[PATCH v3] ungetc: Guarantee single char pushback

[Maciej W. Rozycki]

On Fri, 6 Dec 2024, Siddhesh Poyarekar wrote:

> The C standard requires that ungetc guarantees at least one pushback, so
> put a single byte pushback buffer in the FILE struct to enable that.

 So what would the problem be if we instead replaced:

  int _flags2;

with

  short int _flags2;
  char _short_backupbuf[1];
  char _unused;

or if we wanted to be super cautious and used a union to prevent any issue 
with alignment with some obscure psABI (which I doubt is needed given that 
the preceding member is of the int type), then:

  union
    {
      int _overlay;
      struct
	{
	  short int _flags2;
	  char _short_backupbuf[1];
	  char _unused;
	};
    };

?  There's no need to change existing code that uses _flags2 and there's 
no need to penalise legacy calls.  I guess saving _unused2 space is hardly 
an argument, though I note that placing a char[1] member first will ask 
for rearrangement to avoid wasting space for padding once a wider member 
follows in the future.

 I've run native `powerpc64le-linux-gnu' regression testing and saw no 
issues with either patch attached applied on top of your change.

  Maciej
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