mprotect - how aligning to multiple of pagesize works?
- by user299988
Hi,
I am not understanding the 'aligning allocated memory' part from the mprotect usage.
I am referring to the code example given on http://linux.die.net/man/2/mprotect
char *p;
char c;
/* Allocate a buffer; it will have the default
protection of PROT_READ|PROT_WRITE. */
p = malloc(1024+PAGESIZE-1);
if (!p) {
perror("Couldn't malloc(1024)");
exit(errno);
}
/* Align to a multiple of PAGESIZE, assumed to be a power of two */
p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));
c = p[666]; /* Read; ok */
p[666] = 42; /* Write; ok */
/* Mark the buffer read-only. */
if (mprotect(p, 1024, PROT_READ)) {
perror("Couldn't mprotect");
exit(errno);
}
For my understanding, I tried using a PAGESIZE of 16, and 0010 as address of p.
I ended up getting 0001 as the result of (((int) p + PAGESIZE-1) & ~(PAGESIZE-1)).
Could you please clarify how this whole 'alignment' works?
Thanks,
