What is SIGSEGV run time error in C++?

问题:

I need to know the root cause of the segmentation fault, and also can anyone tell me how to handle it.

回答1:

Wikipedia has the answer, along with a number of other sources.

A segfault basically means you did something bad with pointers. This is probably a segfault:

char *c = NULL;
...
*c; // dereferencing a NULL pointer

Or this:

char *c = "Hello";
...
c[10] = 'z'; // out of bounds, or in this case, writing into read-only memory

Or maybe this:

char *c = new char[10];
...
delete [] c;
...
c[2] = 'z'; // accessing freed memory

Same basic principle in each case - you're doing something with memory that isn't yours.

回答2:

There are various causes of segmentation faults, but fundamentally, you are accessing memory incorrectly. This could be caused by dereferencing a null pointer, or by trying to modify readonly memory, or by using a pointer to somewhere that is not mapped into the memory space of your process (that probably means you are trying to use a number as a pointer, or you incremented a pointer too far). On some machines, it is possible for a misaligned access via a pointer to cause the problem too - if you have an odd address and try to read an even number of bytes from it, for example (that can generate SIGBUS, instead).

回答3:

using an invalid/null pointer? Overrunning the bounds of an array? Kindof hard to be specific without any sample code.

Essentially, you are attempting to access memory that doesn't belong to your program, so the OS kills it.

回答4:

Here is an example of SIGSEGV.

root@pierr-desktop:/opt/playGround# cat test.c
int main()
{
     int * p ;
     * p = 0x1234;
     return 0 ;
}
root@pierr-desktop:/opt/playGround# g++ -o test test.c  
root@pierr-desktop:/opt/playGround# ./test 
Segmentation fault

And here is the detail.

How to handle it?

1. Avoid it as much as possible in the first place.

   Program defensively: use assert(), check for NULL pointer , check for buffer overflow.

   Use static analysis tools to examine your code.

   compile your code with -Werror -Wall.

   Has somebody review your code.

2. When that actually happened.

   Examine you code carefully.

   Check what you have changed since the last time you code run successfully without crash.

   Hopefully, gdb will give you a call stack so that you know where the crash happened.

---

EDIT : sorry for a rush. It should be *p = 0x1234; instead of p = 0x1234;

回答5:

segmentation fault arrives when you access memory which is not declared by the program. You can do this through pointers i.e through memory addresses. Or this may also be due to stackoverflow for eg:

void rec_func() {int q = 5; rec_func();}

int main() {rec_func();}

This call will keep on consuming stack memory until it's completely filled and thus finally stackoverflow happens. Note: it might not be visible in some competitive questions as it leads to timeouterror first but for those in which timeout doesn't happens its a hard time figuring out sigsemv.