42.1中的stdlib.h 和 limits.h有问题？

编程
1 
#include <sys/time.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <inttypes.h>
#include <stdint.h>
#include <limits.h>
#include <unistd.h>
uint_fast32_t  static cirnum=0,openwrong=0, readwrong=0,overflow=0,goodnum=0;
uint_fast32_t oldrandom()
{
	
  struct timeval ti;
  gettimeofday(&ti,NULL);
uint_fast32_t  seeds=ti.tv_usec;
 // #srandom(seed);
  srandom(seeds);
  return random();
	
}

uint_fast32_t    myrandom()
{
 

  uint32_t data;

  int fd=open("/dev/urandom",O_RDONLY);
  if(fd<0 )
  {
  data=oldrandom();
  openwrong++;	
  }
  else if (read(fd,&data,sizeof(data))<=0)
  {
	  
       data=oldrandom();
	   readwrong++;
	  }
  else if(data>SSIZE_MAX||data<0)// 有可能溢出
  {
	  overflow++;
	
     
  } 
  else goodnum++;
  close(fd);
  return data%RAND_MAX;
}
void  circum()
 {
  uint_fast32_t  xr=myrandom();
  long double xrate=xr*1.0/(RAND_MAX-1);
  xrate=xrate*xrate;
  
  uint_fast32_t   yr=myrandom();
  long double yrate=yr*1.0/(RAND_MAX-1);
  yrate=yrate*yrate;
 	if((xrate+yrate)<=1.0)
    cirnum++; 
 }

int main(int argc, char **argv)
{
	
    uint_fast32_t  i,num=1999999;
	double pi;
	for(i=0;i<num;i++)
	  circum();
	printf("the cirnum=%"PRIuFAST32"\n",cirnum);
	pi=(cirnum*1.0/num)*4;
	printf("openwrong=%"PRIuFAST32",readwrong=%"PRIuFAST32",overflow=%"PRIuFAST32",goodnum=%"PRIuFAST32"\n",openwrong,readwrong,overflow,goodnum);
    printf("the pi almost=%lf\n",pi);
	return 0;
}
// 以上代码在 13.2 中是能编译通过并运行的。但是是 42.1 中
/*use@linux-yryd:~/bin> gcc -std=c11 random-pi.c
random-pi.c: In function ‘oldrandom’:
random-pi.c:20:3: warning: implicit declaration of function ‘srandom’ -Wimplicit-function-declaration]
   srandom(seeds);
   ^
random-pi.c:21:3: warning: implicit declaration of function ‘random’ -Wimplicit-function-declaration]
   return random();
   ^
random-pi.c: In function ‘myrandom’:
random-pi.c:43:16: error: ‘SSIZE_MAX’ undeclared (first use in this function)
   else if(data>SSIZE_MAX||data<0)// 有可能溢出
                ^
random-pi.c:43:16: note: each undeclared identifier is reported only once for each function it appears in
suse@linux-yryd:~/bin> 
*/
2

又试了一下，这回不用 -std=c11。编译通过了，这是为什么呢？