diff --git a/Makefile b/Makefile
index fb90f074..c5bd5c05 100644
--- a/Makefile
+++ b/Makefile
@@ -78,7 +78,8 @@ clean:
 	@rm -rf build
 
 fmt:
-	go fmt . ./src/examples/hello ./src/runtime
+	@go fmt . ./src/examples/hello
+	@go fmt ./src/runtime/*.go
 
 gen-device: gen-device-nrf
 
diff --git a/src/runtime/runtime_nrf.go b/src/runtime/runtime_nrf.go
index 0250113a..93af1b1f 100644
--- a/src/runtime/runtime_nrf.go
+++ b/src/runtime/runtime_nrf.go
@@ -2,14 +2,13 @@
 
 package runtime
 
-// #include "runtime_nrf.h"
-import "C"
-
 import (
 	"device/arm"
 	"device/nrf"
 )
 
+func _Cfunc_rtc_sleep(ticks uint32)
+
 const Microsecond = 1
 
 func init() {
@@ -51,7 +50,7 @@ func sleep(d Duration) {
 	for ticks64 != 0 {
 		monotime()                          // update timestamp
 		ticks := uint32(ticks64) & 0x7fffff // 23 bits (to be on the safe side)
-		C.rtc_sleep(C.uint32_t(ticks))      // TODO: not accurate (must be d / 30.5175...)
+		_Cfunc_rtc_sleep(ticks)             // TODO: not accurate (must be d / 30.5175...)
 		ticks64 -= Duration(ticks)
 	}
 }
diff --git a/src/runtime/runtime_unix.go b/src/runtime/runtime_unix.go
index aedb197a..ca062df3 100644
--- a/src/runtime/runtime_unix.go
+++ b/src/runtime/runtime_unix.go
@@ -6,20 +6,28 @@ import (
 	"unsafe"
 )
 
-// #include <stdio.h>
-// #include <stdlib.h>
-// #include <unistd.h>
-// #include <time.h>
-import "C"
-
 const Microsecond = 1
 
+func _Cfunc_putchar(c int) int
+func _Cfunc_usleep(usec uint) int
+func _Cfunc_calloc(nmemb, size uintptr) unsafe.Pointer
+func _Cfunc_exit(status int)
+func _Cfunc_clock_gettime(clk_id uint, ts *timespec)
+
+// TODO: Linux/amd64-specific
+type timespec struct {
+	tv_sec  int64
+	tv_nsec int64
+}
+
+const CLOCK_MONOTONIC_RAW = 4
+
 func putchar(c byte) {
-	C.putchar(C.int(c))
+	_Cfunc_putchar(int(c))
 }
 
 func sleep(d Duration) {
-	C.usleep(C.useconds_t(d))
+	_Cfunc_usleep(uint(d))
 }
 
 // Return monotonic time in microseconds.
@@ -27,17 +35,18 @@ func sleep(d Duration) {
 // TODO: use nanoseconds?
 // TODO: noescape
 func monotime() uint64 {
-	var ts C.struct_timespec
-	C.clock_gettime(C.CLOCK_MONOTONIC, &ts)
+	ts := timespec{}
+	_Cfunc_clock_gettime(CLOCK_MONOTONIC_RAW, &ts)
 	return uint64(ts.tv_sec)*1000*1000 + uint64(ts.tv_nsec)/1000
 }
 
 func abort() {
-	C.abort()
+	// panic() exits with exit code 2.
+	_Cfunc_exit(2)
 }
 
 func alloc(size uintptr) unsafe.Pointer {
-	buf := C.calloc(1, C.size_t(size))
+	buf := _Cfunc_calloc(1, size)
 	if buf == nil {
 		panic("cannot allocate memory")
 	}