add -L limit bitrate option (#14)

src/bwlimit.h

@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-3.0-only */
+#ifndef _BWLIMIT_H_
+#define _BWLIMIT_H_
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <sys/types.h>
+#include <time.h>
+#include <semaphore.h>
+
+struct bwlimit {
+	sem_t		*sem;	/* semaphore */
+	uint64_t	bps;	/* limit bit-rate (bps) */
+	uint64_t	win;	/* window size (msec) */
+	size_t		amt;	/* amount of bytes can be sent in a window */
+
+	ssize_t		credit;	/* remaining bytes can be sent in a window */
+	struct timespec wstart, wend; /* window start time and end time */
+};
+
+int bwlimit_init(struct bwlimit *bw, uint64_t bps, uint64_t win);
+/* if bps is 0, it means that bwlimit is not active. If so,
+ * bwlimit_wait() returns immediately. */
+
+int bwlimit_wait(struct bwlimit *bw, size_t nr_bytes);
+
+
+#endif /* _BWLIMIT_H_ */