Valgrind report use of uninitialized memory

[mingodad]

Build camlrum with -g -O0 and building and and running examples/mls on ubuntu 18.04 64 bits we get this output from valgrind:

valgrind --leak-check=full ./a.out 
==25841== Memcheck, a memory error detector
==25841== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==25841== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==25841== Command: ./a.out
==25841== 
==25841== Conditional jump or move depends on uninitialised value(s)
==25841==    at 0x11C414: darken (major_gc.c:496)
==25841==    by 0x11DC1F: modify (memory.c:200)
==25841==    by 0x123137: mkexnname (sys.c:206)
==25841==    by 0x1232F3: sys_init (sys.c:241)
==25841==    by 0x118AF5: main (main.c:251)
==25841== 
==25841== Conditional jump or move depends on uninitialised value(s)
==25841==    at 0x11C42B: darken (major_gc.c:496)
==25841==    by 0x11DC1F: modify (memory.c:200)
==25841==    by 0x123137: mkexnname (sys.c:206)
==25841==    by 0x1232F3: sys_init (sys.c:241)
==25841==    by 0x118AF5: main (main.c:251)
==25841== 
==25841== Conditional jump or move depends on uninitialised value(s)
==25841==    at 0x11DC64: modify (memory.c:200)
==25841==    by 0x123137: mkexnname (sys.c:206)
==25841==    by 0x1232F3: sys_init (sys.c:241)
==25841==    by 0x118AF5: main (main.c:251)
==25841== 
==25841== Conditional jump or move depends on uninitialised value(s)
==25841==    at 0x11DC77: modify (memory.c:200)
==25841==    by 0x123137: mkexnname (sys.c:206)
==25841==    by 0x1232F3: sys_init (sys.c:241)
==25841==    by 0x118AF5: main (main.c:251)
==25841== 
Makefile      a.out         mls.sml       mls.ui        mls.uo        
Makefile.w32  
==25841== 
==25841== HEAP SUMMARY:
==25841==     in use at exit: 656,664 bytes in 15 blocks
==25841==   total heap usage: 22 allocs, 7 frees, 718,160 bytes allocated
==25841== 
==25841== 30,712 bytes in 1 blocks are possibly lost in loss record 9 of 13
==25841==    at 0x4C32F6D: malloc (vg_replace_malloc.c:380)
==25841==    by 0x1177CF: expandcode (expand.c:363)
==25841==    by 0x111518: interprete (interp.c:181)
==25841==    by 0x118B36: main (main.c:253)
==25841== 
==25841== 266,256 bytes in 1 blocks are possibly lost in loss record 13 of 13
==25841==    at 0x4C32F6D: malloc (vg_replace_malloc.c:380)
==25841==    by 0x1180C2: aligned_malloc (misc.c:143)
==25841==    by 0x11CDCF: init_major_heap (major_gc.c:725)
==25841==    by 0x1247BD: init_gc (gc_ctrl.c:197)
==25841==    by 0x1189BB: main (main.c:224)
==25841== 
==25841== LEAK SUMMARY:
==25841==    definitely lost: 0 bytes in 0 blocks
==25841==    indirectly lost: 0 bytes in 0 blocks
==25841==      possibly lost: 296,968 bytes in 2 blocks
==25841==    still reachable: 359,696 bytes in 13 blocks
==25841==         suppressed: 0 bytes in 0 blocks
==25841== Reachable blocks (those to which a pointer was found) are not shown.
==25841== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==25841== 
==25841== Use --track-origins=yes to see where uninitialised values come from
==25841== For lists of detected and suppressed errors, rerun with: -s
==25841== ERROR SUMMARY: 74 errors from 6 contexts (suppressed: 0 from 0)

[mingodad]

Looking at Camel Light (cl75) I can see that the code that valgrind report is an extension by mosml, the camlrun from cl75 doesn't show any error in valgrind other than memory leaks.

[mingodad]

Using Initialize instead of Modify then valgrind doesn't report invalid memory usage anymore and no signs of undesired side effects so far (building all mosml under valgrind).

------------------------------ src/runtime/sys.c ------------------------------

index 98b3be3..f214d46 100644
@@ -203,7 +203,7 @@ value mkexnname(char* name) {
   Push_roots(r, 1);
   r[0] = copy_string(name);
   ref = alloc_shr(1, Reference_tag);
-  modify(&Field(ref, 0), r[0]);
+  initialize(&Field(ref, 0), r[0]);
   Pop_roots();
   return ref;
 }