Segmentation fault (core dumped) là gì

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A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to lớn access, or attempts to access a memory location in a way that is not allowed (for example, attempting khổng lồ write lớn a read-only location, or to lớn overwrite part of the operating system).Segmentation is one approach to memory management & 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 It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example. Some operating systems still have sầu segmentation at some logical màn chơi although paging is used as the main memory management policy.On Unix-like operating systems, a signal called SIGSEGV is sent lớn a process that accesses an invalid memory address. On Microsoft Windows, a process that accesses invalid memory receives the STATUS_ACCESS_VIOLATION exception. Common causesA few causes of a segmentation fault can be summarized as follows: * attempting to lớn exexinh đẹp a program that does not compile correctly. lưu ý that most compilers will not output a binary given a compile-time error. * a buffer overflow. * using uninitialized pointers. * dereferencing NULL pointers. * attempting to lớn access memory the program does not own. * attempting to lớn alter memory the program does not own (storage violation).Generally, segmentation faults occur because: a pointer is either NULL, points to lớn random memory (probably never initialized to lớn anything), or points to memory that has been freed/deallocated/"deleted".e.g. char *p1 = NULL; // Initialized to null, which is OK, // (but cannot be dereferenced on many systems). char *p2; // Not initialized at all. char *p3 = new char; // Great! it"s allocated, delete p3; // but now it isn"t anymore.Now, dereferencing any of these variables could cause a segmentation fault. Examples Bus error exampleThis is an example of un-aligned memory access, written in the C programming language.#include int main(int argc, char **argv) int *iptr; char *cptr; #if defined(__GNUC__)# if defined(__i386__) /* Enable Alignment Checking on x86 */ __asm__("pushf orl $0x40000,(%esp) popf");# elif defined(__x86_64__) /* Enable Alignment Checking on x86_64 */ __asm__("pushf orl $0x40000,(%rsp) popf");# endif#endif /* malloc() always provides aligned memory */ cptr = malloc(sizeof(int) + 1); /* Increment the pointer by one, making it misaligned */ iptr = (int *) ++cptr; /* Dereference it as an int pointer, causing an unaligned access */ *iptr = 42; return 0;Compiling and running the example on Linux on x86 demonstrates the error:$ gcc -anyêu thích sigbus.c -o sigbus$ ./sigbus Bus error$ gdb ./sigbus(gdb) rProgram received signal SIGBUS, Bus error.0x080483tía in main ()(gdb) x/i $pc0x80483tía : mov DWORD PTR ,0x2a(gdb) p/x $eax$1 = 0x804a009(gdb) p/t $eax và (sizeof(int) - 1)$2 = 1The GDB debugger shows that the immediate value 0x2a is being stored at the location stored in the EAX register, using X86 assembly language. This is an example of register indirect addressing.Printing the low order bits of the address shows that it is not aligned lớn a word boundary ("dword" using x86 terminology). Segmentation fault exampleHere is an example of ANSI C code that should create a segmentation fault on platforms with memory protection: int main(void) char *s = "hello world"; *s = "H"; When the program containing this code is compiled, the string "hello world" is placed in the section of the program binary marked as read-only; when loaded, the operating system places it with other strings & constant data in a read-only segment of memory. When executed, a variable, s, is mix to lớn point khổng lồ the string"s location, & an attempt is made to write an H character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, và running it on a Unix-like operating system produces the following runtime error:$ gcc segfault.c -g -o segfault$ ./segfaultSegmentation faultBacktrace from gdb:Program received signal SIGSEGV, Segmentation fault.0x1c0005c2 in main () at segfault.c:66 *s = "H";The conditions under which segmentation violations occur và how they manifest themselves are specific lớn an operating system.Because a very comtháng program error is a null pointer dereference (a read or write through a null pointer, used in C khổng lồ mean "pointer khổng lồ no object" và as an error indicator), most operating systems bản đồ the null pointer"s address such that accessing it causes a segmentation fault. int *ptr = NULL; *ptr = 1;This sample code creates a null pointer, & tries to lớn assign a value to its non-existent target. Doing so causes a segmentation fault at runtime on many operating systems.Another example is recursion without a base case: int main(void) main(); which causes the staông xã khổng lồ overflow which results in a segmentation fault.