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What is Segmentation Fault?

by nikoo28
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From 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 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 It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, “segmentation fault” being an example.

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.

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 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 the 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 de-reference 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 any more, like here:

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

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

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