Re: [PATCH] Discard any push-back from a memory stream before finishing it (bug 34020)

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 30/03/26 16:07, DJ Delorie wrote:
> Andreas Schwab <schwab@suse.de> writes:
>> This makes sure that the write base is pointing to the main area, not the
>> backup area.
> 
> So I just realized, after working in this code for years, that "backup"
> here doesn't mean "in case we run out" it means "going backwards" :-P No
> wonder it seemed weird to me.
> 
> One question below about fseek, but otherwise OK.

The new testcase triggers some regression on both redhad and Linaro CI [1],
and it seems a real one.


[1] https://patchwork.sourceware.org/project/glibc/patch/mvm7bqtiivh.fsf@suse.de/

> 
>> diff --git a/libio/memstream.c b/libio/memstream.c
>> index 0456adb92f..fcc925df87 100644
>> --- a/libio/memstream.c
>> +++ b/libio/memstream.c
>> @@ -100,6 +100,9 @@ _IO_mem_finish (FILE *fp, int dummy)
>>  {
>>    struct _IO_FILE_memstream *mp = (struct _IO_FILE_memstream *) fp;
>>  
>> +  if (_IO_in_backup (fp))
>> +    _IO_switch_to_main_get_area (fp);
>> +
>>    *mp->bufloc = (char *) realloc (fp->_IO_write_base,
>>  				  fp->_IO_write_ptr - fp->_IO_write_base + 1);
>>    if (*mp->bufloc != NULL)
> 
> Ok.
> 
>> diff --git a/libio/tst-memstream6.c b/libio/tst-memstream6.c
>> +static void
>> +mcheck_abort (enum mcheck_status ev)
>> +{
>> +  printf ("mecheck failed with status %d\n", (int) ev);
>> +  exit (1);
>> +}
> 
> Ok.
> 
>> +void
>> +do_test_ungetc_1 (void)
>> +{
>> +  CHAR_T *buf;
>> +  size_t size;
>> +
>> +  FILE *fp = OPEN_MEMSTREAM (&buf, &size);
>> +  TEST_VERIFY_EXIT (fp != NULL);
>> +
>> +  FPUTC (W('A'), fp);
>> +  fflush (fp);
>> +
>> +  TEST_COMPARE (FGETC (fp), W('A'));
> 
> I would have expected you to need an fseek here.  fflush() on a
> memstream synchronizes the buffer and size pointers but says nothing
> about moving the file pointer.
> 
>> +  TEST_COMPARE (UNGETC (W('B'), fp), W('B'));
>> +
>> +  TEST_COMPARE (fclose (fp), 0);
>> +
>> +  TEST_COMPARE (buf[0], W('A'));
>> +
>> +  free (buf);
>> +}
> 
> Ok.
> 
>> +void
>> +do_test_ungetc_2 (void)
>> +{
>> +  CHAR_T *buf;
>> +  size_t size;
>> +
>> +  FILE *fp = OPEN_MEMSTREAM (&buf, &size);
>> +  TEST_VERIFY_EXIT (fp != NULL);
>> +
>> +  TEST_COMPARE (UNGETC (W('A'), fp), W('A'));
>> +
>> +  TEST_COMPARE (fclose (fp), 0);
>> +
>> +  TEST_COMPARE (buf[0], 0);
>> +
>> +  free (buf);
>> +}
> 
> Ok.
> 
>> +static int
>> +do_test (void)
>> +{
>> +  mcheck_pedantic (mcheck_abort);
>> +
>> +  do_test_ungetc_1 ();
>> +  do_test_ungetc_2 ();
>> +
>> +  return 0;
>> +}
> 
> Ok.
> 
>> diff --git a/libio/tst-wmemstream6.c b/libio/tst-wmemstream6.c
>> +#define TEST_WCHAR
>> +#include <libio/tst-memstream6.c>
> 
> Ok.
> 
>> diff --git a/libio/wmemstream.c b/libio/wmemstream.c
>> index d0c639be70..a0c61a627f 100644
>> --- a/libio/wmemstream.c
>> +++ b/libio/wmemstream.c
>> @@ -101,6 +101,9 @@ _IO_wmem_finish (FILE *fp, int dummy)
>>  {
>>    struct _IO_FILE_wmemstream *mp = (struct _IO_FILE_wmemstream *) fp;
>>  
>> +  if (_IO_in_backup (fp))
>> +    _IO_switch_to_main_wget_area (fp);
>> +
>>    *mp->bufloc = (wchar_t *) realloc (fp->_wide_data->_IO_write_base,
>>  				     (fp->_wide_data->_IO_write_ptr
>>  				      - fp->_wide_data->_IO_write_base + 1)
> 
> Ok.
>