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>
