A call to a function that switches state from Secure to Non-secure is called a non-secure function call. A non-secure function call can only happen through function pointers. This is a consequence of separating secure and non-secure code into separate executable files.

A non-secure function type must only be used as a base type of a pointer.

Example

#include <arm_cmse.h>
typedef void __attribute__((cmse_nonsecure_call)) nsfunc(void);

void default_callback(void) { … }

// fp can point to a secure function or a non-secure function
nsfunc *fp = (nsfunc *) default_callback;   // secure function pointer

void __attribute__((cmse_nonsecure_entry)) entry(nsfunc *callback) {
    fp = cmse_nsfptr_create(callback);      // non-secure function pointer
}

void call_callback(void) {
    if (cmse_is_nsfptr(fp)){
	   fp();           // non-secure function call
    }
    else {
        ((void (*)(void)) fp)();           // normal function call
    }
}