libc: io: implement file io buffering and internal locking for concurrency

lib/libc/io/stdio/fgetc.c

@@ -3,29 +3,25 @@
 #include <errno.h>
 #include <stdio.h>
 
-int fgetc(struct __opaque_file *stream)
+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;
+	char c = 0;
+	int ret = __libc_file_read(stream, &c, 1);
+	if (ret < 1) {
 		return EOF;
 	}
 
-	char c = stream->f_buf[stream->f_buf_readptr++];
 	return c;
 }
+
+int fgetc(struct __opaque_file *stream)
+{
+	__libc_file_lock(stream);
+	int ret = __fgetc(stream);
+	__libc_file_unlock(stream);
+	return ret;
+}