There are many ways to get a segmentation fault, at least in the lower-level languages such as C(++). A common way to get a segmentation fault is to dereference a null pointer:
int *p = NULL;Segmentation fault also happens when you try to write to a portion of memory that was marked as read-only:
*p = 1;
char *str = "Foo"; // Compiler marks the constant string as read-onlyAccessing Dangling pointer also causes segmentation faults like here:
*str = 'b'; // Which means this is illegal and results in a segfault
char *p = NULL;The pointer p dangles because it points to 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 segmentation fault.
{
char c;
p = &c;
}
// Now p is dangling
Another reason for segmentation fault is to recurse without a base case, which causes a stack overflow:
int main() { main(); }
Segmentation fault also happens when accessing a region of memory already freed
Whenever segmentation faults happens, a core is generated by the process in the current directory. By default, most of the Linux distros disable the creation of core. So if you dont find the core file run the following command to enable the creation of core files.
$ ulimit -c unlimitedGenerally core files are created as core.<pid>, pid is the pid of the process that created the core.
Running the above command will set ulimit to unlimited on that terminal. So when you start a new terminal it would not be set. So to make it default in all the new terminals add it into ~/.bashrc.
But to make sense out of the core dump you must compile the program with debugging symbols. Lets take an example program to understand how to use the core dump to the fullest.
$ cat hello.c
#include <stdio.h>Now enable debugging symbols while compiling
int
main ()
{
int *p = NULL;
*p = 2;
return 0;
}
$ gcc -g -O0 hello.c -o helloNow when you run this program, you will receive the following error
$ ./helloYou will find that a core file is created in the current directory.
Segmentation fault (core dumped)
Now run the core with the debugger (on most of the distros gdb is provided if not install it)
$ gdb hello core.5123It will display the line in the program that caused the error. There are many other options that you can use with gdb to debug your issue, we will visit that in future posts.
Some of the best practices to follow while programming in C/C++ is to
- Always check for NULL before dereferencing the pointers. Example
if (ptr == NULL)
//return with error
- Also check for NULL after the call to dynamic memory allocation functions like malloc, calloc, new etc. Example
ptr = (int *) malloc (10 * sizeof (int));
if (ptr == NULL)
//return with error
References
* http://collectd.org/wiki/index.php/Core_file
* http://stackoverflow.com/questions/2346806/what-is-segmentation-fault
* http://en.wikipedia.org/wiki/Segmentation_fault
