How do I use valgrind to find memory leaks

Question

How do I use valgrind to find the memory leaks in a program   Please someone help me and describe the steps to carryout the procedure   I am using Ubuntu 10 04 and I have a program a c  please help me out

User · Answer

You can run   valgrind --leak-check full --log-file  logfile out  -v  your program and its arguments

User · Answer

You can create an alias in  bashrc file as follows  alias vg  valgrind --leak-check full -v --track-origins yes --log-file vg logfile out    So whenever you want to check memory leaks  just do simply  vg    lt name of your executable gt   lt command line parameters to your executable gt    This will generate a Valgrind log file in the current directory

User · Answer

How to Run Valgrind  Not to insult the OP  but for those who come to this question and are still new to Linux mdash you might have to install Valgrind on your system   sudo apt install valgrind    Ubuntu  Debian  etc  sudo yum install valgrind    RHEL  CentOS  Fedora  etc    Valgrind is readily usable for C C   code  but can even be used for other  languages when configured properly  see this for Python    To run Valgrind  pass the executable as an argument  along with any  parameters to the program    valgrind --leak-check full            --show-leak-kinds all            --track-origins yes            --verbose            --log-file valgrind-out txt              executable exampleParam1   The flags are  in short    --leak-check full   each individual leak will be shown in detail  --show-leak-kinds all  Show all of  definite  indirect  possible  reachable  leak kinds in the  full  report  --track-origins yes  Favor useful output over speed  This tracks the origins of uninitialized values  which could be very useful for memory errors  Consider turning off if Valgrind is unacceptably slow  --verbose  Can tell you about unusual behavior of your program  Repeat for more verbosity  --log-file  Write to a file  Useful when output exceeds terminal space    Finally  you would like to see a Valgrind report that looks like this   HEAP SUMMARY      in use at exit  0 bytes in 0 blocks   total heap usage  636 allocs  636 frees  25 393 bytes allocated  All heap blocks were freed -- no leaks are possible  ERROR SUMMARY  0 errors from 0 contexts  suppressed  0 from 0  ERROR SUMMARY  0 errors from 0 contexts  suppressed  0 from 0      I have a leak  but WHERE   So  you have a memory leak  and Valgrind isn t saying anything meaningful   Perhaps  something like this   5 bytes in 1 blocks are definitely lost in loss record 1 of 1    at 0x4C29BE3  malloc  vg replace malloc c 299     by 0x40053E  main  in  home Peri461 Documents executable    Let s take a look at the C code I wrote too    include  lt stdlib h gt   int main         char  string   malloc 5   sizeof char      LEAK  not freed      return 0      Well  there were 5 bytes lost  How did it happen  The error report just says  main and malloc  In a larger program  that would be seriously troublesome to  hunt down  This is because of how the executable was compiled  We can  actually get line-by-line details on what went wrong  Recompile your program  with a debug flag  I m using gcc here    gcc -o executable -std c11 -Wall main c           suppose it was this at first gcc -o executable -std c11 -Wall -ggdb3 main c    add -ggdb3 to it   Now with this debug build  Valgrind points to the exact line of code  allocating the memory that got leaked   The wording is important  it might not  be exactly where your leak is  but what got leaked  The trace helps you find  where    5 bytes in 1 blocks are definitely lost in loss record 1 of 1    at 0x4C29BE3  malloc  vg replace malloc c 299     by 0x40053E  main  main c 4      Techniques for Debugging Memory Leaks  amp  Errors   Make use of www cplusplus com  It has great documentation on C C   functions  General advice for memory leaks    Make sure your dynamically allocated memory does in fact get freed  Don t allocate memory and forget to assign the pointer  Don t overwrite a pointer with a new one unless the old memory is freed   General advice for memory errors    Access and write to addresses and indices you re sure belong to you  Memory  errors are different from leaks  they re often just IndexOutOfBoundsException  type problems  Don t access or write to memory after freeing it   Sometimes your leaks errors can be linked to one another  much like an IDE discovering that you haven t typed a closing bracket yet  Resolving one issue can resolve others  so look for one that looks a good culprit and apply some of these ideas    List out the functions in your code that depend on are dependent on the   offending  code that has the memory error  Follow the program s execution   maybe even in gdb perhaps   and look for precondition postcondition errors  The idea is to trace your program s execution while focusing on the lifetime of allocated memory  Try commenting out the  offending  block of code  within reason  so your code  still compiles   If the Valgrind error goes away  you ve found where it is   If all else fails  try looking it up  Valgrind has documentation too      A Look at Common Leaks and Errors  Watch your pointers  60 bytes in 1 blocks are definitely lost in loss record 1 of 1    at 0x4C2BB78  realloc  vg replace malloc c 785     by 0x4005E4  resizeArray  main c 12     by 0x40062E  main  main c 19    And the code    include  lt stdlib h gt   include  lt stdint h gt   struct  List       int32 t  data      int32 t length     typedef struct  List List   List  resizeArray List  array        int32 t  dPtr   array- gt data      dPtr   realloc dPtr  15   sizeof int32 t      doesn t update array- gt data     return array     int main         List  array   calloc 1  sizeof List        array- gt data   calloc 10  sizeof int32 t        array   resizeArray array        free array- gt data       free array       return 0      As a teaching assistant  I ve seen this mistake often  The student makes use of  a local variable and forgets to update the original pointer  The error here is  noticing that realloc can actually move the allocated memory somewhere else  and change the pointer s location  We then leave resizeArray without telling  array- gt data where the array was moved to   Invalid write  1 errors in context 1 of 1  Invalid write of size 1    at 0x4005CA  main  main c 10   Address 0x51f905a is 0 bytes after a block of size 26 alloc d    at 0x4C2B975  calloc  vg replace malloc c 711     by 0x400593  main  main c 5    And the code    include  lt stdlib h gt   include  lt stdint h gt   int main         char  alphabet   calloc 26  sizeof char         for uint8 t i   0  i  lt  26  i                alphabet   i     A    i              alphabet   26      0     null-terminate the string       free alphabet       return 0      Notice that Valgrind points us to the commented line of code above  The array  of size 26 is indexed  0 25  which is why   alphabet   26  is an invalid  write mdash it s out of bounds  An invalid write is a common result of  off-by-one errors  Look at the left side of your assignment operation   Invalid read  1 errors in context 1 of 1  Invalid read of size 1    at 0x400602  main  main c 9   Address 0x51f90ba is 0 bytes after a block of size 26 alloc d    at 0x4C29BE3  malloc  vg replace malloc c 299     by 0x4005E1  main  main c 6    And the code    include  lt stdlib h gt   include  lt stdint h gt   int main         char  destination   calloc 27  sizeof char        char  source   malloc 26   sizeof char         for uint8 t i   0  i  lt  27  i                destination   i      source   i     Look at the last iteration             free destination       free source       return 0      Valgrind points us to the commented line above  Look at the last iteration here   which is    destination   26      source   26    However    source   26  is  out of bounds again  similarly to the invalid write  Invalid reads are also a  common result of off-by-one errors  Look at the right side of your assignment  operation     The Open Source  U Dys topia  How do I know when the leak is mine  How do I find my leak when I m using  someone else s code  I found a leak that isn t mine  should I do something  All  are legitimate questions  First  2 real-world examples that show 2 classes of  common encounters   Jansson  a JSON library   include  lt jansson h gt   include  lt stdio h gt   int main         char  string        key      value            json error t error      json t  root   json loads string  0   amp error     obtaining a pointer     json t  value   json object get root   key      obtaining a pointer     printf     s   is the value field  n   json string value value      use value      json decref value     Do I free this pointer      json decref root      What about this one  Does the order matter      return 0      This is a simple program  it reads a JSON string and parses it  In the making   we use library calls to do the parsing for us  Jansson makes the necessary  allocations dynamically since JSON can contain nested structures of itself   However  this doesn t mean we decref or  free  the memory given to us from  every function  In fact  this code I wrote above throws both an  Invalid read   and an  Invalid write   Those errors go away when you take out the decref line  for value   Why  The variable value is considered a  borrowed reference  in the Jansson  API  Jansson keeps track of its memory for you  and you simply have to decref  JSON structures independent of each other  The lesson here   read the documentation  Really  It s sometimes hard to understand  but  they re telling you why these things happen  Instead  we have  existing questions about this memory error   SDL  a graphics and gaming library   include  SDL2 SDL h   int main int argc  char  argv          if  SDL Init SDL INIT VIDEO SDL INIT AUDIO     0            SDL Log  Unable to initialize SDL   s   SDL GetError             return 1             SDL Quit        return 0      What s wrong with this code  It consistently leaks  212 KiB of memory for me  Take a moment to think about it  We turn SDL on and then off  Answer  There is nothing wrong   That might sound bizarre at first  Truth be told  graphics are messy and sometimes you have to accept some leaks as being part of the standard library  The lesson here  you need not quell every memory leak  Sometimes you just need to suppress the leaks because they re known issues you can t do anything about   This is not my permission to ignore your own leaks    Answers unto the void  How do I know when the leak is mine    It is   99  sure  anyway   How do I find my leak when I m using someone else s code   Chances are someone else already found it  Try Google  If that fails  use the skills I gave you above  If that fails and you mostly see API calls and little of your own stack trace  see the next question   I found a leak that isn t mine  should I do something   Yes  Most APIs have ways to report bugs and issues  Use them  Help give back to the tools you re using in your project     Further Reading  Thanks for staying with me this long  I hope you ve learned something  as I tried to tend to the broad spectrum of people arriving at this answer  Some things I hope you ve asked along the way  How does C s memory allocator work  What actually is a memory leak and a memory error  How are they different from segfaults  How does Valgrind work  If you had any of these  please do feed your curiousity    More about malloc  C s memory allocator Definition of a segmentation fault Definition of a memory leak Definition of a memory access error How does Valgrind work

User · Answer

Try this   valgrind --leak-check full -v   your program  As long as valgrind is installed it will go through your program and tell you what s wrong  It can give you pointers and approximate places where your leaks may be found  If you re segfault ing  try running it through gdb

[c] How do I use valgrind to find memory leaks?

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