在 contiki os 中对clock_gettime 的未定义引用

Question

I need to generate a random number in C and I did it using srand() and rand() functions. It worked fine when I ran it in the normal compiler, but when I am trying to compile the file in the Contiki OS, it's throwing an 'undefined reference to clock_gettime' error.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
int a;
srand(time(NULL));
a=rand() % 10000 + 2;
char *str = (char*)malloc(a);
int i=0;
for(i=0; i<a-1;i++){
  str[i]='s';
}
str[i] = '\0';
strcat(str,"Hello %d from the client ");
sprintf(buf,str, seq_id);

Can someone help me with this?

Answer 1

By "normal compiler" I presume you mean the native compiler for your host system? In that case the development host is standard hardware with an operating system that provides "wall-clock" time through operating services that ultimately get time from the RTC or NTP.

In embedded systems the standard library is generic and needs to be retargeted to your platform. Typically I/O, heap management and time services need retargeting. In this case you need to define clock_gettime() to resolve the link (or simply redefine an override for time() - as described here ). How you implement it will depend on your specific hardware. Typically you would get time from the RTC, but if your hardware lacks an RTC crystal and a battery backup thereof, it will serve little purpose. Time has to come from somewhere, and the library cannot determine where for your specific hardware.

If you actually never need wall-clock time and are simply following the common idiom of using time(NULL) as a random number generator seed, then rather then fully supporting a time() function you don't need; consider an alternative means of generating a seed such as that described in this Application Note which uses two independent clock sources (VLO and DCO) to generate a random bit sequence. The method is time consuming, so you would normally do it once, then use that as a seed for the standard PRNG via srand() .

There is an implementation of the App Note algorithm at https://github.com/0/msp430-rng/blob/master/rand.c , but it is implemented as an override for the standard rand() . I recommend you rename to something like generate_seed() , then call:

srand( generate_seed() ) ;

Answer 2

My suggestion is to use the chip unique values from the TLV memory that is pre-programmed at the factory. Each chip will contain unique values (such as calibration data), so just hash all TLV memory to arrive at a seed value, then plug that into the rand() function. If you don't have a hash function handy, then just use any CRC32 calculation over all the bytes.

See this code for example how to read from the TLV memory. Keep in mind that every MSP430 family is different, so consult the device datasheet for the exact memory locations.

If your application initializes the Real Time Clock (RTC) hardware inside every MSP430 chip (and updates it through power cycles) then you could add that to the hash result. Just continue the hash (or CRC32) using the RTC date and time registers for your seed value.