何时使用MongoDB或其他面向文档的数据库系统？(When to use MongoDB or other document oriented database systems? [closed])

 通过更多的上下文，代码更有意义。 
 
// used to store pointers to allocated memory
unsigned char *n[NO_OF_POINTERS]; 

int s[5000]; // used to store sizes when testing

....

for(i=0;i<NO_OF_ITERATIONS;i++) {
   k=myrand()%NO_OF_POINTERS; // pick a pointer
   if(n[k]) { // if it was allocated then free it
      // check that the stuff we wrote has not changed
      if(n[k][0]!=(unsigned char)(n[k]+s[k]+k))
         printf("Error when checking first byte!\n");
      if(s[k]>1 && n[k][s[k]-1]!=(unsigned char)(n[k]-s[k]-k))
         printf("Error when checking last byte!\n");
      FREE(n[k]);
   }
   size=randomsize(); // pick a random size
             size=1;
   n[k]=(unsigned char *)MALLOC(size); // do the allocation
   s[k]=size; // remember the size
   n[k][0]=(unsigned char)(n[k]+s[k]+k);  // put some data in the first and
   if(size>1) n[k][size-1]=(unsigned char)(n[k]-s[k]-k); // last byte
}
 
 最后两行使用基于指针值（ n[k] ），分配大小（ s[k] ）和指针索引（ k ）的公式将第一个和最后一个字节设置为值。 这个公式没有意义，它只是计算要存储的值，对于不同的指针分配将是不同的。 
 
 您突出显示的if语句检查在释放内存之前，第一个（ n[k][0] ）和last（ n[k][s[k]-1] ）字节的值没有改变。 该代码基本上是VirtualAlloc和VirtualFree函数的测试工具。 

The code makes more sense with a bit more context.
 
// used to store pointers to allocated memory
unsigned char *n[NO_OF_POINTERS]; 

int s[5000]; // used to store sizes when testing

....

for(i=0;i<NO_OF_ITERATIONS;i++) {
   k=myrand()%NO_OF_POINTERS; // pick a pointer
   if(n[k]) { // if it was allocated then free it
      // check that the stuff we wrote has not changed
      if(n[k][0]!=(unsigned char)(n[k]+s[k]+k))
         printf("Error when checking first byte!\n");
      if(s[k]>1 && n[k][s[k]-1]!=(unsigned char)(n[k]-s[k]-k))
         printf("Error when checking last byte!\n");
      FREE(n[k]);
   }
   size=randomsize(); // pick a random size
             size=1;
   n[k]=(unsigned char *)MALLOC(size); // do the allocation
   s[k]=size; // remember the size
   n[k][0]=(unsigned char)(n[k]+s[k]+k);  // put some data in the first and
   if(size>1) n[k][size-1]=(unsigned char)(n[k]-s[k]-k); // last byte
}
 
The last two lines set the first and the last byte to values using a formula based on the pointer value (n[k]), the size of the allocation (s[k]), and the pointer index (k). There is no meaning to this formula, it's just a calculation of a value to be stored that will be different for different pointer allocations.
 
The if statements you highlighted check that the values of the first (n[k][0]) and last (n[k][s[k]-1]) bytes haven't changed before freeing the memory. The code is basically a test harness for the VirtualAlloc and VirtualFree functions.