What is a segmentation fault?

What is a segmentation fault? Is it different in C and C++? How are segmentation faults and dangling pointers related?

Segmentation fault is a specific kind of error caused by accessing memory that “does not belong to you.” It’s a helper mechanism that keeps you from corrupting the memory and introducing hard-to-debug memory bugs. Whenever you get a segfault you know you are doing something wrong with memory – accessing a variable that has already been freed, writing to a read-only portion of the memory, etc. Segmentation fault is essentially the same in most languages that let you mess with memory management, there is no principal difference between segfaults in C and C++.

There are many ways to get a segfault, at least in the lower-level languages such as C(++). A common way to get a segfault is to dereference a null pointer:

int *p = NULL;
*p = 1;

Another segfault happens when you try to write to a portion of memory that was marked as read-only:

char *str = "Foo"; // Compiler marks the constant string as read-only
*str = 'b'; // Which means this is illegal and results in a segfault

Dangling pointer points to a thing that does not exist anymore, like here:

char *p = NULL;
{
    char c;
    p = &c;
}
// Now p is dangling

The pointer p dangles because it points to the character variable c that ceased to exist after the block ended. And when you try to dereference dangling pointer (like *p='A'), you would probably get a segfault.

It would be worth noting that segmentation fault isn't caused by directly accessing another process memory (this is what I'm hearing sometimes), as it is simply not possible. With virtual memory every process has its own virtual address space and there is no way to access another one using any value of pointer. Exception to this can be shared libraries which are same physical address space mapped to (possibly) different virtual addresses and kernel memory which is even mapped in the same way in every process (to avoid TLB flushing on syscall, I think). And things like shmat ;) - these are what I count as 'indirect' access. One can, however, check that they are usually located long way from process code and we are usually able to access them (this is why they are there, nevertheless accessing them in a improper way will produce segmentation fault).

Still, segmentation fault can occur in case of accessing our own (process) memory in improper way (for instance trying to write to non-writable space). But the most common reason for it is the access to the part of the virtual address space that is not mapped to physical one at all.

And all of this with respect to virtual memory systems.

A segmentation fault is caused by a request for a page that the process does not have listed in its descriptor table, or an invalid request for a page that it does have listed (e.g. a write request on a read-only page).

A dangling pointer is a pointer that may or may not point to a valid page, but does point to an "unexpected" segment of memory.

To be honest, as other posters have mentioned, Wikipedia has a very good article on this so have a look there. This type of error is very common and often called other things such as Access Violation or General Protection Fault.

They are no different in C, C++ or any other language that allows pointers. These kinds of errors are usually caused by pointers that are

1. Used before being properly initialised
2. Used after the memory they point to has been realloced or deleted.
3. Used in an indexed array where the index is outside of the array bounds. This is generally only when you're doing pointer math on traditional arrays or c-strings, not STL / Boost based collections (in C++.)

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).

Segmentation fault is also caused by hardware failures, in this case the RAM memories. This is the less common cause, but if you don't find an error in your code, maybe a memtest could help you.

The solution in this case, change the RAM.

edit:

Here there is a reference: Segmentation fault by hardware

Wikipedia's Segmentation_fault page has a very nice description about it, just pointing out the causes and reasons. Have a look into the wiki for a detailed description.

In computing, a segmentation fault (often shortened to segfault) or access violation is a fault raised by hardware with memory protection, notifying an operating system (OS) about a memory access violation.

The following are some typical causes of a segmentation fault:

• Dereferencing NULL pointers – this is special-cased by memory management hardware
• Attempting to access a nonexistent memory address (outside process's address space)
• Attempting to access memory the program does not have rights to (such as kernel structures in process context)
• Attempting to write read-only memory (such as code segment)

These in turn are often caused by programming errors that result in invalid memory access:

• Dereferencing or assigning to an uninitialized pointer (wild pointer, which points to a random memory address)

• Dereferencing or assigning to a freed pointer (dangling pointer, which points to memory that has been freed/deallocated/deleted)

• A buffer overflow.

• A stack overflow.

• Attempting to execute a program that does not compile correctly. (Some compilers will output an executable file despite the presence of compile-time errors.)

Segmentation fault occurs when a process (running instance of a program) is trying to access read-only memory address or memory range which is being used by other process or access the non-existent (invalid) memory address. Dangling Reference (pointer) problem means that trying to access an object or variable whose contents have already been deleted from memory, e.g:

int *arr = new int[20];
delete arr;
cout<<arr[1];  //dangling problem occurs here

In simple words: segmentation fault is the operating system sending a signal to the program saying that it has detected an illegal memory access and is prematurely terminating the program to prevent memory from being corrupted.

There are several good explanations of "Segmentation fault" in the answers, but since with segmentation fault often there's a dump of the memory content, I wanted to share where the relationship between the "core dumped" part in Segmentation fault (core dumped) and memory comes from:

> From about 1955 to 1975 - before semiconductor memory - the dominant technology in computer memory used tiny magnetic doughnuts strung on copper wires. The doughnuts were known as "ferrite cores" and main memory thus known as "core memory" or "core".

Taken from here.

"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).

There are enough definitions of segmentation fault, I would like to quote few examples which I came across while programming, which might seem like silly mistakes, but will waste a lot of time.

1. You can get a segmentation fault in below case while argument type mismatch in printf:

#include <stdio.h>
int main(){   
  int a = 5;
  printf("%s",a);
  return 0;
}

output : Segmentation Fault (SIGSEGV)

2. When you forgot to allocate memory to a pointer, but try to use it.

#include <stdio.h> 
typedef struct{
  int a;
} myStruct;   
int main(){
  myStruct *s;
  /* few lines of code */
  s->a = 5;
  return 0;
}

output : Segmentation Fault (SIGSEGV)

Consider the following snippets of Code,

SNIPPET 1

int *number = NULL;
*number = 1;

SNIPPET 2

int *number = malloc(sizeof(int));
*number = 1;

I'd assume you know the meaning of the functions: malloc() and sizeof() if you are asking this question.

Now that that is settled, SNIPPET 1 would throw a Segmentation Fault Error. while SNIPPET 2 would not.

Here's why.

The first line of snippet one is creating a variable(*number) to store the address of some other variable but in this case it is initialized to NULL. on the other hand, The second line of snippet two is creating the same variable(*number) to store the address of some other and in this case it is given a memory address(because malloc() is a function in C/C++ that returns a memory address of the computer)

The point is you cannot put water inside a bowl that has not been bought OR a bowl that has been bought but has not been authorized for use by you. When you try to do that, the computer is alerted and it throws a SegFault error.

You should only face this errors with languages that are close to low-level like C/C++. There is an abstraction in other High Level Languages that ensure you do not make this error.

It is also paramount to understand that Segmentation Fault is not language-specific.

Simple meaning of Segmentation fault is that you are trying to access some memory which doesn't belong to you. Segmentation fault occurs when we attempt to read and/or write tasks in a read only memory location or try to freed memory. In other words, we can explain this as some sort of memory corruption.

Below I mention common mistakes done by programmers that lead to Segmentation fault.

• Use scanf() in wrong way(forgot to put &).

int num;
scanf("%d", num);// must use &num instead of num

• Use pointers in wrong way.

int *num; 
printf("%d",*num); //*num should be correct as num only
//Unless You can use *num but you have to point this pointer to valid memory address before accessing it.

• Modifying a string literal(pointer try to write or modify a read only memory.)

char *str;  
  
//Stored in read only part of data segment
str = "GfG";      
  
//Problem:  trying to modify read only memory
*(str+1) = 'n';

• Try to reach through an address which is already freed.

// allocating memory to num 
int* num = malloc(8); 
*num = 100; 
      
// de-allocated the space allocated to num 
free(num); 
      
// num is already freed there for it cause segmentation fault
*num = 110; 

• Stack Overflow -: Running out of memory on the stack
• Accessing an array out of bounds'
• Use wrong format specifiers when using printf() and scanf()'

> In computing, a segmentation fault or access violation is a fault, or failure condition, raised by hardware with memory protection, > notifying an operating system the software has attempted to access a > restricted area of memory. -WIKIPEDIA

You might be accessing the computer memory with the wrong data type. Your case might be like the code below:

#include <stdio.h>

int main(int argc, char *argv[]) {
	
	char A = 'asd';
	puts(A);
	
	return 0;
	
}

'asd' -> is a character chain rather than a single character char data type. So, storing it as a char causes the segmentation fault. Stocking some data at the wrong position.

Storing this string or character chain as a single char is trying to fit a square peg in a round hole.

Terminated due to signal: SEGMENTATION FAULT (11)

Segm. Fault is the same as trying to breath in under water, your lungs were not made for that. Reserving memory for an integer and then trying to operate it as another data type won't work at all.

A segmentation fault or access violation occurs when a program attempts to access a memory location that is not exist, or attempts to access a memory location in a way that is not allowed.

 /* "Array out of bounds" error 
   valid indices for array foo
   are 0, 1, ... 999 */
   int foo[1000];
   for (int i = 0; i <= 1000 ; i++) 
   foo[i] = i;

Here i[1000] not exist, so segfault occurs.

Causes of segmentation fault:

it arise primarily due to errors in use of pointers for virtual memory addressing, particularly illegal access.

De-referencing NULL pointers – this is special-cased by memory management hardware.

Attempting to access a nonexistent memory address (outside process’s address space).

Attempting to access memory the program does not have rights to (such as kernel structures in process context).

Attempting to write read-only memory (such as code segment).

A segmentation fault (sometimes known as a segfault) happens when your program tries to access memory that it is not permitted to access.In other words, when your program attempts to access memory that exceeds the boundaries set by the operating system for your program.And it is a common circumstance that causes programs to crash; it is frequently related with a file called core.

Program memory is divided into different segments:

• a text segment for program instructions
• a data segment for variables and arrays defined at compile time
• a stack segment for temporary (or automatic) variables defined in subroutines and functions
• a heap segment for variables allocated during runtime by functions, such as malloc (in C) and allocate (in Fortran).

When a reference to a variable falls beyond the segment where that variable exists, or when a write is attempted to a place that is in a read-only segment, a segfault occurs. In reality, segfaults are nearly typically caused by attempting to read or write a non-existent array member, failing to correctly define a pointer before using it, or (in C applications) inadvertently using the value of a variable as an address (see the scan example below).

*Calling memset(), for example, would cause a program to segfault:

memset((char *)0x0, 1, 100);

*The three examples below show the most frequent sorts of array-related segfaults:

Case A

/* "Array out of bounds" error valid indices for array foo are 0, 1, ... 999 */
int foo[1000]; for (int i = 0; i <= 1000 ; i++) foo[i] = i;

Case B

/* Illegal memory access if value of n is not in the range 0, 1, ... 999 */ 
int n; int foo[1000]; for (int i = 0; i < n ; i++) foo[i] = i;

Case C

/* Illegal memory access because no memory is allocated for foo2 */
float *foo, *foo2; foo = (float*)malloc(1000); foo2[0] = 1.0;

• In case A, array foo is defined for index = 0, 1, 2, ... 999. However, in the last iteration of the for loop, the program tries to access foo[1000]. This will result in a segfault if that memory location lies outside the memory segment where foo resides. Even if it doesn't cause a segfault, it is still a bug.
• In case B, integer n could be any random value. As in case A, if it is not in the range 0, 1, ... 999, it might cause a segfault. Whether it does or not, it is certainly a bug.
• In case C, allocation of memory for variable foo2 has been overlooked, so foo2 will point to a random location in memory. Accessing foo2[0] will likely result in a segfault.

*Another typical programming issue that causes segfaults is a failure to use pointers properly. The C function scanf(), for example, requires the address of a variable as its second parameter; hence, the following will very certainly cause the program to fail with a segfault:

int foo = 0; scanf("%d", foo); 
/* Note missing & sign ; correct usage would have been &foo */

Although the variable foo may be created at memory position 1000, the preceding function call would attempt to read integer values into memory location 0 in accordance with the definition of foo.

A segfault occurs when a software attempts to operate on a memory region in an unauthorized manner (for example, attempts to write a read-only location would result in a segfault).When your application runs out of stack space, segfaults can occur. This might be due to your shell setting the stack size limit too low, rather than a fault in your software.

Dangling Pointers point to something that no longer exists. A dangling pointer is an example of this.

char *ptr = NULL;
{
char c;
ptr = &c; //After the block is over, ptr will be a dangling pointer.
}

When the block concludes, the scope of variable c expires. Because it now points to something that doesn't exist, the 'ptr' will become a dangling pointer.

But when you try to access memory that doesn't belong to you or when you try to write to a read-only area, you get a segmentation fault.

char *str ="Testing Seg fault.";
*str= "I hate Seg fault :( ";

The'str' will be made a constant by the compiler. You are altering the read-only part when you try to update the value, resulting in a segmentation fault.So there's a clear distinction between a segmentation fault and dangling pointers.