ungetc.c\stdio\src - musl - musl - an implementation of the standard library for Linux-based systems

#include "stdio_impl.h"
int ungetc(int c, FILE *f)
{
	if (c == EOF) return c;
	FLOCK(f);
	if ((!f->rend && __toread(f)) || f->rpos <= f->buf - UNGET) {
		FUNLOCK(f);
		return EOF;
	}
	*--f->rpos = c;
	f->flags &= ~F_EOF;
	FUNLOCK(f);
	return c;
}