Fix #14743 (New check: ftell() result is unspecified when file is opened in mode "t") by damorinCan · Pull Request #8360 · cppcheck-opensource/cppcheck

damorinCan · 2026-03-21T16:45:33Z

Some legacy tools stopped suddenly working after 20+ years.

https://stackoverflow.com/questions/79762122/ftell-no-more-returning-the-correct-offset-on-a-text-file-with-windows-11-ente

Trac ticket: https://trac.cppcheck.net/ticket/14743#ticket

https://stackoverflow.com/questions/79762122/ftell-no-more-returning-the-correct-offset-on-a-text-file-with-windows-11-ente

chrchr-github · 2026-03-21T18:29:28Z

Thanks for your contribution.
As far as I can tell, this has nothing to do with Windows 11, but rather the C11 spec of ftell(), right? So the title of the PR should be adjusted.
Please add a test.

damorinCan · 2026-03-21T21:42:01Z

So the title of the PR should be adjusted.

Title now including reference to C11 and removed reference to Windows 11.

Please add a test.

I added a test in test/testio.cpp. Anything missing in it ?

chrchr-github · 2026-03-21T21:48:12Z

Please add a test.

I added a test in test/testio.cpp. Anything missing in it ?

Thanks, I must have missed that.

chrchr-github · 2026-03-24T08:08:23Z

Ping @danmar

danmar

I feel we need to have a ticket for this in trac

it feels like it should be added to the release notes as a "new check" ?

danmar · 2026-04-13T10:56:59Z

I have tried to add some descriptions about some of our checkers here:
https://github.com/danmar/cppcheck/tree/main/man/checkers

Ideally that would contain descriptions for all our checkers but due to lack-of-resources it does not.

If you can add a corresponding file for this checking that would be great.

- Fix the uncrustify check (removed https reference to Microsoft). New changes: - Added missing string related to new check - Added checker description for ftellTextModeFile - Updated copyright.

sonarqubecloud · 2026-04-18T16:33:26Z

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

danmar · 2026-04-24T20:15:31Z

We need a ticket in trac for this.

Can you please tell me what you think the summary and description should be and we can create it..

damorinCan · 2026-05-02T17:38:04Z

We need a ticket in trac for this.

Can you please tell me what you think the summary and description should be and we can create it..

Thanks, I lost my login/password (last time was years ago). What about this:

[ftell() is no more returning the correct offset on a text file with Windows 11 compared to Linux.

danmar · 2026-05-02T20:14:01Z

Thanks, I lost my login/password (last time was years ago). What about this:

do you remember your username? If you send me a new htpasswd hash via email I can update..

danmar · 2026-05-11T09:33:26Z

The PR title should be something like:

Fix #14743 (ftell() not consistent in C11)

I would like that the PR title and ticket title are consistent. and well imho the fundamental issue is not that linux/windows does not have the same behavior. the fundamental issue is that the behavior is implementation defined. it's possible you can have different behavior when using the same compiler but different volume type, compiler flags, etc..

damorinCan · 2026-05-11T22:16:00Z

Hi, I tried to edit the ticket title but it's not possible so I made the PR title the same as the ticket. Robert Le lun. 11 mai 2026 à 05:33, Daniel Marjamäki ***@***.***> a écrit :

…

*danmar* left a comment (cppcheck-opensource/cppcheck#8360) <#8360 (comment)> The PR title should be something like: Fix #14743 (ftell() not consistent in C11) I would like that the PR title and ticket title are consistent. — Reply to this email directly, view it on GitHub <#8360 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AB2AXDMB3H3KXMOCKNN3DTD42GM7ZAVCNFSM6AAAAACW2EKWFWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DIMJZGMYDOMRVHE> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>. You are receiving this because you authored the thread.Message ID: ***@***.***>

danmar · 2026-05-12T19:39:11Z

+void CheckIO::ftellFileError(const Token *tok)
+{
+    reportError(tok, Severity::portability,
+                "ftellTextModeFile", "The ftell function obtains the current value of the file position indicator"


sorry but I want to change...

This is not a good warning message. How about:

ftell() result is unspecified when file is opened in mode "t"

I want that the standard text is quoted in the ftellTextModeFile.md file.

danmar · 2026-05-13T07:07:02Z

Could you add a note about this in the releasenotes? In my opinion we can say it's a new check. Technically it's not but it's a completely new type of warning.

danmar · 2026-05-13T07:11:46Z

+
+        position = fseek(f, 0, SEEK_END);
+        fstat(f, &st);
+        printf( "Position  %d\n", ftell(f);


I want to have a buggy code that shows why we should warn. There should be a mistake.

It seems clear to me that the developer of this code understands that the output of ftell() is a position and that it will not necessarily be the size.

danmar · 2026-05-13T07:26:37Z

+        printf( "Position  %d\n", ftell(f);
+        printf( "File size %d\n, st.st_size);
+        fclose(f);
+     }


I suggest such bad code:

Suggested change

}

// Wrong way to get file size:

FILE *f = fopen("Example.txt", "rt");

fseek(f, 0, SEEK_END);

printf("File size: %d\n", ftell(f));

fclose(f);

ftell() have a different behavior in Windows 11 than before.

14d63ab

https://stackoverflow.com/questions/79762122/ftell-no-more-returning-the-correct-offset-on-a-text-file-with-windows-11-ente

damorinCan changed the title ~~ftell() have a different behavior in Windows 11 than before.~~ ftell() not consistent in C11 Mar 21, 2026

danmar reviewed Apr 9, 2026

View reviewed changes

Comment thread lib/checkio.cpp Outdated

Comment thread test/testio.cpp Outdated

Applied comments.

1876e70

Major issue:

89e02be

- Fix the uncrustify check (removed https reference to Microsoft). New changes: - Added missing string related to new check - Added checker description for ftellTextModeFile - Updated copyright.

danmar reviewed Apr 24, 2026

View reviewed changes

Comment thread man/checkers/ftellTextModeFile.md Outdated

Comment thread lib/checkio.h Outdated

danmar reviewed Apr 24, 2026

View reviewed changes

Comment thread test/testio.cpp Outdated

Comment thread man/checkers/ftellTextModeFile.md Outdated

Applied latest comment from danmar.

c103823

damorinCan changed the title ~~ftell() not consistent in C11~~ ftell() not consistent in between Linux/Unix and Windows when file is opened in mode "t" May 11, 2026

danmar changed the title ~~ftell() not consistent in between Linux/Unix and Windows when file is opened in mode "t"~~ Fix #14743 (ftell() not consistent when file is opened in mode "t") May 12, 2026

danmar reviewed May 12, 2026

View reviewed changes

danmar changed the title ~~Fix #14743 (ftell() not consistent when file is opened in mode "t")~~ Fix #14743 (New check: ftell() result is unspecified when file is opened in mode "t") May 13, 2026

danmar reviewed May 13, 2026

View reviewed changes

-     }
+    // Wrong way to get file size:
+    FILE *f = fopen("Example.txt", "rt");
+    fseek(f, 0, SEEK_END);
+    printf("File size: %d\n", ftell(f));
+    fclose(f);

Conversation

damorinCan commented Mar 21, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

chrchr-github commented Mar 21, 2026

Uh oh!

damorinCan commented Mar 21, 2026

Uh oh!

chrchr-github commented Mar 21, 2026

Uh oh!

chrchr-github commented Mar 24, 2026

Uh oh!

danmar left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

danmar commented Apr 13, 2026

Uh oh!

sonarqubecloud Bot commented Apr 18, 2026

Quality Gate passed

Uh oh!

danmar commented Apr 24, 2026

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

damorinCan commented May 2, 2026

Uh oh!

danmar commented May 2, 2026

Uh oh!

danmar commented May 11, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

damorinCan commented May 11, 2026 via email

Uh oh!

danmar May 12, 2026

Choose a reason for hiding this comment

Uh oh!

danmar commented May 13, 2026

Uh oh!

danmar May 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

danmar May 13, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

damorinCan commented Mar 21, 2026 •

edited

Loading

danmar commented May 11, 2026 •

edited

Loading

danmar May 13, 2026 •

edited

Loading