read from /dev/urandom 返回值异常

 #include<stdio.h>
 #include<iostream>
 #include <fcntl.h>
 #include <sys/mman.h>
 using namespace std;
 int main(int argc,char* argv[])
 {
     ssize_t bytesToWrite = **;
     const char *pathZero = "/dev/zero";
     int fdZero = open(pathZero, O_RDONLY);
     void *pageBuffPtr = mmap(NULL, bytesToWrite, PROT_READ | PROT_WRITE, MAP_PRIVATE, fdZero, );
     int randomData = open("/dev/urandom", O_RDONLY);
     if (randomData < )
     {
         // something went wrong
     }
     else
     {
         char* myRandomData;
         ssize_t result = read(randomData, pageBuffPtr, bytesToWrite);
         if (result < )
         {
             // something went wrong
         }
         cout <<"len:"<<bytesToWrite << endl;
         cout <<"result:"<<result << endl;
     }
     return ;
 }

以上代码运行在suse12上，len和result总是差1

后来查看urandom的man手册中有这么一句：Since Linux 3.16, a read(2) from /dev/urandom will return at most 32 MB

即最大只能读 32MB -1