This is xnu-10002.1.13. See this file in:
#include <libkern/libkern.h>
#include <libkern/section_keywords.h>
#include <libkern/img4/interface.h>
#if defined(SECURITY_READ_ONLY_LATE)
SECURITY_READ_ONLY_LATE(const img4_interface_t *) img4if = NULL;
#else
const img4_interface_t *img4if = NULL;
#endif
void
img4_interface_register(const img4_interface_t *i4)
{
if (img4if) {
panic("img4 interface already set");
}
img4if = i4;
}