【Valgrind】How to check if we reading uninitialized memory in 10 min

 1 #include <stdio.h>

 2 #include <stdlib.h>

 3 

 4 int main(int argc, char** argv)

 5 {

 6   int i;

 7   int a[10];

 8   for (i = 0; i < 9; i++)

 9     a[i] = i;

10 

11   for (i = 0; i < 10; i++){

12     printf("%d ", a[i]);

13   }

14   printf("\n");

15   return 0;

16 }

Let's build and run:

$ gcc -O0 -g un_uninit.c -o un_uninit

$ ./un_uninit

0 1 2 3 4 5 6 7 8 32767

It seems not as expect, and reading some memory that is randomized. Let's use Valgrind to test:

 1 $ valgrind ./un_uninit

 2 ==3863== Memcheck, a memory error detector

 3 ==3863== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.

 4 ==3863== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info

 5 ==3863== Command: ./un_uninit

 6 ==3863==

 7 ==3863== Conditional jump or move depends on uninitialised value(s)

 8 ==3863==    at 0x4E8183E: vfprintf (vfprintf.c:1660)

 9 ==3863==    by 0x4E8B748: printf (printf.c:33)

10 ==3863==    by 0x4005CD: main (un_uninit.c:12)

11 ==3863==

12 ==3863== Use of uninitialised value of size 8

13 ==3863==    at 0x4E80CFB: _itoa_word (_itoa.c:179)

14 ==3863==    by 0x4E849A6: vfprintf (vfprintf.c:1660)

15 ==3863==    by 0x4E8B748: printf (printf.c:33)

16 ==3863==    by 0x4005CD: main (un_uninit.c:12)

17 ==3863==

18 ==3863== Conditional jump or move depends on uninitialised value(s)

19 ==3863==    at 0x4E80D05: _itoa_word (_itoa.c:179)

20 ==3863==    by 0x4E849A6: vfprintf (vfprintf.c:1660)

21 ==3863==    by 0x4E8B748: printf (printf.c:33)

22 ==3863==    by 0x4005CD: main (un_uninit.c:12)

23 ==3863==

24 ==3863== Conditional jump or move depends on uninitialised value(s)

25 ==3863==    at 0x4E849F2: vfprintf (vfprintf.c:1660)

26 ==3863==    by 0x4E8B748: printf (printf.c:33)

27 ==3863==    by 0x4005CD: main (un_uninit.c:12)

28 ==3863==

29 ==3863== Conditional jump or move depends on uninitialised value(s)

30 ==3863==    at 0x4E81909: vfprintf (vfprintf.c:1660)

31 ==3863==    by 0x4E8B748: printf (printf.c:33)

32 ==3863==    by 0x4005CD: main (un_uninit.c:12)

33 ==3863==

34 ==3863== Conditional jump or move depends on uninitialised value(s)

35 ==3863==    at 0x4E8198C: vfprintf (vfprintf.c:1660)

36 ==3863==    by 0x4E8B748: printf (printf.c:33)

37 ==3863==    by 0x4005CD: main (un_uninit.c:12)

38 ==3863==

39 0 1 2 3 4 5 6 7 8 15

40 ==3863==

41 ==3863== HEAP SUMMARY:

42 ==3863==     in use at exit: 0 bytes in 0 blocks

43 ==3863==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated

44 ==3863==

45 ==3863== All heap blocks were freed -- no leaks are possible

46 ==3863==

47 ==3863== For counts of detected and suppressed errors, rerun with: -v

48 ==3863== Use --track-origins=yes to see where uninitialised values come from

49 ==3863== ERROR SUMMARY: 8 errors from 6 contexts (suppressed: 0 from 0)

View Code

From the error msg, we find there is "Use of uninitialised value of size 8" in line 12.

Let's assume we still cannot be sure if i or a[i] is the root cause, and use '--track-origins=yes' valgrind command line:

 1 $ valgrind --track-origins=yes ./un_uninit

 2 ==3917== Memcheck, a memory error detector

 3 ==3917== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.

 4 ==3917== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info

 5 ==3917== Command: ./un_uninit

 6 ==3917==

 7 ==3917== Conditional jump or move depends on uninitialised value(s)

 8 ==3917==    at 0x4E8183E: vfprintf (vfprintf.c:1660)

 9 ==3917==    by 0x4E8B748: printf (printf.c:33)

10 ==3917==    by 0x4005CD: main (un_uninit.c:12)

11 ==3917==  Uninitialised value was created by a stack allocation

12 ==3917==    at 0x40057D: main (un_uninit.c:5)

13 ==3917==

14 ==3917== Use of uninitialised value of size 8

15 ==3917==    at 0x4E80CFB: _itoa_word (_itoa.c:179)

16 ==3917==    by 0x4E849A6: vfprintf (vfprintf.c:1660)

17 ==3917==    by 0x4E8B748: printf (printf.c:33)

18 ==3917==    by 0x4005CD: main (un_uninit.c:12)

19 ==3917==  Uninitialised value was created by a stack allocation

20 ==3917==    at 0x40057D: main (un_uninit.c:5)

21 ==3917==

22 ==3917== Conditional jump or move depends on uninitialised value(s)

23 ==3917==    at 0x4E80D05: _itoa_word (_itoa.c:179)

24 ==3917==    by 0x4E849A6: vfprintf (vfprintf.c:1660)

25 ==3917==    by 0x4E8B748: printf (printf.c:33)

26 ==3917==    by 0x4005CD: main (un_uninit.c:12)

27 ==3917==  Uninitialised value was created by a stack allocation

28 ==3917==    at 0x40057D: main (un_uninit.c:5)

29 ==3917==

30 ==3917== Conditional jump or move depends on uninitialised value(s)

31 ==3917==    at 0x4E849F2: vfprintf (vfprintf.c:1660)

32 ==3917==    by 0x4E8B748: printf (printf.c:33)

33 ==3917==    by 0x4005CD: main (un_uninit.c:12)

34 ==3917==  Uninitialised value was created by a stack allocation

35 ==3917==    at 0x40057D: main (un_uninit.c:5)

36 ==3917==

37 ==3917== Conditional jump or move depends on uninitialised value(s)

38 ==3917==    at 0x4E81909: vfprintf (vfprintf.c:1660)

39 ==3917==    by 0x4E8B748: printf (printf.c:33)

40 ==3917==    by 0x4005CD: main (un_uninit.c:12)

41 ==3917==  Uninitialised value was created by a stack allocation

42 ==3917==    at 0x40057D: main (un_uninit.c:5)

43 ==3917==

44 ==3917== Conditional jump or move depends on uninitialised value(s)

45 ==3917==    at 0x4E8198C: vfprintf (vfprintf.c:1660)

46 ==3917==    by 0x4E8B748: printf (printf.c:33)

47 ==3917==    by 0x4005CD: main (un_uninit.c:12)

48 ==3917==  Uninitialised value was created by a stack allocation

49 ==3917==    at 0x40057D: main (un_uninit.c:5)

50 ==3917==

51 0 1 2 3 4 5 6 7 8 15

52 ==3917==

53 ==3917== HEAP SUMMARY:

54 ==3917==     in use at exit: 0 bytes in 0 blocks

55 ==3917==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated

56 ==3917==

57 ==3917== All heap blocks were freed -- no leaks are possible

58 ==3917==

59 ==3917== For counts of detected and suppressed errors, rerun with: -v

60 ==3917== ERROR SUMMARY: 8 errors from 6 contexts (suppressed: 0 from 0)

Now, everything is clear by "Uninitialised value was created by a stack allocation" in line 5(this line number should cover all the varable in stack).

 

reference:

http://pages.cs.wisc.edu/~bart/537/valgrind.html