lib: c: io: implement standard FILE and DIR interfaces

lib/libc/io/stdio/fgetc.c

@@ -0,0 +1,31 @@
+#include "file.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+int fgetc(struct __opaque_file *stream)
+{
+	if (stream->f_flags & (FILE_EOF | FILE_ERR)) {
+		return EOF;
+	}
+
+	size_t available = __libc_file_available(stream);
+	if (available == 0) {
+		int err = __libc_file_refill(stream);
+		if (err != SUCCESS) {
+			__set_errno(err);
+			stream->f_flags |= FILE_ERR;
+			return EOF;
+		}
+
+		available = __libc_file_available(stream);
+	}
+
+	if (available == 0) {
+		stream->f_flags |= FILE_EOF;
+		return EOF;
+	}
+
+	char c = stream->f_buf[stream->f_buf_readptr++];
+	return c;
+}