What is "Segmentation fault (core dumped)?" [duplicate]
I am trying to write a C program in linux that having sqrt of the argument, Here’s the code:
After I type in my input at the «shell> » prompt, gcc gives me the following error:
1 Answer 1
«Segmentation fault» means that you tried to access memory that you do not have access to.
The first problem is with your arguments of main . The main function should be int main(int argc, char *argv[]) , and you should check that argc is at least 2 before accessing argv[1] .
Also, since you’re passing in a float to printf (which, by the way, gets converted to a double when passing to printf ), you should use the %f format specifier. The %s format specifier is for strings ( ‘\0’ -terminated character arrays).
Segmentation fault core dumped c что это
segmentation fault (core dumped) It’s an error that used to cause me a lot of headaches. Of course, it also has a Chinese form,Segfault (core has been transferred)。
This kind of error may appear in various programs. Every time I use the software name plus the error message to search. For some common software, I can find some solutions, so I can solve them in the way they provide. But for some relatively niche software, there is almost no solution.
Until I learned "c and pointer", I created a segfault myself, and I was finally no longer afraid of this error (of course, it was limited to not being afraid).
The following line of code, I think and printf("hello world!\n") As important. "Hello world!" allows you to get started with programming, and understanding the code below will allow you to go deeper into programming.
We can copy it to a error.c In, then compile and run
In the code, we create a pointer variable called a, and the following assignment statement stores 12 at the memory address pointed to by a. From a logical point of view, there seems to be no problem. The problem behind it, and listen to me slowly.
a is a pointer variable. It is like a normal variable. It also has a specific memory address. It also stores a value in that memory pass, which is an address. For an ordinary variable, if they are not initialized when they are declared, then they will have a default value, for example int i; The result on my machine is 0. So for a pointer variable, if it is not initialized when it is declared, what is its value? I tested it with the following code
During the three runs, the value of i is always 0, and the value of a is 0x7ffee20e7dc0, 0x7ffc1d202d10, 0x7fff50972560. These all correspond to an address on the memory. Correct *a The process of assigning values is to assign values to their corresponding memory addresses, which poses a problem. If these memories are legal to be operated by us, then this operation will proceed smoothly without error, but there may be some problems that you can’t imagine. But if the memory is protected or the corresponding address does not exist (in most cases), then an error will pop up on the Linux system.
Now we know that segfault (core dumped) is caused by a program accessing/using a memory address that it cannot use, so what should we do in the future when this error occurs in the program?
How to diagnose and locate segmentation faults in x86 assembly
Segmentation faults commonly occur when programs attempt to access memory regions that they are not allowed to access. This article provides an overview of segmentation faults with practical examples. We will discuss how segmentation faults can occur in x86 assembly as well as C along with some debugging techniques.
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What are segmentation faults in x86 assembly?
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).
Let us consider the following x86 assembly example.
message db «Welcome to Segmentation Faults! «
mov ecx, message
As we can notice, the preceding program calls the subroutine _printMessage when it is executed. When we read this program without executing, it looks innocent without any evident problems. Let us assemble and link it using the following commands.
Now, let us run the program and observe the output.
As we can notice in the preceding excerpt, there is a segmentation fault when the program is executed.
How to detect segmentation faults in x86 assembly
Segmentation faults always occur during runtime but they can be detected at the code level. The previous sample program that was causing the segmentation faults, is due to lack of an exit routine within the program. So when the program completes executing the code responsible for printing the string, it doesn’t know how to exit and thus lands on some invalid memory address.
Another way to detect segmentation faults is to look for core dumps. Core dumps are usually generated when there is a segmentation fault. Core dumps provide the situation of the program at the time of the crash and thus we will be able to analyze the crash. Core dumps must be enabled on most systems as shown below.
When a segmentation fault occurs, a new core file will be generated as shown below.
Welcome to Segmentation Faults! Segmentation fault (core dumped)
core seg seg.nasm seg.o
As shown in the proceeding excerpt, there is a new file named core in the current directory.
How to fix segmentation faults x86 assembly
Segmentation faults can occur due to a variety of problems. Fixing a segmentation fault always depends on the root cause of the segmentation fault. Let us go through the same example we used earlier and attempt to fix the segmentation fault. Following is the original x86 assembly program causing a segmentation fault.
message db «Welcome to Segmentation Faults! «
mov ecx, message
As mentioned earlier, there isn’t an exit routine to gracefully exit this program. So, let us add a call to the exit routine immediately after the control is returned from _printMessage. This looks as follows
message db «Welcome to Segmentation Faults! «
mov ecx, message
_exit:
mov eax, 1
mov ebx, 0
int 0x80
Notice the additional piece of code added in the preceding excerpt. When _printMessage completes execution, the control will be transferred to the caller and call _exit instruction will be executed, which is responsible for gracefully exiting the program without any segmentation faults. To verify, let us assemble and link the program using the following commands.
Run the binary and we should see the following message without any segmentation fault.
Welcome to Segmentation Faults!
As mentioned earlier, the solution to fix a segmentation fault always depends on the root cause.
How to fix segmentation fault in c
Segmentation faults in C programs are often seen due to the fact that C programming offers access to low-level memory. Let us consider the following example written in C language.
str = «test string»;
The preceding program causes a segmentation fault when it is run. The string variable str in this example stores in read-only part of the data segment and we are attempting to modify read-only memory using the line *(str+1) = ‘x’;
Similarly, segmentation faults can occur when an array out of bound is accessed as shown in the following example.
This example also leads to a segmentation fault. In addition to it, if the data being passed to the test variable is user-controlled, it can lead to stack-based buffer overflow attacks. Running this program shows the following error due to a security feature called stack cookies.
*** stack smashing detected ***: terminated
Aborted (core dumped)
The preceding excerpt shows that the out-of-bound access on an array can also lead to segfaults. Fixing these issues in C programs again falls back to the reason for the Segfault. We should avoid accessing protected memory regions to minimize segfaults.
How to debug segmentation fault
Let us go through our first x86 example that was causing a segfault to get an overview of debugging segmentation faults using gdb. Let us begin by running the program, so we can get the core dump when the segmentation fault occurs.
Welcome to Segmentation Faults! Segmentation fault (core dumped)
Now, a core dump should have been generated. Let us load the core dump along with the target executable as shown in the following command. Loading the executable along with the core dump makes the debugging process much easier.
GEF for linux ready, type `gef’ to start, `gef config’ to configure
78 commands loaded for GDB 9.1 using Python engine 3.8
[*] 2 commands could not be loaded, run `gef missing` to know why.
Core was generated by `./print’.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x0804901c in ?? ()
As we can notice in the preceding output, the core dump is loaded using GDB and the segmentation fault occurred at the address 0x0804901c. To confirm this, we can check the output of info registers.
ecx 0x804a000 0x804a000
esp 0xffbe8aa0 0xffbe8aa0
eip 0x804901c 0x804901c
eflags 0x10202 [ IF RF ]
As highlighted, the eip register contains the same address. This means, the program attempted to execute the instruction at this address and it has resulted in a segmentation fault. Let us go through the disassembly and understand where this instruction is.
First, let us get the list of functions available and identify which function possibly caused the segfault.
All defined functions:
0x08049000 _printMessage
0x08049017 _start
As highlighted in the preceding excerpt, the _printMessage and _start functions’ address ranges are close to the address that caused the segmentation fault. So, let us begin with the disassembly of the function _printMessage.
Dump of assembler code for function _printMessage:
0x08049000 <+0>: mov eax,0x4
0x08049005 <+5>: mov ebx,0x1
0x0804900a <+10>: mov ecx,0x804a000
0x0804900f <+15>: mov edx,0x20
0x08049014 <+20>: int 0x80
0x08049016 <+22>: ret
End of assembler dump.
Let us set a breakpoint at ret instruction and run the program. The following command shows how to setup the breakpoint.
Breakpoint 1 at 0x8049016
Type run to start the program execution.
0x804900f <_printMessage+15> mov edx, 0x20
0x8049014 <_printMessage+20> int 0x80
→ 0x8049016 <_printMessage+22> ret
↳ 0x804901c add BYTE PTR [eax], al
0x804901e add BYTE PTR [eax], al
0x8049020 add BYTE PTR [eax], al
0x8049022 add BYTE PTR [eax], al
0x8049024 add BYTE PTR [eax], al
As we can notice in the preceding excerpt, when the ret instruction gets executed, the control gets passed to the region not controlled by the program code leading to unauthorized memory access and thus a segmentation fault.
Intro to x86 Disassembly
Conclusion
This article has outlined some basic concepts around segmentation faults in x86 assembly and how one can use them for debugging programs. We have seen various simple examples to better understand the concepts. We briefly discussed core dumps, which can help us to detect and analyze program crashes.
Debugging Segmentation Faults and Pointer Problems
By Alex Allain
For new programmers, debugging errors associated with pointers can be a nightmare. «Segmentation Fault (core dumped)» is a pretty vague error message, and it’s even worse when strange bugs start appearing that don’t cause segmentation faults — but that result in things like memory getting overwritten in unexpected ways. Fortunately, tools like Cee Studio, from our sponsor, provide new, easier ways to debug memory errors and buffer overflows. Cee Studio is a web-based compiler designed to make finding segfaults easy by providing instant and informative feedback on misuses of memory. But even without specialized tools, finding problems with pointers is easier than you think.
This tutorial assumes that you have a basic knowledge of pointers such as can be acquired by reading a pointer tutorial. It would help to be running a system that has a debugger such as GDB, or to at least have sufficient familiarity with GDB-like debuggers to understand the examples presented.
What is a segmentation fault?
When your program runs, it has access to certain portions of memory. First, you have local variables in each of your functions; these are stored in the stack. Second, you may have some memory, allocated during runtime (using either malloc, in C, or new, in C++), stored on the heap (you may also hear it called the «free store»). Your program is only allowed to touch memory that belongs to it — the memory previously mentioned. Any access outside that area will cause a segmentation fault. Segmentation faults are commonly referred to as segfaults.
There are four common mistakes that lead to segmentation faults: dereferencing NULL, dereferencing an uninitialized pointer, dereferencing a pointer that has been freed (or deleted, in C++) or that has gone out of scope (in the case of arrays declared in functions), and writing off the end of an array.
A fifth way of causing a segfault is a recursive function that uses all of the stack space. On some systems, this will cause a «stack overflow» report, and on others, it will merely appear as another type of segmentation fault.
The strategy for debugging all of these problems is the same: load the core file into GDB, do a backtrace, move into the scope of your code, and list the lines of code that caused the segmentation fault.
For instance, running on a Linux system, here’s an example session: This just loads the program called example using the core file called «core». The core file contains all the information needed by GDB to reconstruct the state of execution when the invalid operation caused a segmentation fault.
Once we’ve loaded up gdb, we get the following: So, execution stopped inside the function called foo() on line 4, which happened to be the assignment of the number 3 to the location pointed to by x. This is a goldmine of information: we already know exactly where the problem happened and which pointer was involved.
Since this is a somewhat contrived example, we can immediately find the error. The pointer x is initialized to 0, equivalent to NULL (in fact, NULL is a stand-in for 0), and we know that it’s a no-no to then try to access that pointer.
But what if it weren’t so obvious? Simply printing the value of the pointer can often lead to the solution. In this case: Printing out x reveals that it points to memory address 0x0 (the 0x indicates that the value following it is in hexadecimal, traditional for printing memory addresses). The address 0x0 is invalid — in fact, it’s NULL. If you dereference a pointer that stores the location 0x0 then you’ll definitely get a segmentation fault, just as we did.
If we’d gotten something more complicated, such as execution crashing inside a system call or library function (perhaps because we passed an uninitialized pointer to fgets), we’d need to figure out where we called the library function and what might have happened to cause a segfault within it. Here’s an example from another debugging session: This time, the segfault occurred because of something inside strcat. Does this mean the library function did something wrong? Nope! It means that we probably passed a bad value to the function. To debug this, we need to see what we passed into strcat.
So let’s see what function call we made that led to the segfault. Backtrace lists the function calls that had been made at the time the program crashed. Each function is directly above the function that called it. So foo was called by main in this case. The numbers on the side (#0, #1, #2) also indicate the order of calls, from most recent to longest ago.
To move from viewing the state within each function (encapsulated in the idea of a stack frame), we can use the up and down commands. Right now, we know we’re in the strcat stack frame, which contains all of the local variables of strcat, because it’s the top function on the stack. We want to move «up» (toward the higher numbers); this is the opposite of how the stack is printed. This helps a little — we know that we have a variable called x and a constant string. We should probably lookup the strcat function at this point to make sure that we got the order of arguments correct. Since we did, the problem must be with x. There it is again: a NULL pointer. The strcat function must be derefencing a NULL pointer that we gave it, and even though it’s a library function, it doesn’t do anything magical.
NULL pointers are generally pretty easy to work with — once we’ve found one, we know that somewhere along the line, we didn’t allocate some memory that we should have. It’s just a question of where. A common mistake is to not check the return from malloc to make sure that the system isn’t out of memory. Another common mistake is to assume that a function that calls malloc doesn’t return NULL even though it returns the result of malloc. Note that in C++, when you call new, it will throw an exception, bad_alloc, if sufficient memory cannot be allocated. Your code should be prepared to handle this situation cleanly, and if you choose to catch the exception and return NULL inside a function that ordinarily returns a new’ed pointer, this advice still holds. We did a good thing by checking to make sure that malloc succeeds before using the memory in create_memory, but we don’t check to make sure that create_memory returns a valid pointer! Shame on us. This is a bug that won’t catch you until you’re running your code on a real system unless you explicitly test your code in low memory situations.
Dereferencing an Uninitialized Pointer
Figuring out whether or not a pointer has been initialized is a bit harder than figuring out whether a pointer is NULL. The best way to avoid using an uninitialized pointer is to set your pointers to NULL when you declare them (or immediately initialize them). That way, if you do use a pointer that hasn’t had memory allocated for it, you will immediately be able to tell.
If you don’t set your pointers to NULL when you declare them, then you’ll have a much harder time of it (remember that non-static variables aren’t automatically initialized to anything in C or C++). You might need to figure out if 0x4025e800 is valid memory. One way you can get a sense of this in GDB is by printing out the addresses stored in other pointers you’ve allocated. If they’re fairly close together, you’ve probably correctly allocated memory. Of course, there’s no guarantee that this rule of thumb will hold on all systems.
In some cases, your debugger can tell you that an address is invalid based on the value stored in the pointer. For instance, in the following example, GDB indicates that the char* x, which I set to point to the memory address «30», is not accessible. Generally, though, the best way to handle such a situation is just to avoid having to rely on memory’s being close together or obviously invalid. Set your variables to NULL from the beginning.
Dereferencing Freed Memory
This is another tricky bug to find because you’re working with memory addresses that look valid. 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. If you declare a local array such as then the array, x, will no longer be valid once the function returns. 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.g., char x[10]).
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. This approach can get tricky when you’re dealing with a lot of dynamically allocated memory and it’s not entirely clear what you should watch. To simplify things, use simple test cases, keep working with the same inputs, and turn off randomized seeds if you’re using random numbers!
Stack Overflows
A stack overflow isn’t the same type of pointer-related problem as the others. In this case, you don’t need to have a single explicit pointer in your program; you just need a recursive function without a base case. Nevertheless, this is a tutorial about segmentation faults, and on some systems, a stack overflow will be reported as a segmentation fault. (This makes sense because running out of memory on the stack will violate memory segmentation.)
To diagnose a stack overflow in GDB, typically you just need to do a backtrace: If you find a single function call piling up an awfully large number of times, this is a good indication of a stack overflow.
Typically, you need to analyze your recursive function to make sure that all the base cases (the cases in which the function should not call itself) are covered correctly. For instance, in computing the factorial function In this case, the base case of n being zero is covered, but what about n < 0? On «valid» inputs, the function will work fine, but not on «invalid» inputs like -1.
You also have to make sure that your base case is reachable. Even if you have the correct base case, if you don’t correctly progress toward the base case, your function will never terminate.
Summary
While segmentation faults can be nasty and difficult to track down when you are first learning to program, over time you will start to see them as falling into a small number of patterns that are relatively easy to track down. This tutorial hasn’t covered every possible scenario for causing segmentation faults, but it touches on many of the basic problems you may encounter.