glibc 里面rand函数的实现

luoyanghero

1. #include <stdlib.h>
   
2. 
   
3. 
   
4. /* This algorithm is mentioned in the ISO C standard, here extended
   
5.    for 32 bits.  */
   
6. int
   
7. rand_r (unsigned int *seed)
   
8. {
   
9.   unsigned int next = *seed;
   
10.   int result;
    
11. 
    
12.   next *= 1103515245;
    
13.   next += 12345;
    
14.   result = (unsigned int) (next / 65536) % 2048;
    
15. 
    
16.   next *= 1103515245;
    
17.   next += 12345;
    
18.   result <<= 10;
    
19.   result ^= (unsigned int) (next / 65536) % 1024;
    
20. 
    
21.   next *= 1103515245;
    
22.   next += 12345;
    
23.   result <<= 10;
    
24.   result ^= (unsigned int) (next / 65536) % 1024;
    
25. 
    
26.   *seed = next;
    
27. 
    
28.   return result;
    
29. }
    

复制代码

Source: [color=var(--blue-700)]https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/rand_r.c;hb=HEAD

As you can see, it's simply multiply with an addition and a shift. The values are carefully chosen to make sure that you get no repeat of the output for RAND_MAX iterations.

Note that this is an old implementation which has been replaced by a more complex algorithm: [color=var(--blue-700)]https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD

If the link if broken, Google for "glibc rand_r"

https://stackoverflow.com/questions/1026327/what-common-algorithms-are-used-for-cs-rand