a = fopen(\"aa\", \"r\");\n"
" do {} while (fclose(a));\n"
"}");
ASSERT_EQUALS("[test.cpp:3:18]: (error) Used file that is not opened. [useClosedFile]\n", errout_str());

// #6823
check("void foo() {\n"
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