svfwscanf replaces getwc and ungetwc_r. The comments in the code talk about avoiding file operations, but they also need to bypass the mbtowc calls as svfwscanf operates on wchar_t, not multibyte data, which is a more important reason here; they would not work correctly otherwise. The ungetwc replacement has code which uses the 3 byte FILE _ubuf field, but if wchar_t is 32-bits, this field is not large enough to hold even one wchar_t value. Building in this mode generates warnings about array overflow: In file included from ../../newlib/libc/stdio/svfiwscanf.c:35: ../../newlib/libc/stdio/vfwscanf.c: In function '_sungetwc_r.isra': ../../newlib/libc/stdio/vfwscanf.c:316:12: warning: array subscript 4294967295 is above array bounds of 'unsigned char[3]' [-Warray-bounds] 316 | fp->_p = &fp->_ubuf[sizeof (fp->_ubuf) - sizeof (wchar_t)]; | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In file included from ../../newlib/libc/stdio/stdio.h:46, from ../../newlib/libc/stdio/vfwscanf.c:82, from ../../newlib/libc/stdio/svfiwscanf.c:35: ../../newlib/libc/include/sys/reent.h:216:17: note: while referencing '_ubuf' 216 | unsigned char _ubuf[3]; /* guarantee an ungetc() buffer */ | ^~~~~ However, the vfwscanf code *never* ungets data before the start of the scanning operation, and *always* ungets data which matches the input at that point, so the code always hits the block which backs up over the input data and never hits the block which uses the _ubuf field. In addition, the svfwscanf code will always start with the unget buffer empty, so the ungetwc replacement never needs to support an unget buffer at all. Simplify the code by removing support for everything other than backing up over the input data, leaving the check to make sure it doesn't get underflowed in case the vfscanf code has a bug in it. Signed-off-by: Keith Packard