CODEWITHSUNDEEP
csegmentation-fault
anarchokawaii
anarchokawaii

Reputation: 116

memory can be used past the break point?

How come this works

char p;
p = sbrk(0);
brk(p+1);
*p = 1;
printf("%p %d %p\n",p,*p,sbrk(0));
p++;
*p = 5;
printf("%p %d\n",p,*p);
p++;
*p = 6;
printf("%p %d\n",p,*p);
p++;
*p = 1;
printf("%p %d\n",p,*p);
p++;
*p = 38;
printf("%p %d\n",p,*p);
p++;
*p = 61;
printf("%p %d %p\n",p,*p,sbrk(0));

but when i remove the brk(p+1); it causes a segfault? i also tried removing the first *p = x; before the first p++; i've also tried brk(p-1) and that caused a seg fault. what exactly is going on here? how come i can control all the memory if i just do brk(p+1)? but get a segfault if i don't.

Upvotes: 3

Views: 41

Answers (1)

Crowman
Crowman

Reputation: 25908

brk() and sbrk() allocate memory in page-sized pieces. So brk(p+1) allocates a whole new page, even though you asked for a single byte. So the few bytes you are writing easily fit within the page you just allocated, and you don't segfault.

Upvotes: 5

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