Fixing Segmentation faults in C

Question

I am writing a cross-platform C   program for Windows and Unix  On the Window side  the code will compile and execute no problem  On the Unix side  it will compile however when I try to run it  I get a segmentation fault  My initial hunch is that there is a problem with pointers   What are good methodologies to find and fix segmentation fault errors

User · Answer

I don t know of any methodology to use to fix things like this   I don t think it would be possible to come up with one either for the very issue at hand is that your program s behavior is undefined  I don t know of any case when SEGFAULT hasn t been caused by some sort of UB    There are all kinds of  methodologies  to avoid the issue before it arises   One important one is RAII   Besides that  you just have to throw your best psychic energies at it

User · Answer

Compile your application with -g  then you ll have debug symbols in the binary file  Use gdb to open the gdb console  Use file and pass it your application s binary file in the console  Use run and pass in any arguments your application needs to start  Do something to cause a Segmentation Fault  Type bt in the gdb console to get a stack trace of the Segmentation Fault

User · Answer

Sometimes the crash itself isn t the real cause of the problem-- perhaps the memory got smashed at an earlier point but it took a while for the corruption to show itself   Check out valgrind  which has lots of checks for pointer problems  including array bounds checking    It ll tell you where the problem starts  not just the line where the crash occurs

User · Answer

Yes  there is a problem with pointers   Very likely you re using one that s not initialized properly  but it s also possible that you re messing up your memory management with double frees or some such   To avoid uninitialized pointers as local variables  try declaring them as late as possible  preferably  and this isn t always possible  when they can be initialized with a meaningful value   Convince yourself that they will have a value before they re being used  by examining the code   If you have difficulty with that  initialize them to a null pointer constant  usually written as NULL or 0  and check them   To avoid uninitialized pointers as member values  make sure they re initialized properly in the constructor  and handled properly in copy constructors and assignment operators   Don t rely on an init function for memory management  although you can for other initialization   If your class doesn t need copy constructors or assignment operators  you can declare them as private member functions and never define them   That will cause a compiler error if they re explicitly or implicitly used   Use smart pointers when applicable   The big advantage here is that  if you stick to them and use them consistently  you can completely avoid writing delete and nothing will be double-deleted   Use C   strings and container classes whenever possible  instead of C-style strings and arrays   Consider using  at i  rather than  i   because that will force bounds checking   See if your compiler or library can be set to check bounds on  i   at least in debug mode   Segmentation faults can be caused by buffer overruns that write garbage over perfectly good pointers   Doing those things will considerably reduce the likelihood of segmentation faults and other memory problems   They will doubtless fail to fix everything  and that s why you should use valgrind now and then when you don t have problems  and valgrind and gdb when you do

User · Answer

Before the problem arises  try to avoid it as much as possible    Compile and run your code as often as you can  It will be easier to locate the faulty part  Try to encapsulate low-level   error prone routines so that you rarely have to work directly with memory  pay attention to the modelization of your program  Maintain a test-suite  Having an overview of what is currently working  what is no more working etc  will help you to figure out where the problem is  Boost test is a possible solution  I don t use it myself but the documentation can help to understand what kind of information must be displayed     Use appropriate tools for debugging  On Unix      GDB can tell you where you program crash and will let you see in what context  Valgrind will help you to detect many memory-related errors  With GCC you can also use mudflap With GCC  Clang and since October experimentally MSVC you can use Address Memory Sanitizer  It can detect some errors that Valgrind doesn t and the performance loss is lighter  It is used by compiling with the-fsanitize address flag    Finally I would recommend the usual things  The more your program is readable  maintainable  clear and neat  the easiest it will be to debug

User · Answer

On Unix you can use valgrind to find issues  It s free and powerful  If you d rather do it yourself you can overload the new and delete operators to set up a configuration where you have 1 byte with 0xDEADBEEF before and after each new object  Then track what happens at each iteration   This can fail to catch everything  you aren t guaranteed to even touch those bytes  but it has worked for me in the past on a Windows platform

[c++] Fixing Segmentation faults in C++

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