diff --git a/pigpio.c b/pigpio.c
index 6d9ceca..fc57d68 100644
--- a/pigpio.c
+++ b/pigpio.c
@@ -8828,6 +8828,7 @@ static void stopHardwarePWM(void)
 int gpioSetMode(unsigned gpio, unsigned mode)
 {
    int reg, shift, old_mode;
+   static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
 
    DBG(DBG_USER, "gpio=%d mode=%d", gpio, mode);
 
@@ -8839,6 +8840,8 @@ int gpioSetMode(unsigned gpio, unsigned mode)
    if (mode > PI_ALT3)
       SOFT_ERROR(PI_BAD_MODE, "gpio %d, bad mode (%d)", gpio, mode);
 
+   pthread_mutex_lock(&mutex);
+
    reg   =  gpio/10;
    shift = (gpio%10) * 3;
 
@@ -8853,6 +8856,8 @@ int gpioSetMode(unsigned gpio, unsigned mode)
 
    gpioReg[reg] = (gpioReg[reg] & ~(7<<shift)) | (mode<<shift);
 
+   pthread_mutex_unlock(&mutex);
+
    return 0;
 }