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calignment

How do I get pointer to beginning of a page

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.

Solution

  • 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);