diff --git a/Makefile b/Makefile
index 398ccd1..23f1bcb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 all: tardis novdso.so
 
 tardis: tardis.c
-	gcc tardis.c -o tardis -lm -Wall -Ofast -std=c99
+	gcc tardis.c -o tardis -lm -Wall -Ofast -std=c99 -DPID_MAX=$(shell cat /proc/sys/kernel/pid_max)
 
 novdso.so: novdso.c
 	gcc -std=c99 -Wall -fPIC -shared -o novdso.so novdso.c
diff --git a/tardis.c b/tardis.c
index acaae74..1529495 100644
--- a/tardis.c
+++ b/tardis.c
@@ -17,7 +17,9 @@
 #define MICROSECONDS 1000000
 #define NANOSECONDS (MICROSECONDS*1000)
 #define NUM_SYSCALLS 512 // this is higher than the real number, but shouldn't matter
+#ifndef PID_MAX
 #define PID_MAX 32768 // XXX: assumption
+#endif
 #define NUM_CLKIDS 16 // XXX: incorrect, but "works" anyway
 
 double starttimes[NUM_CLKIDS], delayfactor, timefactor;