The best way to handle such a situation is again preventative: set your pointer to point to NULL as soon as you've freed it. That way, if you do try to use it later, then you'll have another "dereferencing NULL" bug, which should be much easier to track. Another form of this bug is the problem of dealing with memory that has gone out of scope.
This is a really tricky bug to find because once again the memory address will look valid when you print it out in GDB. In fact, your code might even work sometimes or just display weird behavior by printing whatever happens to be on the stack in the location that used to be the memory of the array x. Generally, the way you'll know if you have this kind of bug is that you'll get garbage when you print out the variable even though you know that it's initialized. Watch out for the pointers returned from functions.
If that pointer is causing you trouble, check the function and look for whether the pointer is pointing to a local variable in the function.
Note that it is perfectly fine to return a pointer to memory allocated in the function using new or malloc, but not to return a pointer to a statically declared array e.
Tools such as Valgrind can be immensely helpful in tracking down these bugs because they watch memory to ensure that it's valid. If it isn't, Valgrind will alert you. Our Valgrind tutorial goes into more detail about finding this sort of bug. Of course, the best solution is simply to avoid ever doing anything like this.
Technically, you could use a static buffer, which would allow you to have a permanent buffer you could pass around. But this is only asking for trouble if you later decide, for whatever reason, that you don't need it to be static if you forget why you made it static in the first place, for instance. Writing off the end of the array Generally, if you're writing off the bounds of an array, then the line that caused the segfault in the first place should be an array access.
There are a few times when this won't actually be the case -- notably, if the fact that you wrote off an array causes the stack to be smashed -- basically, overwriting the pointer that stores where to return after the function completes. Of course, sometimes, you won't actually cause a segfault writing off the end of the array. Instead, you might just notice that some of your variable values are changing periodically and unexpectedly.
This is a tough bug to crack; one option is to set up your debugger to watch a variable for changes and run your program until the variable's value changes. Your debugger will break on that instruction, and you can poke around to figure out if that behavior is unexpected. To simplify things, use simple test cases, keep working with the same inputs, and turn off randomized seeds if you're using random numbers! By clicking sign up, you agree to receive emails from Techopedia and agree to our Terms of Use and Privacy Policy.
A segmentation fault segfault is an error returned by hardware with memory protection that tells the operating system that a memory access violation has occurred. The operating system usually reacts by telling the offending process about the error through a signal and then the OS performs some sort of corrective action.
This happens because a process or program tries to access memory that is not assigned to it. By: Justin Stoltzfus Contributor, Reviewer.
By: Satish Balakrishnan. Dictionary Dictionary Term of the Day. Gorilla Glass. Unexpected end of JSON input while parsing near ' There is likely additional logging output above lite server this operation is rejected by user system npm npm ERR! OutOfMemoryError no error message pop os theem. Social Twitter Facebook. Legal Privacy Policy Terms. Ask Question. Asked 11 years, 8 months ago. Active 2 months ago. Viewed k times. Improve this question. Jonathan Leffler k gold badges silver badges bronze badges.
Rajendra Uppal Rajendra Uppal Just a memory dump when something goes wrong! Typically called by attempting to dereference a null pointer, so a segmentation fault is often analogous to a Java NullPointerException. Segmentation comes from Memory Segmentation. You're accessing a segment of memory that does not belong to you.
Add a comment. Active Oldest Votes. Improve this answer. Community Bot 1 1 1 silver badge. No warnings on compile. No segfault. The code can run fine on a production system for years, you alter another part of the code, change compiler or something else and BOOOOOM!
Sorry for the bump but just a side note Not least because there are systems out there that operate without memory protection, thus cannot tell whether a piece of memory actually "belongs to you", and thus don't know segfaults, only undefined behaviour ChrisHuang-Leaver, normally when you get out of scope, the compiler has to recover some stack space to free the unused stack space, but this doesn't happen always with gcc being one of this compilers.
Also, the allocated stack space is normally reused again, so I have heard of no operating systems that return unused stack pages to the system, making that space subject for a SIGSEGV , so I won't expect such a signal from mangling with the stack.
Show 4 more comments. And all of this with respect to virtual memory systems. This is what I had on mind in "And things like shmat ; - these are what I count as 'indirect' access. In a virtual memory operating system there's no way normally, so please, operating system implementors, don't flame me for this for a process to access another process virtual memory, not being some kind of memory attach system call that allows you to access.
Virtual memory addresses normally mean different things depending on the process being considered. Ignacio Vazquez-Abrams Ignacio Vazquez-Abrams k gold badges silver badges bronze badges. These kinds of errors are usually caused by pointers that are Used before being properly initialised Used after the memory they point to has been realloced or deleted.
Used in an indexed array where the index is outside of the array bounds. Component 10 Component 10 9, 5 5 gold badges 43 43 silver badges 59 59 bronze badges. According to Wikipedia : A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed for example, attempting to write to a read-only location, or to overwrite part of the operating system.
Fabio says Reinstate Monica 4, 7 7 gold badges 38 38 silver badges 52 52 bronze badges.
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