ungetc: Guarantee single char pushback

The C standard requires that ungetc guarantees at least one pushback,
but the malloc call to allocate the pushback buffer could fail, thus
violating that requirement.  Fix this by adding a single byte pushback
buffer in the FILE struct that the pushback can fall back to if malloc
fails.

The side-effect is that if the initial malloc fails and the 1-byte
fallback buffer is used, future resizing (if it succeeds) will be
2-bytes, 4-bytes and so on, which is suboptimal but it's after a malloc
failure, so maybe even desirable.

A future optimization here could be to have the pushback code use the
single byte buffer first and only fall back to malloc for subsequent
calls.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Maciej W. Rozycki <macro@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/libio/oldfileops.c b/libio/oldfileops.c
index 8f775c9094..03f4d76a57 100644
--- a/libio/oldfileops.c
+++ b/libio/oldfileops.c
@@ -1,4 +1,5 @@
 /* Copyright (C) 1993-2024 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -311,7 +312,7 @@ _IO_old_file_underflow (FILE *fp)
       /* Maybe we already have a push back pointer.  */
       if (fp->_IO_save_base != NULL)
 	{
-	  free (fp->_IO_save_base);
+	  _IO_free_backup_buf (fp, fp->_IO_save_base);
 	  fp->_flags &= ~_IO_IN_BACKUP;
 	}
       _IO_doallocbuf (fp);
@@ -464,7 +465,7 @@ _IO_old_file_seekoff (FILE *fp, off64_t offset, int dir, int mode)
       /* It could be that we already have a pushback buffer.  */
       if (fp->_IO_read_base != NULL)
 	{
-	  free (fp->_IO_read_base);
+	  _IO_free_backup_buf (fp, fp->_IO_read_base);
 	  fp->_flags &= ~_IO_IN_BACKUP;
 	}
       _IO_doallocbuf (fp);