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Determining the root cause – debugging the bug – can be simple in some cases, where the program will consistently cause a segmentation fault (e.g., dereferencing a null pointer), while in other cases the bug can be difficult to reproduce and depend on memory allocation on each run (e.g., dereferencing a dangling pointer).
LLDB supports debugging of programs written in C, ... Since the example code, when executed, generates a segmentation fault, lldb can be used to inspect the problem:
(gdb) run Starting program: /path/example Program received signal SIGSEGV, Segmentation fault. 0x0000000000400527 in foo_len (s=0x0) at example.c:7 7 return strlen (s); (gdb) print s $ 1 = 0x0 The problem is present in line 7, and occurs when calling the function strlen (because its argument, s , is NULL ).
Protection may encompass all accesses to a specified area of memory, write accesses, or attempts to execute the contents of the area. An attempt to access unauthorized [a] memory results in a hardware fault, e.g., a segmentation fault, storage violation exception, generally causing abnormal termination of the offending process.
The Unix kernel maintains internal consistency and runtime correctness with assertions as the fault detection mechanism. The basic assumption is that the hardware and the software should perform correctly and a failure of an assertion results in a panic, i.e. a voluntary halt to all system activity. [5]
If the memory has been reallocated to another process, then attempting to dereference the dangling pointer can cause segmentation faults (UNIX, Linux) or general protection faults (Windows). If the program has sufficient privileges to allow it to overwrite the bookkeeping data used by the kernel's memory allocator, the corruption can cause ...
On x86 there exists an older memory management mechanism known as segmentation. If the application loads a segment register with the selector of a non-present segment (which under POSIX-compliant OSes can only be done with assembly language), the exception is generated. Some OSes used that for swapping, but under Linux this generates SIGBUS.
Failures to adhere to this pattern, such as memory usage after a call to free (dangling pointer) or before a call to malloc (wild pointer), calling free twice ("double free"), etc., usually causes a segmentation fault and results in a crash of the program. These errors can be transient and hard to debug – for example, freed memory is usually ...