[musl] Bug: ftell() after fopen(..., "ab") returns 0 unless an explicit fseek() is used first

[musl] Bug: ftell() after fopen(..., "ab") returns 0 unless an explicit fseek() is used first

[Buchholz, Robert]

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I'm running Alpine Linux with MUSL as its standard library within a Docker container based on the official openjdk:16-jdk-alpine3.13 Alpine image.

I've used the following minimal test case:
========
#include <stdio.h>

int main()
{
  FILE* f = fopen("foo", "wb");
  fwrite("42", 3, 1, f);
  fclose(f);

  f = fopen("foo", "ab");
  printf("%d ", (int)ftell(f));
  fwrite("42", 3, 1, f);
  fseek(f, 0, SEEK_END);
  printf("%d\n", (int)ftell(f));
}
=======
On my Alpine/MUSL setup this prints "0 6" while on Ubuntu 22.04 with glibc it prints "3 6" - which I'm assuming is the expected output. So, with MUSL the second fwrite() correctly appends to the existing file, but ftell() nevertheless reports that the initial position within the file after opening it for appending is 0 and not 3.

My GCC version is "gcc (Alpine 10.2.1_pre1) 10.2.1 20201203", ldd confirms that the compiled output is linked only against "/lib/ld-musl-x86_64.so.1" and not against glibc.

I'm not subscribed to the MUSL mailing list, please CC me on replies.

Best Regards,
Robert Buchholz

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Re: [musl] Bug: ftell() after fopen(..., "ab") returns 0 unless an explicit fseek() is used first

[Florian Weimer]

* Robert Buchholz:

> On my Alpine/MUSL setup this prints "0 6" while on Ubuntu 22.04 with
> glibc it prints "3 6" - which I'm assuming is the expected output.

Both behaviors are acceptable.  C11 (7.21.3p1) says this:

| If a file can support positioning requests (such as a disk file, as
| opposed to a terminal), then a file position indicator associated with
| the stream is positioned at the start (character number zero) of the
| file, unless the file is opened with append mode in which case it is
| implementation-defined whether the file position indicator is initially
| positioned at the beginning or the end of the file.

musl is likely not the only libc with this type of behavior, given that
it's explicitly allowed by the standard.

Thanks,
Florian

Re: [musl] Bug: ftell() after fopen(..., "ab") returns 0 unless an explicit fseek() is used first

[Rich Felker]

On Thu, Aug 11, 2022 at 01:36:33PM +0200, Florian Weimer wrote:
> * Robert Buchholz:
> 
> > On my Alpine/MUSL setup this prints "0 6" while on Ubuntu 22.04 with
> > glibc it prints "3 6" - which I'm assuming is the expected output.
> 
> Both behaviors are acceptable.  C11 (7.21.3p1) says this:
> 
> | If a file can support positioning requests (such as a disk file, as
> | opposed to a terminal), then a file position indicator associated with
> | the stream is positioned at the start (character number zero) of the
> | file, unless the file is opened with append mode in which case it is
> | implementation-defined whether the file position indicator is initially
> | positioned at the beginning or the end of the file.
> 
> musl is likely not the only libc with this type of behavior, given that
> it's explicitly allowed by the standard.

Indeed. We simply match the underlying fd behavior here because it's
the "default" behavior without incurring extra cost. Since
applications can't assume one behavior or the other, ones that care to
have the position start at the end have to do the fseeko to SEEK_END
themselves.

Rich