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

lib/libc/io/stdio/setvbuf.c

@@ -0,0 +1,39 @@
+#include "file.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+extern int setvbuf(
+	struct __opaque_file *restrict stream,
+	char *restrict buf,
+	int mode,
+	size_t size)
+{
+	switch (mode) {
+	case _IOFBF:
+	case _IOLBF:
+	case _IONBF:
+		break;
+	default:
+		return __set_errno(EINVAL);
+	}
+
+	if (stream->f_prev != 0) {
+		return __set_errno(EINVAL);
+	}
+
+	int err = SUCCESS;
+	if (buf) {
+		err = __libc_ringbuf_resize_static(&stream->f_buf, buf, size);
+	} else {
+		err = __libc_ringbuf_resize(&stream->f_buf, size);
+	}
+
+	if (err != SUCCESS) {
+		return __set_errno(err);
+	}
+
+	stream->f_buffer_mode = mode;
+
+	return 0;
+}