diff --git a/defs.h b/defs.h
index 4e06a92..22e847b 100644
--- a/defs.h
+++ b/defs.h
@@ -712,13 +712,13 @@
 extern int loop_ioctl(struct tcb *, long, long);
 extern int ptp_ioctl(struct tcb *, long, long);
 
-extern int tv_nz(struct timeval *);
-extern int tv_cmp(struct timeval *, struct timeval *);
-extern double tv_float(struct timeval *);
-extern void tv_add(struct timeval *, struct timeval *, struct timeval *);
-extern void tv_sub(struct timeval *, struct timeval *, struct timeval *);
-extern void tv_mul(struct timeval *, struct timeval *, int);
-extern void tv_div(struct timeval *, struct timeval *, int);
+extern int tv_nz(const struct timeval *);
+extern int tv_cmp(const struct timeval *, const struct timeval *);
+extern double tv_float(const struct timeval *);
+extern void tv_add(struct timeval *, const struct timeval *, const struct timeval *);
+extern void tv_sub(struct timeval *, const struct timeval *, const struct timeval *);
+extern void tv_mul(struct timeval *, const struct timeval *, int);
+extern void tv_div(struct timeval *, const struct timeval *, int);
 
 /* Strace log generation machinery.
  *
diff --git a/util.c b/util.c
index ff18b87..33482d5 100644
--- a/util.c
+++ b/util.c
@@ -79,13 +79,13 @@
 }
 
 int
-tv_nz(struct timeval *a)
+tv_nz(const struct timeval *a)
 {
 	return a->tv_sec || a->tv_usec;
 }
 
 int
-tv_cmp(struct timeval *a, struct timeval *b)
+tv_cmp(const struct timeval *a, const struct timeval *b)
 {
 	if (a->tv_sec < b->tv_sec
 	    || (a->tv_sec == b->tv_sec && a->tv_usec < b->tv_usec))
@@ -97,13 +97,13 @@
 }
 
 double
-tv_float(struct timeval *tv)
+tv_float(const struct timeval *tv)
 {
 	return tv->tv_sec + tv->tv_usec/1000000.0;
 }
 
 void
-tv_add(struct timeval *tv, struct timeval *a, struct timeval *b)
+tv_add(struct timeval *tv, const struct timeval *a, const struct timeval *b)
 {
 	tv->tv_sec = a->tv_sec + b->tv_sec;
 	tv->tv_usec = a->tv_usec + b->tv_usec;
@@ -114,7 +114,7 @@
 }
 
 void
-tv_sub(struct timeval *tv, struct timeval *a, struct timeval *b)
+tv_sub(struct timeval *tv, const struct timeval *a, const struct timeval *b)
 {
 	tv->tv_sec = a->tv_sec - b->tv_sec;
 	tv->tv_usec = a->tv_usec - b->tv_usec;
@@ -125,7 +125,7 @@
 }
 
 void
-tv_div(struct timeval *tv, struct timeval *a, int n)
+tv_div(struct timeval *tv, const struct timeval *a, int n)
 {
 	tv->tv_usec = (a->tv_sec % n * 1000000 + a->tv_usec + n / 2) / n;
 	tv->tv_sec = a->tv_sec / n + tv->tv_usec / 1000000;
@@ -133,7 +133,7 @@
 }
 
 void
-tv_mul(struct timeval *tv, struct timeval *a, int n)
+tv_mul(struct timeval *tv, const struct timeval *a, int n)
 {
 	tv->tv_usec = a->tv_usec * n;
 	tv->tv_sec = a->tv_sec * n + tv->tv_usec / 1000000;