Segmentation fault

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A segmentation fault (often shortened to segfault) is a particular error condition that can occur during the operation of computer software. 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). Systems based on processors like the Motorola 68000 tend to refer to these events as address or bus errors.

Segmentation is one approach to memory management and protection in the operating system. It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example. Some operating systems still have segmentation at some logical level although paging is used as the main memory management policy.

On Unix-like operating systems, a process that accesses an invalid memory address receives the SIGSEGV signal. On Microsoft Windows, a process that accesses invalid memory receives the STATUS_ACCESS_VIOLATION exception.

[edit] Example

Here is an example of ANSI C code that should create a segmentation fault on platforms with memory protection:

 int main(){
     char *s = "hello world";
     *s = 'H';
 }

When the program containing this code is compiled, the string "hello world" is placed in the section of the program binary marked as read-only; when loaded, the operating system places it with other strings and constant data in a read-only segment of memory. When executed, a variable, s, is set to point to the string's location, and an attempt is made to write an H character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, and running it on a Unix-like operating system, produces the following runtime error:

$ gcc segfault.c -g -o segfault
$ ./segfault
Segmentation fault

Backtrace from gdb:

Program received signal SIGSEGV, Segmentation fault.
0x1c0005c2 in main () at segfault.c:6
6               *s = 'H';

The conditions under which segmentation violations occur and how they manifest themselves are specific to an operating system.

/* C++ way */
#include <iostream>
using namespace std;
int main(){
	char *s = "hello world";
	cout << s << endl;
	*s = 'H';
	cout << s << endl;
	return 0;
}

/* C way */
#include <stdio.h>
int main(){
	char *s = "hello world";
	printf("%s\n", s);
	*s = 'H';
	printf(s);
	return 0;
}

On Windows machines the "read/write" operations on hard-coded text string are available. System does not see anything wrong within the above code. Just the first letter of a text string "hello world" is replaced with another letter 'H' (for both C & C++). No memory violation occurs. But, if you start to move the 's' pointer forward (like using s++ directive inside a loop) and you pass over the final NULL value, that ends up each text string (including "Hello World"), then if you try to write another letter, you should receive the "Segmentation fault error".

     /* Move 20 characters forward... */
     s = s + 20;
     *s = 'H';

(Used Borland's Free Command Line Tools for tests.)

Because a very common program error is a null pointer dereference (a read or write through the null pointer, a null pointer, used in C to mean "pointer to no object" and as an error indicator), most operating systems map the null pointer's address such that accessing it causes a segmentation fault.

 int* ptr = NULL;
 *ptr = 1;

This sample code creates a null pointer, and tries to assign a value to its non-existent target. Doing so causes a segmentation fault at runtime on many operating systems.

Another way to cause a segmentation fault is to recurse without a base case, which causes a stack overflow:

 int main(){
    main();
}

A few causes of segmentation fault can be summarized as follows,

(1) attempt to execute a program that does not compile correctly. Note that most compilers will not output a binary given a compile-time error.

(2) buffer overruns

(3) using uninitialized pointers

(4) dereference NULL pointers

(5) attempt to address memory the program does now own.

(6) failure to check for data validity before using it.

(7) failure to check for file open success before attempting to use the file pointer.

Generally, segmentation faults are because a pointer of yours is either NULL, or points to random memory (probably never initialized to anything), or points to memory that was deleted.

e.g.

   char* p1 = NULL;   // initialized to null, which is good but not dereferencable on many systems
   char* p2;  // not initialized at all
   char* p3  = new char[20];  // great, it's allocated
   delete [] p3; // but now it isn't anymore

Now, referencing any of these variables could cause a segmentation fault.

[edit] See also

• Buffer overflow
• Bus error
• Core dump
• General protection fault
• Segfault, a humor website named after this error.

[edit] External links

• A FAQ: User contributed answers regarding the definition of a segmentation fault
• A "null pointer" explained
• Answer to: NULL is guaranteed to be 0, but the null pointer is not?