syspower.c

#include <CoreFoundation/CoreFoundation.h>
#include <IOKit/IOKitLib.h>

typedef struct {
  char key[4];
  char ignored[24];
  uint32_t size;
  char ignored2[10];
  char cmd;
  char ignored3[5];
  uint16_t val;
  char ignored4[30];
} smc_msg_t;

static uint16_t smbus_read_int(io_connect_t connect) {
	smc_msg_t in = {{'R', 'T', 'S', 'P'}};
	smc_msg_t out = {{0}};
	size_t outSz = sizeof(out);

	in.cmd = 5;
  in.size = 4;

  kern_return_t ret = IOConnectCallStructMethod(connect, 2, &in, sizeof(in), &out, &outSz);
  if (ret) {
    fprintf(stderr, "IOConnectCallStructMethod error %d", ret);
    exit(2);
  }
  return (out.val>>8) | (out.val<<8);
}

int main(int argc, char *argv[]) {
	const io_service_t smcService = IOServiceGetMatchingService(
		kIOMasterPortDefault, IOServiceMatching("AppleSMC")
	);

	io_connect_t connect = 0;
    if (IOServiceOpen(smcService, mach_task_self_, 0, &connect) != kIOReturnSuccess) {
		fputs("Failed to open the SMC.", stderr);
		exit(1);
	}

	while (true) {
		printf("%.2f\n", smbus_read_int(connect) / 100.0);
		sleep(1);
	}
}