Darren Tucker wrote this message on Fri, Feb 17, 2023 at 19:07 +1100:
> I've got a bit of a puzzle and I was wondering if anyone has any insight
> as to what's going on.
>
> I added some code to ssh's known_hosts handling that checks if the last
> byte in the file is a newline, and if not, it adds one before writing
> the new record. I also wrote a regression test for this and in most
> cases this works fine.
>
> On some platforms (Solaris, OpenIndiana and AIX) however, the test fails
> because it adds two newlines instead of the expected one. Basically if
> I fseek to the end, read a byte and write a byte the first byte will
> be duplicated. I reduced it to this test case:
>
> #include <stdio.h>
> int main(void)
> {
> FILE *f = fopen("testfile", "w");
> putc('A', f);
> fclose(f);
>
> f = fopen("testfile", "a+"); /* same behaviour for r+ */
> fseek(f, -1L, SEEK_END);
> printf("c=%d\n", fgetc(f));
> /* fseek(f, 0, SEEK_END); -- with this it behaves as expected */
> /* fflush(f); -- this too */
> fputc('B', f);
> }
>
> $ gcc test.c && ./a.out; od -x -c testfile
> c=65
> 0000000 4141 0042
> A A B
> 0000003
>
> I wrote two bytes and read one but somehow ended up with three bytes in
> the file? (This example is from Solaris 11 but AIX does the same thing).
>
> On most platforms this behaves as expected:
>
> $ cc test.c && ./a.out; od -x -c testfile
> c=65
> 0000000 4241
> A B
> 0000002
>
> Now I could just add the fseek, but as far as I can tell I shouldn't
> have to, and I don't understand why. I've read the specs and the man
> pages and haven't found anything that would explain this behaviour.
>
> I'm curious if anyone has a) any insight as to what's going on, or b)
> any additional examples of where it fails?
Can you check the return value of fseek to see if it's non-zero? It
should be zero if successful, but likely non-zero on the failing
systems.
I decided to check the C standard for fseek's behavior, and it looks
like SEEK_END is not specified on text streams, only binary streams
(at least in C99 and C2X).
For text streams, this is what it says is supported (C99 7.19.9.2):
For a text stream, either offset shall be zero, or offset shall be a
value returned by an earlier successful call to the ftell function on
a stream associated with the same file and whence shall be SEEK_SET.
Does it work if you open the file in binary mode? a+b or r+b?
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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