To use Branch Target Identification (BTI), you must compile your application with ‑‑branch_protection=bti or --branch_protection=bti+pac-ret, and enable BTI in the startup code by setting the BTI_EN bit in the processor CONTROL register. If you are using the cstartup.s file, the best way of implementing this is by defining the function __low_level_init. For more information about cstartup.s and __low_level_init, see System startup and termination.

#include <arm_acle.h>
void __low_level_init(void)
{
  unsigned int old_ctrl = __arm_rsr("CONTROL");
  __arm_wsr("CONTROL", old_ctrl | 0x10);
  // Enables BTI by setting bit #4 in CONTROL
}

To use Pointer Authentication (PAC), you must compile your application with ‑‑branch_protection=pac-ret or --branch_protection=bti+pac-ret, and initialize an authentication key and enable PAC in the startup code. If you are using the cstartup.s file, the best way of implementing this is by defining the function __low_level_init. For more information about cstartup.s and __low_level_init, see System startup and termination.

void __low_level_init(void)
{
  // Set up a 128-bit key in 4 parts:
  __arm_wsr("PAC_KEY_P_0", key[0]);
  __arm_wsr("PAC_KEY_P_1", key[1]);
  __arm_wsr("PAC_KEY_P_2", key[2]);
  __arm_wsr("PAC_KEY_P_3", key[3]);
  unsigned int old_ctrl = __arm_rsr("CONTROL");
  __arm_wsr("CONTROL", old_ctrl | 0x40);
  // Enables PAC by setting bit #6 in CONTROL
}