在os x上未定义strtok\u

我正在尝试使用C11函数 strtok_s ,应在中定义 <string.h> ,但当我尝试使用它时,clang给了我一个链接错误。编译此程序:

#include <string.h>

int main() {
    char * buffer;
    char * state;
    rsize_t strmax = 0;
    char * fpl = strtok_s(buffer, &strmax, "\n", &state);
}

给我这个错误:

repro.c:7:15: warning: implicit declaration of function 'strtok_s' is invalid in
  C99 [-Wimplicit-function-declaration]
    char * fpl = strtok_s(buffer, &strmax, "\n", &state);
                 ^
repro.c:7:9: warning: incompatible integer to pointer conversion initializing
  'char *' with an expression of type 'int' [-Wint-conversion]
    char * fpl = strtok_s(buffer, &strmax, "\n", &state);
           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.
Undefined symbols for architecture x86_64:
  "_strtok_s", referenced from:
  _main in repro-3dcfc9.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

我正在OS X 10.12.6上使用clang 8.1.0。我做错了什么?