My device is Apple MacBook M1
I am writing a "malloc" function using brk and sbrk, however when I am advancing my unit test I found some strange things about sbrk when I need to use it to extend my heap.
The issue:
Here I will give a minimized example about it, and recreate the "abnormal" scenario
// 1st example
// if I just use sbrk for a large number like:
assert(sbrk(4096*1000000000) != (void *) -1); // sbrk return -1 if no more space
// the assert holds
// second example
// if I apply a not big number like
assert(sbrk(40960032) != (void *) -1);
// the assert failed(or like 40960040 and so on)
It seems that the sbrk can give way more space than this(from the 1st example), however it just failed.
My attempts:
1.Reach to the manual, and the man sbrk says:
BUGS
Setting the break may fail due to a temporary lack of swap space. It is
not possible to distinguish this from a failure caused by exceeding the
maximum size of the data segment without consulting getrlimit.
so it may be a temporary lack of swap space, so I tried:
printf("init %p\n", sbrk(0));
sbrk(40960032);
if(sbrk(1)!=(void*)-1){
printf("now %p\n", sbrk(0));
}
and it outputs
init 0x102648000
now 0x102648001
which shows the sbrk(40960032) also do not work
I also tried step sbrk(1) for 40960032 times and it will start to return -1 from a certain position, but i did not find the pattern
Can anyone give me some tips and I would be grateful
