如何获得指向页面开头的指针

Question

How do I get a pointer to the beginning of a page?

I tried the following to no success:

#define PAGESIZE 4096

 bool is_page_aligned(void *p)
 {
    return !((long int)p & 0xFFF);
 }

 int main(void)
 {
     bool res;
     void *buffer;

     buffer = malloc(PAGESIZE*2);
     printf("%p\n", (void *) &buffer);
     res = is_page_aligned(&buffer);
     fputs(res ? "true\n" : "false\n", stdout);
     return 0;
}

I'm trying to mitigate TLB misses. Any possible assistance is greatly appreciated.

Answer 1

To get the address of the beginning of the page containing the address a , divide by the page size and then multiply by the page size.

long int page_beginning = PAGESIZE * (a / PAGESIZE);

This works because of the truncation performed during integer division.

You can also subtract the modulus:

long int page_beginning = a - (a % PAGESIZE);