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

lib/libc/io/stdio/vprintf.c

@@ -0,0 +1,27 @@
+#include "file.h"
+
+#include <stddef.h>
+#include <stdio.h>
+
+extern int __libc_fctprintf(
+	void (*out)(char character, void *arg),
+	void *arg,
+	const char *format,
+	va_list va);
+extern int __fputc(int c, struct __opaque_file *stream);
+
+static inline void _out_file(char character, void *arg)
+{
+	struct __opaque_file *fp = arg;
+	__fputc(character, fp);
+}
+
+int vprintf(const char *format, va_list arg)
+{
+	__libc_file_lock(stdout);
+
+	int ret = __libc_fctprintf(_out_file, stdout, format, arg);
+
+	__libc_file_unlock(stdout);
+	return ret;
+}