Segmentation fault - Wikipedia. In computing, a segmentation fault (often shortened to segfault) or access violation is a fault raised by hardware with memory protection, notifying an operating system (OS) about a memory access violation; on x. The OS kernel will, in response, usually perform some corrective action, generally passing the fault on to the offending process by sending the process a signal. Processes can in some cases install a custom signal handler, allowing them to recover on their own. They arise primarily due to errors in use of pointers for virtual memory addressing, particularly illegal access. Another type of memory access error is a bus error, which also has various causes, but is today much rarer; these occur primarily due to incorrect physical memory addressing, or due to misaligned memory access . For example, the Rust programming language employs an 'Ownership'. Thus attempting to read outside of the program's address space, or writing to a read- only segment of the address space, results in a segmentation fault, hence the name. On systems using hardware memory segmentation to provide virtual memory, a segmentation fault occurs when the hardware detects an attempt to refer to a non- existent segment, or to refer to a location outside the bounds of a segment, or to refer to a location in a fashion not allowed by the permissions granted for that segment. On systems using only paging, an invalid page fault generally leads to a segmentation fault, and segmentation faults and page faults are both faults raised by the virtual memory management system. Segmentation faults can also occur independently of page faults: illegal access to a valid page is a segmentation fault, but not an invalid page fault, and segmentation faults can occur in the middle of a page (hence no page fault), for example in a buffer overflow that stays within a page but illegally overwrites memory. At the hardware level, the fault is initially raised by the memory management unit (MMU) on illegal access (if the referenced memory exists), as part of its memory protection feature, or an invalid page fault (if the referenced memory does not exist). If the problem is not an invalid logical address but instead an invalid physical address, a bus error is raised instead, though these are not always distinguished. At the operating system level, this fault is caught and a signal is passed on to the offending process, activating the process's handler for that signal. Different operating systems have different signal names to indicate that a segmentation fault has occurred. On Unix- like operating systems, a signal called SIGSEGV (abbreviated from segmentation violation) is sent to the offending process. On Microsoft Windows, the offending process receives a STATUS. The proximate cause is a memory access violation, while the underlying cause is generally a software bug of some sort. Determining the root cause . GDB: Program received signal: '0' & Memory Leak. Tue Aug 25 21:52:15 unknown SpringBoard. I've now got it compiling for OS 3.0 in GCC 4.2 with 0 errors!
Dereferencing a null pointer will always result in a segmentation fault, but wild pointers and dangling pointers point to memory that may or may not exist, and may or may not be readable or writable, and thus can result in transient bugs. For example: char*p. NULL; // Null pointerchar*p. Wild pointer: not initialized at all. Initialized pointer to allocated memory// (assuming malloc did not fail)free(p. Now, dereferencing any of these variables could cause a segmentation fault: dereferencing the null pointer generally will cause a segfault, while reading from the wild pointer may instead result in random data but no segfault, and reading from the dangling pointer may result in valid data for a while, and then random data as it is overwritten. Handling. A core file may be generated to aid debugging, and other platform- dependent actions may also be performed. For example, Linux systems using the grsecurity patch may log SIGSEGV signals in order to monitor for possible intrusion attempts using buffer overflows. Examples. At the level of code errors, this occurs when the program writes to part of its own code segment or the read- only portion of the data segment, as these are loaded by the OS into read- only memory. Here is an example of ANSI C code that will generally cause a segmentation fault on platforms with memory protection. It attempts to modify a string literal, which is undefined behavior according to the ANSI C standard. 0 scribble deallocated CF memory 1. 1 level 0 group 0x76E0BFE9 'com.apple.hitoolbox.windowgroups.root' 2 level 0 group 0x76E47A89 'com.apple.HIToolbox. Most compilers will not catch this at compile time, and instead compile this to executable code that will crash: intmain(void). 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. Segmentation fault. Backtrace of the core file from GDB: Programreceivedsignal. SIGSEGV,Segmentationfault. H'; This code can be corrected by using an array instead of a character pointer, as this allocates memory on stack and initializes it to the value of the string literal: chars. Doing so causes a segmentation fault at runtime on many operating systems. Dereferencing a null pointer and then assigning to it (writing a value to a non- existent target) also usually causes a segmentation fault: The following code includes a null pointer dereference, but when compiled will often not result in a segmentation fault, as the value is unused and thus the dereference will often be optimized away by dead code elimination: Buffer overflow. In this case, the behavior of unreachable code (the return statement) is undefined, so the compiler can eliminate it and use a tail call optimization that might result in no stack usage. Other optimizations could include translating the recursion into iteration, which given the structure of the example function would result in the program running forever, while probably not overflowing its stack. Anyone else seeing this? Mainly happens when dismissing a send level view controller. I am not using IB to construct my UIView's.
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