pinpointing conditional jump or move depends on uninitialized value s valgrind message

Question

So I ve been getting some mysterious uninitialized values message from valgrind and it s been quite the mystery as of where the bad value originated from   Seems that valgrind shows the place where the unitialised value ends up being used  but not the origin of the uninitialised value     11366   Conditional jump or move depends on uninitialised value s    11366      at 0x43CAE4F    printf fp  in  lib tls i686 cmov libc-2 7 so    11366      by 0x43C6563  vfprintf  in  lib tls i686 cmov libc-2 7 so    11366      by 0x43EAC03  vsnprintf  in  lib tls i686 cmov libc-2 7 so    11366      by 0x42D475B   within  usr lib libstdc   so 6 0 9    11366      by 0x42E2C9B  std  ostreambuf iterator lt char  std  char traits lt char gt   gt  std  num put lt char  std  ostreambuf iterator lt char  std  char traits lt char gt   gt   gt    M insert float lt double gt  std  ostreambuf iterator lt char  std  char traits lt char gt   gt   std  ios base amp   char  char  double  const  in  usr lib libstdc   so 6 0 9    11366      by 0x42E31B4  std  num put lt char  std  ostreambuf iterator lt char  std  char traits lt char gt   gt   gt   do put std  ostreambuf iterator lt char  std  char traits lt char gt   gt   std  ios base amp   char  double  const  in  usr lib libstdc   so 6 0 9    11366      by 0x42EE56F  std  ostream amp  std  ostream   M insert lt double gt  double   in  usr lib libstdc   so 6 0 9    11366      by 0x81109ED  Snake  SnakeBody  syncBodyPos    ostream 221    11366      by 0x810B9F1  Snake  Snake  update    snake cpp 257    11366      by 0x81113C1  SnakeApp  updateState    snakeapp cpp 224    11366      by 0x8120351  RoenGL  updateState    roengl cpp 1180    11366      by 0x81E87D9  Roensachs  update    rs cpp 321    As can be seen  it gets quite cryptic   especially because when it s saying by Class  MethodX  it sometimes points straight to ostream etc  Perhaps this is due to optimization     11366      by 0x81109ED  Snake  SnakeBody  syncBodyPos    ostream 221    Just like that  Is there something I m missing  What is the best way to catch bad values without having to resort to super-long printf detective work   Update   I found out what was wrong  but the strange thing is  valgrind did not report it when the bad value was first used  It was used in a multiplication function   movespeed   stat speedfactor   speedfac   currentbendfactor val    Where speedfac was an unitialised float  However  at that time it was not reported and not until the value is to be printed that I get the error   Is there a setting for valgrind to change this behavior

User · Accepted Answer

Use the valgrind option --track-origins yes to have it track the origin of uninitialized values   This will make it slower and take more memory  but can be very helpful if you need to track down the origin of an uninitialized value   Update  Regarding the point at which the uninitialized value is reported  the valgrind manual states      It is important to understand that your program can copy around junk  uninitialised  data as much as it likes  Memcheck observes this and keeps track of the data  but does not complain  A complaint is issued only when your program attempts to make use of uninitialised data in a way that might affect your program s externally-visible behaviour    From the Valgrind FAQ      As for eager reporting of copies of uninitialised memory values  this has been suggested multiple times  Unfortunately  almost all programs legitimately copy uninitialised memory values around  because compilers pad structs to preserve alignment  and eager checking leads to hundreds of false positives  Therefore Memcheck does not support eager checking at this time

User · Answer

What this means is that you are trying to print out output a value which is at least partially uninitialized  Can you narrow it down so that you know exactly what value that is  After that  trace through your code to see where it is being initialized  Chances are  you will see that it is not being fully initialized   If you need more help  posting the relevant sections of source code might allow someone to offer more guidance   EDIT  I see you ve found the problem  Note that valgrind watches for Conditional jump or move based on unitialized variables  What that means is that it will only give out a warning if the execution of the program is altered due to the uninitialized value  ie  the program takes a different branch in an if statement  for example   Since the actual arithmetic did not involve a conditional jump or move  valgrind did not warn you of that  Instead  it propagated the  uninitialized  status to the result of the statement that used it   It may seem counterintuitive that it does not warn you immediately  but as mark4o pointed out  it does this because uninitialized values get used in C all the time  examples  padding in structures  the realloc   call  etc   so those warnings would not be very useful due to the false positive frequency

[c++] pinpointing "conditional jump or move depends on uninitialized value(s)" valgrind message

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