Strange results when echoing the memory address of some pointers

[jovial]

Trying to print the memory address of some pointers yields strange results. Yet it is fine if I assign the result of the cast to a variable and then print the result.

type CArray*{.unchecked.}[T] = array[0..0, T]

proc main =
  var a = cast[CArray[ptr int]](alloc0(4 * sizeof(ptr int)))
  for i in 0..3:
    a[i] = cast[ptr int](alloc0(sizeof(int)))
    a[i][] = i

  let before = cast[int](a[2])
  echo cast[int](a[2])

  #let after = cast[int](a[2])
  echo cast[int](a[2])
  #assert before == after

main()

example output

140209307041920
0

Think this was leading to some other issues due to memory corruption.

[jovial]

Seems to be the cast to the CArray that causes this as it works if I cast to array[4,ptr int] instead. Is there a mistake in my code somewhere?

[refi64]

For me, under the latest devel, this just crashes:

ryan@DevPC-LX:~/nim$ ./tst
139911396708480
139911396708480
Traceback (most recent call last)
tst.nim(16)              tst
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
ryan@DevPC-LX:~/nim$ 

But building with -d:release works:

ryan@DevPC-LX:~/nim$ ./tst
139818517663872
139818517663872
ryan@DevPC-LX:~/nim$ 

Here's the diff:

< static N_INLINE(NI, addInt)(NI a, NI b);
< N_NOINLINE(void, raiseOverflow)(void);
29,31d26
< static N_INLINE(void, nimFrame)(TFrame* s);
< N_NOINLINE(void, stackoverflow_18601)(void);
< static N_INLINE(void, popFrame)(void);
38,85d32
< extern TFrame* frameptr_16042;
< 
< static N_INLINE(NI, addInt)(NI a, NI b) {
<   NI result;
<   result = 0;
<   result = (NI)((NU64)(a) + (NU64)(b));
<   {
<       NIM_BOOL LOC3;
<       LOC3 = 0;
<       LOC3 = (0 <= (NI)(result ^ a));
<       if (LOC3) goto LA4;
<       LOC3 = (0 <= (NI)(result ^ b));
<       LA4: ;
<       if (!LOC3) goto LA5;
<       goto BeforeRet;
<   }
<   LA5: ;
<   raiseOverflow();
<   BeforeRet: ;
<   return result;
< }
< 
< static N_INLINE(void, nimFrame)(TFrame* s) {
<   NI LOC1;
<   LOC1 = 0;
<   {
<       if (!(frameptr_16042 == NIM_NIL)) goto LA4;
<       LOC1 = 0;
<   }
<   goto LA2;
<   LA4: ;
<   {
<       LOC1 = ((NI) ((NI16)((*frameptr_16042).calldepth + ((NI16) 1))));
<   }
<   LA2: ;
<   (*s).calldepth = ((NI16) (LOC1));
<   (*s).prev = frameptr_16042;
<   frameptr_16042 = s;
<   {
<       if (!((*s).calldepth == ((NI16) 2000))) goto LA9;
<       stackoverflow_18601();
<   }
<   LA9: ;
< }
< 
< static N_INLINE(void, popFrame)(void) {
<   frameptr_16042 = (*frameptr_16042).prev;
< }
93d39
<   nimfr("main", "tst.nim")
95,96d40
<   nimln(4, "tst.nim");
<   nimln(4, "tst.nim");
104d47
<       nimln(1576, "system.nim");
107d49
<           nimln(1577, "system.nim");
110d51
<               nimln(1577, "system.nim");
112d52
<               nimln(1576, "system.nim");
114,115d53
<               nimln(6, "tst.nim");
<               nimln(6, "tst.nim");
119d56
<               nimln(7, "tst.nim");
121,122c58
<               nimln(1579, "system.nim");
<               res_88052 = addInt(res_88052, 1);
---
>               res_88052 += 1;
126d61
<   nimln(9, "tst.nim");
128,129d62
<   nimln(10, "tst.nim");
<   nimln(10, "tst.nim");
133,134d65
<   nimln(13, "tst.nim");
<   nimln(13, "tst.nim");
138d68
<   popFrame();
181,182d110
<   nimfr("tst", "tst.nim")
<   nimln(16, "tst.nim");
184d111
<   popFrame();

[jovial]

I believe the mistake is in my code. I should have been casting to a ptr CArray:

type CArray*{.unchecked.}[T] = array[0..0, T]

proc main =
  let arrayAsPtr = alloc0(4 * sizeof(ptr int))
  var a = cast[ptr CArray[ptr int]](arrayAsPtr)
  for i in 0..3:
    a[i] = cast[ptr int](alloc0(sizeof(int)))
    a[i][] = i

  let before = cast[int](a[2])
  echo cast[int](a[2])

  #let after = cast[int](a[2])
  echo cast[int](a[2])
  #assert before == after

main()

see: http://forum.nimrod-lang.org/t/567#3035 for more details.

[jangko]

no longer a valid issue for nim 0.12.0

[reactormonk]

What would the expected output be?

[jangko]

@reactormonk: both of the echos output the same value

[reactormonk]

Works with -d:release, but not without. Close it?

[jangko]

need more investigation, my system: windows 7-32bit, minGW/GCC 4.8.1, both mode with/without -:release, don't have problem don't know about other system/platform, need more feedback from other users

[dom96]

Works with both -d:release and without for me (Mac OS X/clang/Nim 0.12.1). Closing.