Segfault error in linux

How to catch segmentation fault in linux?

A segmentation fault, also known as a segfault, is a common error in Linux and other Unix-based operating systems. It occurs when a program attempts to access a memory address that it is not allowed to access, either because it is trying to access memory that it has not been allocated, or because it has exceeded the bounds of a memory buffer. In either case, the result is a crash, and the program is terminated by the operating system. If you are developing software in C++ on Linux, it is important to know how to handle and catch these errors, so that you can diagnose the cause and prevent future crashes.

Method 1: Signal Handlers

To catch a segmentation fault in C++ on Linux using signal handlers, you can use the sigaction function to set up a handler for the SIGSEGV signal. Here are the steps to do this:

1. Define a signal handler function that will be called when the SIGSEGV signal is raised. This function should take an integer argument that represents the signal number.

void segfault_handler(int signal_number) < std::cerr 

1. In your main function, set up the signal handler using sigaction . This function takes a pointer to a struct sigaction that describes the action to be taken when the signal is raised. Here, we set the sa_handler field to our segfault_handler function.

1. Now, if a segmentation fault occurs during the execution of your program, the segfault_handler function will be called. You can add any additional handling that you need to this function, such as logging or cleanup.

Note that it is generally not a good idea to try to recover from a segmentation fault in your program. It usually indicates a serious error that should be fixed in the code. However, catching the signal can be useful for logging or other diagnostic purposes.

Method 2: Core Dumps

To catch a segmentation fault in C++ on Linux, you can use core dumps. Core dumps are files that contain the memory image of a program at the time it crashed. You can use a debugger to analyze the core dump and find the cause of the segmentation fault.

Here are the steps to enable core dumps and analyze them:

This command sets the maximum size of core dump files to unlimited.

The -g option tells the compiler to include debugging symbols in the executable.

1. Run the program and wait for it to crash.
2. When the program crashes, a core dump file will be created in the current directory. The filename will be core. , where is the process ID of the crashed program.
3. Use a debugger to analyze the core dump. For example, you can use GDB:

This will start GDB and load the core dump file. You can use GDB commands to analyze the memory image and find the cause of the segmentation fault.

Here is an example program that intentionally causes a segmentation fault:

#include int main()  int* ptr = nullptr; *ptr = 42; // This will cause a segmentation fault std::cout  <"Hello, world!"  ::endl; return 0; >

When you run this program with core dumps enabled and analyze the resulting core dump file with GDB, you will see that the segmentation fault was caused by a null pointer dereference.

Method 3: Valgrind

To catch segmentation faults in C++ on Linux, you can use the Valgrind tool. Here are the steps to do so:

Install Valgrind on your Linux system using your package manager. For example, on Ubuntu, you can use the following command:

sudo apt-get install valgrind

g++ -g my_program.cpp -o my_program

==12345== Invalid read of size 4 ==12345== at 0x4005E5: my_function (my_program.cpp:10) ==12345== by 0x4006A2: main (my_program.cpp:20) ==12345== Address 0x0 is not stack'd, malloc'd or (recently) free'd

Here is an example C++ program that causes a segmentation fault:

#include int main()  int* p = nullptr; *p = 42; std::cout  <*p  ::endl; return 0; >

When run with Valgrind, the output will be:

==12345== Invalid write of size 4 ==12345== at 0x4005C5: main (my_program.cpp:5) ==12345== Address 0x0 is not stack'd, malloc'd or (recently) free'd

This output indicates that there was an invalid write of size 4 in main at line 5 of my_program.cpp , which attempted to write to a null pointer.

Method 4: Address Sanitizer (ASan)

To catch segmentation faults in C++ on Linux using Address Sanitizer (ASan), you can follow these steps:

sudo apt-get install build-essential llvm clang libclang-dev

clang++ -g -fsanitize=address -fno-omit-frame-pointer -o executable_name source_file.cpp

1. Run your program as usual. If a segmentation fault occurs, ASan will print a detailed error message with a stack trace.

Here's an example program that will trigger a segmentation fault:

Compile and run this program with ASan enabled:

clang++ -g -fsanitize=address -fno-omit-frame-pointer -o segfault_example segfault_example.cpp ./segfault_example

==15757==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000401c6a bp 0x7ffc6e1f6c60 sp 0x7ffc6e1f6c50 T0) ==15757==The signal is caused by a READ memory access. ==15757==Hint: address points to the zero page. #0 0x401c69 in main (/path/to/segfault_example+0x401c69) #1 0x7f4f9f9c82e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082e0) #2 0x401b39 in _start (/path/to/segfault_example+0x401b39) AddressSanitizer can not provide additional info. SUMMARY: AddressSanitizer: SEGV (/path/to/segfault_example+0x401c69) in main ==15757==ABORTING

ASan has detected the segmentation fault and provided a detailed error message with a stack trace, including the line number in the source code where the error occurred.

In summary, using Address Sanitizer (ASan) is an effective way to catch segmentation faults in C++ on Linux. By following the steps outlined above and compiling your code with the -fsanitize=address flag, ASan will provide detailed error messages with stack traces when a segmentation fault occurs.

Method 5: Memory Pool Allocators

Memory pool allocators are a useful tool for managing memory in C++ programs. They can also be used to catch segmentation faults, which occur when a program attempts to access memory that it is not allowed to access. Here's how to catch segmentation faults in C++ using memory pool allocators:

Step 1: Include the Necessary Libraries

#include #include #include #include #include #include

Step 2: Define the Memory Pool Allocator

template class MemoryPoolAllocator < public: MemoryPoolAllocator(size_t size = 100) < pool_ = (T*)malloc(sizeof(T) * size); size_ = size; head_ = 0; >~MemoryPoolAllocator() < free(pool_); >T* allocate() < if (head_ >= size_) < return nullptr; >return &pool_[head_++]; > void deallocate(T* ptr) < if (ptr == nullptr) < return; >if (ptr < pool_ || ptr >= pool_ + size_) < return; >head_--; > private: T* pool_; size_t size_; size_t head_; >;

Step 3: Define the Signal Handler

void signal_handler(int sig) < std::cerr free(symbols); _exit(1); >

Step 4: Set the Signal Handler

Step 5: Use the Memory Pool Allocator

int main() < set_signal_handler(); MemoryPoolAllocatorallocator; int* ptr = allocator.allocate(); *ptr = 42; // Access memory out of bounds *(ptr + 1) = 43; allocator.deallocate(ptr); return 0; >

When you run this program, it will catch the segmentation fault and print out a stack trace. This can be useful for debugging your program and identifying the source of the error.

Note that this is just one way to catch segmentation faults in C++. There are other methods available, such as using try-catch blocks or using tools like Valgrind. However, memory pool allocators can be a useful tool for managing memory and can also be used to catch segmentation faults.

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Interpreting segfault messages

This is a segfault due to following a null pointer trying to find code to run (that is, during an instruction fetch).

If this were a program, not a shared library

Run addr2line -e yourSegfaultingProgram 00007f9bebcca90d (and repeat for the other instruction pointer values given) to see where the error is happening. Better, get a debug-instrumented build, and reproduce the problem under a debugger such as gdb.

Since it's a shared library

You're hosed, unfortunately; it's not possible to know where the libraries were placed in memory by the dynamic linker after-the-fact. Reproduce the problem under gdb .

What the error means

Here's the breakdown of the fields:

• address (after the at ) - the location in memory the code is trying to access (it's likely that 10 and 11 are offsets from a pointer we expect to be set to a valid value but which is instead pointing to 0 )
• ip - instruction pointer, ie. where the code which is trying to do this lives
• sp - stack pointer
• error - An error code for page faults; see below for what this means on x86 (link).

/* * Page fault error code bits: * * bit 0 == 0: no page found 1: protection fault * bit 1 == 0: read access 1: write access * bit 2 == 0: kernel-mode access 1: user-mode access * bit 3 == 1: use of reserved bit detected * bit 4 == 1: fault was an instruction fetch * bit 5 == 1: protection keys block access * bit 15 == 1: SGX MMU page-fault */ 

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What is a segmentation fault?

In Ubuntu I have faced the segmentation fault error many times. What is a segmentation fault and when does it occur?

Just to cover it up: I had a similar problem, whereas my segfaults were not reliably reproduceable and they came from (almost) random applications. Found out that most likely my memory is broken. So if quite any program causes segfaults, one might have a closer look at the RAM.

3 Answers 3

An error saying segmentation fault (or segfault, or SIGSEGV) in Ubuntu and other Unix-like operating systems, or saying general protection fault in Windows, is when a program attempts to access a part of memory that cannot be accessed, or which the program is prohibited from accessing. A segmentation fault is a kind of program crash, that is, an abnormal termination of a program. See the Wikipedia articles on crashes, memory protection, segmentation fault, general protection fault, and SIGSEGV for more information (and a more textured understanding of the topic than is presented here).

A segmentation fault is almost always due to a bug in the program where it occurs. I am guessing most or all of your segmentation faults are happening from the same application. Please provide more details about the circumstances under which segmentation faults are happening on your machine, and what program is crashing. Please also provide the full and exact text of the error message you're receiving and any other messages that appear before it. This should make it possible for us to provide detailed advice specific to your problem (rather than just general information about what a segmentation fault is).

The best way for you to provide this information is for you to edit your question to include it. Alternatively, if you want this question to be just about segmentation faults in general, you could post a new question to ask about what specifically is causing your segmentation faults (if you do this, make sure to provide all these details in your new question).

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