segfault pointer assignment

Understanding and Solving Segfaults in C: A Comprehensive Guide

Abstract: Explore the concept of segfaults in C programming, learn how to identify and solve them using tools like gdb and valgrind. Even the simplest program can segfault, and it's important to understand why. #include #include typedef struct object { int x; } object;

Understanding and Solving Segfaults: A Comprehensive Guide

In the world of programming, there are few things more frustrating than dealing with a segmentation fault, or "segfault". These errors can be difficult to understand and even more difficult to solve. However, with a solid understanding of what causes segfaults and how to approach solving them, you'll be well-equipped to tackle these issues head-on.

What is a Segmentation Fault?

A segmentation fault is a type of error that occurs when a program attempts to access memory that it does not have permission to access. This can happen for a variety of reasons, such as trying to access memory that has already been freed, or attempting to access a memory location that is outside of the bounds of the array. When a segfault occurs, the operating system will send a SIGSEGV signal to the offending process, causing it to terminate.

Common Causes of Segfaults

There are several common causes of segmentation faults, including:

• Accessing memory that has already been freed
• Accessing a memory location outside of the bounds of an array
• Dereferencing a null pointer
• Using a dangling pointer

How to Debug Segfaults

Debugging segfaults can be challenging, but there are several steps you can take to make the process easier:

• Reproduce the error: In order to debug a segfault, you first need to be able to reproduce it. This means running the program and observing the error. If you're unable to reproduce the error, it will be very difficult to figure out what's causing it.
• Use a debugger: A debugger is a tool that allows you to step through your code and inspect variables as you go. This can be incredibly helpful when debugging segfaults, as it allows you to see exactly where the error is occurring and what variables are involved.
• Check for null pointers: Dereferencing a null pointer is a common cause of segfaults. Make sure that all of your pointers are properly initialized and that you're not trying to dereference a null pointer.
• Check for array bounds: Accessing memory outside of the bounds of an array is another common cause of segfaults. Make sure that you're not trying to access memory that is outside of the array.
• Check for memory leaks: If your program is leaking memory, it can lead to segfaults. Use a memory profiling tool to check for memory leaks and fix any that you find.

Example of a Simple Program that Segfaults

Here's an example of a simple C program that will segfault:

In this example, we first allocate memory for an object and assign a value to it. We then free the memory and attempt to assign a new value to the object. However, since we've already freed the memory, this will cause a segfault.

Segmentation faults can be frustrating, but with a solid understanding of what causes them and how to debug them, you'll be well-equipped to tackle these errors head-on. By following the steps outlined in this guide, you'll be able to quickly and efficiently identify and fix segfaults in your code.

Debugging: The 9 Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems , by David J. Agans

The Art of Debugging with GDB, DDD, and Eclipse , by Norman Matloff and Peter Jay Salzman

Segfault Handling , from the GNU C Library Manual

How to Debug a Segmentation Fault , from the Red Hat Developer Blog

Online Resources:

Segfault , a Stack Exchange site dedicated to debugging and fixing segmentation faults

Valgrind , a powerful memory profiling tool that can help you detect and fix memory leaks and other memory-related issues

This article has a minimum of 800 words.

Tags: :  C programming Debugging Software Development

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Posted on Oct 27, 2023

How to Debug Segmentation Fault in C++

Hello, Dev.to community! Encountering a segmentation fault in C++ can be daunting, but fear not! Debugging this common error is much more manageable when you know the right tools and techniques. In this article, we'll explore what causes segmentation faults and how to diagnose them.

What is a Segmentation Fault?

A segmentation fault (or segfault ) occurs when your program tries to read or write to a memory location it's not allowed to access. Common causes include dereferencing a NULL pointer, accessing an array out of its bounds, or using memory that has already been freed.

Steps to Debug a Segmentation Fault

1. Compiler Warnings

Before diving deep into debugging, ensure that you compile your program with warnings enabled:

Often, the compiler can provide hints or even directly point out problematic areas in the code.

2. Use a Debugger (like GDB) The GNU Debugger ( GDB ) is a powerful tool for debugging C++ programs:

Compile your program with debugging information:

Run your program inside GDB:

If a segmentation fault occurs, GDB will provide the line number and function where the segfault happened. This information is instrumental in identifying the root cause.

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3. Inspect Stack Trace When your program crashes inside GDB, you can view the stack trace with:

This command provides a breakdown of function calls leading up to the crash, helping you trace back to the problematic code.

4. AddressSanitizer AddressSanitizer is a runtime memory error detector. It's beneficial for detecting out-of-bounds accesses and use-after-free bugs:

Compile your program with AddressSanitizer :

Run your program:

If there are any memory violations, AddressSanitizer will provide detailed reports, making it easier to pinpoint and resolve the issues.

• Code Review and Simplification If you're still unable to locate the issue, simplify your code:

Comment out sections of your code to isolate the problem area. Double-check array indices, pointer initializations, and dynamic memory allocations/deallocations. Ensure all memory accessed is properly initialized. Conclusion While segmentation faults can initially seem mystifying, with the right tools and strategies, they become much more approachable. Remember always to compile with warnings, use debuggers like GDB, leverage tools like AddressSanitizer, and when in doubt, simplify and review your code.

Happy debugging and coding!

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Segmentation Fault in C/C++

Segmentation Fault, sometimes known as segfault , is a common error in C and C++ programming. Consider your program a neighborhood, with each memory segment representing a house. A segmentation fault occurs when a program attempts to access a memory location that it is not permitted to access, resulting in a crash. This might be caused by dereferencing a null pointer or accessing memory outside the limits of an array. Your program must respect memory boundaries to keep these unwanted guests out.

Common Causes of Segmentation Fault in C/C++

Segmentation faults, often known as segfaults, are feared failures in C/C++ programming that may leave even the most experienced coders scratching their heads. These errors arise when a program attempts to access a memory region it is not permitted to access, resulting in an abrupt termination. In this section, we'll solve the enigma of segmentation errors by delving into their core causes in general terms.

Dereferencing a Null Pointer :

Dereferencing a null pointer is among the most prevalent causes of segmentation problems. A null pointer points to nothing and holds memory addresses. When you try to access memory referred to by a null pointer, the system throws a segfault, prohibiting you from accessing invalid memory.

Buffer Overflow :

Buffer overflows occur when a program puts more data into a buffer than it can hold. This can overwrite neighboring memory regions, including critical control data, resulting in unexpected behavior and, eventually, a segmentation failure.

Stack Overflow :

A stack is used by every program to keep track of function calls and local variables. A stack overflow occurs when a function call or variable declaration utilizes more stack space than is available. As a result of the program's foray into prohibited memory areas, a segmentation fault occurs.

Accessing Out-of-Bounds Memory :

In C/C++, arrays and pointers lack built-in bounds checking. Accessing memory outside of an array's allotted space might cause a segfault. This is most commonly seen when iterating through an array and exceeds its size.

Read-Only Memory Access :

Attempting to change read-only memory (constants, string literals, etc.) results in a segmentation fault. This protection prevents unintentional alteration of data that should not be altered.

Uninitialized Pointers :

Using uninitialized pointers or addressing de-allocated memory might cause a segfault. Before accessing a pointer, it should always be allocated a proper memory address.

Mismatched Memory Management :

Combining various memory allocation and de-allocation routines (for example, malloc with free or new with delete ) might result in memory corruption and, ultimately, a segmentation fault.

Shared Library Issues :

Segfaults can also be caused by inconsistencies or incompatibilities in shared library versions. If your program uses external libraries, ensure they are compatible to avoid unexpected crashes.

Hardware Violations :

Hardware-related problems such as incorrect instructions or memory protection methods occasionally cause segmentation failures. While uncommon, these cases necessitate further examination.

Recursion without Base Case :

Omitting a base case in a recursive function can result in indefinite recursion and eventually stack Overflow, resulting in a segfault.

Common Segmentation Fault Scenarios

Let's look at some of the most typical segmentation fault scenarios developers experience to help us solve the problem.

Null Pointer Dereferencing

Attempting to access memory through a null pointer is a common problem. A segmentation fault occurs when a program attempts to read or write to an address with no value. This is frequently caused by failing to correctly initialize pointers before usage or accessing objects that have been destroyed or never created.

• Example : Dereferencing a null pointer, such as *ptr , is trying to access an address labeled nowhere , causing a runtime error or crash due to the absence of valid data at that location.

Array Index Out of Bounds

Arrays provide organized data storage, but accessing items beyond their assigned boundaries can be disastrous. Segmentation faults frequently occur when the program attempts to read or write to memory locations outside the array range. Vigilance in array indexing is crucial to prevent these errors.

Stack Overflow

The stack, a memory area utilized for function calls and local variables, has a finite space. A stack overflow might occur due to erroneous recursion or excessive local variable allocations. When the capacity of the stack is exceeded, the program receives a segmentation fault.

Buffer overflows occur when data written to a buffer exceeds its allocated size, encroaching into adjacent memory regions. These regions might hold critical data or code. When overwritten, data corruption occurs, leading to unintended program behavior, crashes, or even security vulnerabilities due to the execution of malicious code injected into adjacent memory areas.

Uninitialized Pointers

Using pointers that haven't been assigned a valid memory address can lead to unpredictable behavior, including segmentation faults. Avoiding such circumstances is made easier by properly initializing pointers before using them.

Read-Only Memory Access

To avoid unintentional change, modern operating systems frequently label particular portions of memory as read-only. A segmentation error occurs when a program attempts to write to a read-only memory area.

Shared Libraries and Dependencies

Memory access conflicts caused by mismatched or incompatible versions of shared libraries or dependencies can result in segmentation faults. Such problems may be avoided by using the correct library versions and dependencies.

Memory Leaks

Memory leaks can gradually deplete memory, resulting in segmentation errors. Unreleased memory fragments might cause the program to become unstable and crash.

Buffer Overflow

Buffer overflows occur when data written to a buffer surpasses its allocated size, spilling into adjacent memory regions. This can corrupt nearby data or overwrite critical control structures, potentially leading to crashes or security vulnerabilities. Exploiting this vulnerability, malicious actors can inject malicious code and gain unauthorized access or control over a program or system.

How to Fix Segmentation Faults?

While segmentation errors might be irritating, understanding their origins and adopting methodical debugging approaches can help you handle them efficiently.

Check for Uninitialized Pointers and Variables

Uninitialized pointers or variables are frequently the source of segmentation errors. Before attempting to access memory through pointers, ensure they are all correctly initialized. It's also critical to initialize variables to avoid problems.

Memory Allocation and De-allocation

Memory allocation and de-allocation errors can result in segmentation problems. Ensure that memory is adequately allocated using functions such as malloc or new and de-allocated using free or delete when no longer required. Using de-allocated memory might lead to unpredictable behavior and segmentation errors.

Array Bounds and Buffer Overflow

Accessing arrays outside of their limits can result in segmentation problems. Always check that array indices are in the correct range. Avoid buffer overflows, which can cause memory corruption and segmentation problems. To avoid these problems, use functions like strcpy and memcpy with caution.

Null Pointers and Dangling Pointers

Attempting to access or alter memory using null or dangling pointers might result in segmentation faults. Check for null pointers regularly and guarantee that pointers do not point to de-allocated memory.

Stack and Heap Confusion

Segmentation faults can also be caused by improper stack and heap memory management. Returning pointers to stack-allocated memory from a function should be avoided since this memory is invalidated when the function finishes. Heap memory, on the other hand, persists until explicitly de-allocated.

Debugging Tools

When a segmentation failure occurs, use debugging tools such as gdb (GNU Debugger) to examine the status of your program. These tools give useful information about the call stack, variable values, and memory contents at the moment of the issue, which aids in determining the root cause.

Debugging tools like gdb play a crucial role in diagnosing issues by allowing programmers to inspect memory contents, track program execution, and analyze variables. They provide a deeper understanding of program state, aiding in identifying errors and optimizing code for more efficient and reliable software development.

Comment Out Sections

If you need to figure out which portion of your code generates the segmentation error, consider gradually commenting out bits of code and reintroducing them. This elimination method can aid in identifying the troublesome code fragment.

Print Debug Statements

Strategically placed print statements can aid in locating the source of the problem. Outputting variable values and markers throughout your code might lead you to a failure point.

It offers crucial insights into program execution by displaying variable values and execution paths. These statements serve as valuable tools for identifying errors, tracking program flow, and validating code assumptions during the debugging process.

Examples of Common Segmentation Faults

Let's dive into a few real-world scenarios with code-based examples.

1. Null Pointer Dereference

In this except, the pointer ptr does not point to a valid memory address, but we are attempting to assign a value. This causes an immediate segmentation fault.

2. Buffer Overflow

The strcpy function writes more characters to the buffer than it can handle, resulting in a memory overflow. A segmentation fault may result from this overwrite of contiguous memory.

3. Uninitialized Pointer

Operating on an uninitialized pointer, such as uninitializedPtr , produces unpredictable behavior, frequently resulting in segmentation failures when the program attempts to access memory without appropriate initialization.

4. Out-of-Bounds Array Access

It is accessing an index outside the array's limits, as seen with arr[5] , which attempts to read memory not allocated for the array. This results in a segmentation fault.

5. Writing to Read-Only Memory

In this instance, we are attempting to alter a string literal stored in read-only memory, such as roStr , which results in a segmentation fault owing to the read-only memory protection mechanism.

• Segmentation faults are mostly caused by a program attempting to access a memory region outside its permitted bounds. This can occur while working with pointers or arrays.
• Dereferencing a null pointer will always result in a segmentation fault. Before using pointers, make sure they are correctly initialized.
• Understanding the difference between stack and heap memory is critical. Stack overflow, frequently caused by excessive recursion, can result in segmentation failures.
• Using variables without first initializing them might result in unexpected behavior and segmentation errors. Make it a practice to initialize variables as they are declared.
• Overwriting contiguous memory with more data than a buffer might cause segmentation faults. When working with strings and arrays, use caution.
• Buffer overflows occur when data written to a buffer surpasses its allocated size, spilling into adjacent memory regions.
• Memory leaks can gradually deplete memory, resulting in segmentation errors.

How To Debug Crashes And Segfaults

Written by Nick Otter.

Introduction

How to configure ulimit, how to test core dump file creation.

When an application crashes or a segfault occurs, how can this be investigated? The answer is by core dump.

I recommend core dump use only on a case by case basis to debug. Persistent configuration leads to disk space hogging and security vulnerabilities as secrets can be exposed in the memory capture files.

A core dump is a memory dump of a program terminated by the operating system due to a fault. A typical reason is due to pointer errors - a program trying to access memmory it’s not allowed to or doesn’t have access to, causing a kill signal to be sent by the os. These errors are known as segfaults.

Requirements

Let’s get started. ulimit will need to be configured to increase resource limits on this system. Using the -c argument will allow use to see the current core file creation limits. By default, core file creation is set to 0 to save on system performance.

• Check current core dump file size settings: $ ulimit -c 0
• Set to a greater size of 0 by passing a size value with the -c flag. In this case, I’m happy to go unlimited for the fun of it. $ ulimit -c unlimited
• Remove any conflicting ulimit configuration in persistent configuration files: /etc/security/limits.conf /etc/init.d/functions /etc/profile
• Set core dump file path and naming convention (see core man page ). $ echo "/tmp/core-%e.%p.%h.%t" > /proc/sys/kernel/core_pattern

Now, let’s test core dump file creation by sending a segfault signal to a running process.

Check core dump file path.

Check dump file has been created at path.

How to read the core dump file

Core dump files can be read with gdb . Let’s read the one that was just created. For better info and not to face loads of ????? in the gdb trace, install the debug version of the binary (and any shared libraries the binary uses) that caused the core dump. In this case I installed the package firefox-debuginfo.x86_64 , the binary that caused the core dump was firefox .

With gdb, we’ll be able to see the stack trace of the program and where the exception was raised that caused the core dump. Let’s see if we can trace it through.

Ok. let’s break this down

Ok. So.. on first look. If the program failed on the shared library function raise ( ) then we could make an assumption that it’s failed gracefully? As opposed to being a bug. Let’s investigate further.

Let’s take a closer look at some of the backtrace bt . Backtrace shows all the executed frames leading up to the currently executing frame ( #0 ), which is where the error occurred. Using the debug binary we should be able to see the functions called leading up to the crash, the arguments passed to them and their respective line number and source file.

Great… more info, also looks like some firefox source files paths have appeared too. Frame #1 in the lead up to the crash supports that the process failed gracefully. FatalSignalHandler and seeing a couple of arguments signo= and signum both with the value of 11 * . From looking at this backtrace, we can assume that the firefox binary has a function to handle receiving a kill signal, and we could assume that the signal number sent to this function was 11 ? A bug might not look like this, FatalSignalHandler is a pretty appropriate function name to find one frame before a crash - probably isn’t a crash (check out Brendan Gregg’s GDB Debugging: ncurses as a nice example of finding a bug with gdb). It seems like the segfault must have been caused by something else (e.g. us sending it to the process, which we know is the case). Let’s see if in gdb we can actually look at that source code for FatalSignalHandler and continue.

• A quick look at the Linux signal man page the signal number for a segfault is… 11 .

With disas we can see the dump of assembler code for a specific funtion in the trace (wow). In this assembler code dump, we are looking for a => before the memory address - this marks the segmentation fault if there is one.

Right, let’s check it out for raise :

And then for FatalSignalHandler (parent being nsProfileLock ):

Hmmm no => anywhere in sight. Does this mean there was no segfault inside the program stack? Yes! But for the sake of argument, let’s say there was segfault ( => ) at frame <+51> .

We can see the jmp assembler instruction, and we can see the binary function FatalSignalHandler that caused the (imaginary) segfault. If we want to do further investigation, we can take a look at the source code of the binary the function was called in within gdb , side by side to the trace with disas/s .

Let’s take a look at the source code that was run at frame <+51> . This will be the source code of where our imaginary segfault came from. Also, scrolling down to frame <+189> of the trace, we can see the source code to handle the SIGSEGV signal we sent at line 58 of this article which includes a break .

Look for <+51> and <189> ( nopl at frame 53 is just an assembler function for do nothing…). We can now see the source code after <+51> as it was run.

Thanks. Example of using gdb and strace to find the cause of a segmentation fault , GDB Debugging: ncurses and other articles were useful to write this. This was written by Nick Otter.

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Debugging Segmentation Faults in C++

11 minute read

Published: October 01, 2020

Segmentation faults in C++ are a sign that you are trying to do hard things. Congratulations! Now, let’s take a peek at how to start debugging.

Never underestimate the easiest option. If trying to run a.out results in a seg fault, try running it through Valgrind to see if you can gain some insight.

Segmentation fault: 11

valgrind ./a.out

==57489== Memcheck, a memory error detector ==57489== Copyright (C) 2002-2017, and GNU GPL’d, by Julian Seward et al. ==57489== Using Valgrind-3.16.0.GIT and LibVEX; rerun with -h for copyright info ==57489== Command: ./a.out ==57489== –57489– run: /usr/bin/dsymutil “./a.out” warning: no debug symbols in executable (-arch x86_64) 0 1 2 3 4 5 ==57489== Invalid read of size 4 ==57489== at 0x100003C5B: operator«(std::ostream&, SimpleGrid const&) (in ./a.out) ==57489== by 0x100003933: main (in ./a.out) ==57489== Address 0x1200000002 is not stack’d, malloc’d or (recently) free’d ==57489== ==57489== ==57489== Process terminating with default action of signal 11 (SIGSEGV) ==57489== Access not within mapped region at address 0x1200000002 ==57489== at 0x100003C5B: operator«(std::ostream&, SimpleGrid const&) (in ./a.out) ==57489== by 0x100003933: main (in ./a.out) ==57489== If you believe this happened as a result of a stack ==57489== overflow in your program’s main thread (unlikely but ==57489== possible), you can try to increase the size of the ==57489== main thread stack using the –main-stacksize= flag. ==57489== The main thread stack size used in this run was 8388608. ==57489== ==57489== HEAP SUMMARY: ==57489== in use at exit: 90,922 bytes in 164 blocks ==57489== total heap usage: 178 allocs, 14 frees, 95,666 bytes allocated ==57489== ==57489== LEAK SUMMARY: ==57489== definitely lost: 0 bytes in 0 blocks ==57489== indirectly lost: 0 bytes in 0 blocks ==57489== possibly lost: 4,392 bytes in 5 blocks ==57489== still reachable: 86,530 bytes in 159 blocks ==57489== suppressed: 0 bytes in 0 blocks ==57489== Rerun with –leak-check=full to see details of leaked memory ==57489== ==57489== For lists of detected and suppressed errors, rerun with: -s ==57489== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 1 from 1) Segmentation fault: 11

Bold font added above for instructive purposes. This Valgrind output indicates that my program is trying to access something outside a mapped region. This almost certainly comes from defining an array of size n and trying to access element n+1 which would be index n . So, in just a second or two, we know to check our array bounds, most likely due to an improperly defined for loop.

For a bit more insight, recompile your program with the -g flag and you’ll probably get the line number where the violation occurred.

GDB is a powerful debugger that allows a programmer to step through their code line by line and probe any variable for its value at that step of execution. It has a lot of capability beyond what can be addressed in a simple primer. A very useful cheat sheet that I always have a printed copy of on my desk can be found here https://darkdust.net/files/GDB Cheat Sheet.pdf .

First and foremost, GDB will need some specific information injected into the executable that needs to be debugged. This requires compiling all of our code with the -g flag. The best way to do this is by adding it to your CXXFLAGS variable in your Makefile. That ensures that all automatically created *.o files are also built using the -g flag. You should also delete the old *.o files before rebuilding. This is a great time to run make clean , assuming you have a well-defined clean rule in your Makefile.

Now that you have recompiled with -g flags, you can fire up the debugger. The -tui flag below opens the source code in the top half of the screen which is great for adding some context to where the program is in the execution. gdb -tui a.out

If files were built with -g , the symbol table should load and GDB is ready to use. The first thing that is typically needed is to add one or more breakpoints. These can be added to method names, or to lines of code (if you specify a line that cannot break, the next breakable line is used).

Breaking on a specific line of source code

To halt execution at line 12 of test.cpp, you would simply type break test.cpp:12 . Now when you type run , GDB starts executing and will halt at line 12 (or the next breakable line) so that you can inspect the status of your program.

Breaking on a method

If you suspect a specific method or function in your code, you can halt execution and inspect whenever that method is called. In order to inspect the method Search within the BinarySearch class, I would type break BinarySearch::Search(int*, int, int, int) . As you might have guessed, this is a great time for tab-completion which GDB is great at. Just start typing the class or method name and hit tab in order to fill in the rest. Now when you type run , GDB starts executing and will halt the first (and every) time that method is called.

Stepping through

The commands needed to restart execution after a breakpoint has been reached are:

• next - This goes to the next line of code, but will not dive into functions.
• step - This goes to the next line of instruction. This might be inside of a function call or elsewhere in the code tree.
• continue - This runs the program until the end of execution, or until a breakpoint is reached.

Looking at values

The simple act of stepping through code often helps figure out the problem. “This should not go inside that if statement”…well, it does, so figure out why.

Sometimes, it is not enough to just know the current location of your code execution. Luckily, we can print out values of our variables. print myVar would print the value of myVar . print &myVar would print the address of myVar . Pointers will print the address by default. To see their value, you would need to dereference, e.g. print *myPointer . As you might have noticed, the print statement is identical to sticking in std::cout statements, except that you don’t need to guess in advance, you can poke around until you find the values you need. Print will also let you call methods, so print myObject.GetSomeData() would print the results of the method call.

When breaking on a method call, the arguments passed to the call are printed automatically. For the BinarySearch breakpoint we mentioned earlier, when the breakpoint is triggered, the output might read:

Breakpoint 1, BinarySearch::Search (listOfNumbers=0x7fffffffc720, left=7, right=10, searchKey=10) at binary-search.cpp:5

“Did this line run?” You can always try sending output to screen in a bunch of places to approximate the power you can get via step-through debugging with a tool such as gdb . If you have peppered more than a couple such statements in your code for the purpose of tracking down a single bug, you’re likely to be better off just using GDB. If you have a bunch of output statements in your code as a standard part of your development process, you are almost certainly missing out on the real power of unit tests and TDD in particular.

Depending on where your output statements are, and where the seg fault occurs, it is possible that output is still in the buffer when the crash occurs. This can be misleading since you won’t see the output and might think the error must have occurred first. To flush the buffer you can add the std::endl output sequence to your cout s. This should ensure that the output that gets generated does indeed make it to your screen rather than dying on the way. So, replace output such as

Commenting Out Code

The “ old reliable ”, never forget that a program with zero lines of code doesn’t have any seg faults. Commenting out blocks of code can sometimes be a very useful strategy to narrowing down the location of a bug. Lines 100-200 are commented out but you still have the seg fault, it must be someplace else.

When using this approach, don’t forget about the power of binary search. Searching for a seg fault in 100 lines of code shouldn’t require “toggling” smaller and smaller blocks of code on and off more than a handful of times.

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Identify what's causing segmentation faults (segfaults)

On this page:

Examples of common segfaults

Find out-of-bounds array references, check shell limits, use debuggers to diagnose segfaults.

A segmentation fault (aka segfault) is a common condition that causes programs to crash; they are often associated with a file named core . Segfaults are caused by a program trying to read or write an illegal memory location.

Program memory is divided into different segments: a text segment for program instructions, a data segment for variables and arrays defined at compile time, a stack segment for temporary (or automatic) variables defined in subroutines and functions, and a heap segment for variables allocated during runtime by functions, such as malloc (in C) and allocate (in Fortran).

A segfault occurs when a reference to a variable falls outside the segment where that variable resides, or when a write is attempted to a location that is in a read-only segment. In practice, segfaults are almost always due to trying to read or write a non-existent array element, not properly defining a pointer before using it, or (in C programs) accidentally using a variable's value as an address ( see the scanf example below ).

• For example, calling memset() as shown below would cause a program to segfault: memset((char *)0x0, 1, 100);

In case A, array foo is defined for index = 0, 1, 2, ... 999 . However, in the last iteration of the for loop, the program tries to access foo[1000] . This will result in a segfault if that memory location lies outside the memory segment where foo resides. Even if it doesn't cause a segfault, it is still a bug.

In case B, integer n could be any random value. As in case A, if it is not in the range 0, 1, ... 999 , it might cause a segfault. Whether it does or not, it is certainly a bug.

In case C, allocation of memory for variable foo2 has been overlooked, so foo2 will point to a random location in memory. Accessing foo2[0] will likely result in a segfault.

The variable foo might be defined at memory location 1000 , but the above function call would try to read integer data into memory location 0 according to the definition of foo .

• A segfault will occur when a program attempts to operate on a memory location in a way that is not allowed (for example, attempts to write a read-only location would result in a segfault).
• Segfaults can also occur when your program runs out of stack space. This may not be a bug in your program, but may be due instead to your shell setting the stack size limit too small.

Most Fortran compilers have an option that will insert code to do bounds checking on all array references during runtime. If an access falls outside the index range defined for an array, the program will halt and tell you where this occurs. For most Fortran compilers, the option is -C , or -check followed by a keyword. See your compiler's user guide to get the exact option. Use bounds checking only when debugging, since it will slow down your program. Some C compilers also have a bounds checking option.

As noted in the last example above , some segfault problems are not due to bugs in your program, but are caused instead by system memory limits being set too low. Usually it is the limit on stack size that causes this kind of problem. To check memory limits, use the ulimit command in bash or ksh , or the limit command in csh or tcsh . Try setting the stacksize higher, and then re-run your program to see if the segfault goes away.

If you can't find the problem any other way, you might try a debugger. For example, you could use GNU's well-known debugger GDB to view the backtrace of a core file dumped by your program; whenever programs segfault, they usually dump the content of (their section of the) memory at the time of the crash into a core file. Start your debugger with the command gdb core , and then use the backtrace command to see where the program was when it crashed. This simple trick will allow you to focus on that part of the code.

If using backtrace on the core g file doesn't find the problem, you might have to run the program under debugger control, and then step through the code one function, or one source code line, at a time. To do this, you will need to compile your code without optimization, and with the -g flag, so information about source code lines will be embedded in the executable file.

This is document aqsj in the Knowledge Base. Last modified on 2023-10-02 13:25:14 .

Debugging Segfaults

Template designed by Alpha Studio

How Does a Segfault Happen?

Referencing address 0, referencing illegal address that is non-zero, using gdb to debug a segfault, segfaults are always a problem.

Copyright © 2012, original design by Alpha Studio transfigured by R. Heckendorn

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Why do I get a segmentation fault when writing to a string?

The following code receives seg fault on line 2:

While this works perfectly well:

Tested with MSVC and GCC.

See the C FAQ, Question 1.32

Q: What is the difference between these initializations? char a[] = "string literal"; char *p = "string literal"; My program crashes if I try to assign a new value to p[i]. A: A string literal (the formal term for a double-quoted string in C source) can be used in two slightly different ways: As the initializer for an array of char, as in the declaration of char a[] , it specifies the initial values of the characters in that array (and, if necessary, its size). Anywhere else, it turns into an unnamed, static array of characters, and this unnamed array may be stored in read-only memory, and which therefore cannot necessarily be modified. In an expression context, the array is converted at once to a pointer, as usual (see section 6), so the second declaration initializes p to point to the unnamed array's first element. Some compilers have a switch controlling whether string literals are writable or not (for compiling old code), and some may have options to cause string literals to be formally treated as arrays of const char (for better error catching).

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Hi in my program for some reason when I assign somthing to a certain pointer my program segfaults. I have tested this by calling exit(0) just before the assignment and it exits fine, just after the line and it segfaults. I have tried making a variable with a different name that is of the same type, and assigning that and it works fine. So my question is, when could setting a pointer to point at somthing directly cause a segfault? Ovbiously if I assigned it wrong and then tried to use what it pointed to it could segfault but actualy making it point at somthing is causing the problem! This crashes: Code: struct Slevel *level;struct Slevel *test_level; /* Load the map if needed*/ if (level == NULL) { test_level = load_level("levels/test.txt"); level = test_level; exit(0); } but this doesn't Code: struct Slevel *level;struct Slevel *test_level; /* Load the map if needed*/ if (level == NULL) { test_level = load_level("levels/test.txt"); exit(0); level = test_level; } My code is an absolute mess at the moment, so if you could tell me an example that just setting a pointer could cause a segfault it would get me started as to what to look for. Thanks

The pointer assignment can never segfault. ( It probably can in special circumstances, but I think it's pretty safe to say that it's not the case here. ). Try replacing the single / by a double // in your string literal. As it stands now, there is a tab ( /t ) in your path. That will probably fail somewhere down the line. Edit: forget that tab stuff, I thought it was the other slash: \t

hth -nv She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate." When in doubt, read the FAQ . Then ask a smart question .

Man this is wierd, didn't think it coiuld really cause it! I think I'm going to try and split my code up into seperate files before trying to debug it. (its like 1100 lines long and its getting hard to find stuff)

The two versions with exit(0) don't prove that the assignment causes a segfault.( as nvoigt pointed out this is safe ). It proves that the segfault happens in the function load_level(..). guess the file "levels/test.txt" doesn't exist or is somewhere else. Kurt

Your probably right that the level loading stuff is to blame although your wrong about the file name, it definately finds that and I have it printing out variables as it loads them from files. Also I put an exit() at the end of load_level() just before it returned and it didnt segfault then. The thing that really confuses me is I put the exit() a few lines further down after a function (and after the pointer is assigned) and it stops it from seg faulting! Wish I could get the dev-c++ debuger working.

Last edited by kzar; 09-15-2005 at 08:56 AM .

Originally Posted by ZuK It proves that the segfault happens in the function load_level(..). This doesn't really mean anything though. Just because this is where it crashes, doesn't mean this is where the problem is. If you've trashed your memory by munging up your pointers some place else, it could just be that this is where it happens to manifest itsefl. The actual problem could be, and most likely is, elsewhere. Quzah.

Hope is the first step on the road to disappointment.

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This pointer assignment should produce a segfault?

Isn't this supposed to produce a segfault? When I run the code the output is 5. Example: http://ideone.com/gV2Nv

• segmentation-fault

• undefined behavior != segfault. –  Brian Roach Commented May 8, 2012 at 4:26

3 Answers 3

Returning an pointer to an local variable in the function is an Undefined Behavior .

Undefined Behavior does not warranty a segmentation fault. It merely means that any behavior is possible and the program can behave in any way.

It is a common misconception that Undefined Behavior means the code should produce a segmentation fault, the truth is that standard does not require any specific behavior in cases where the code invokes an Undefined Behavior and hence the name.

C++ Standard section 1.3.24 states:

Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

No, it's supposed to result in undefined behavior.

The fact that that memory address is popped from the top of the stack when it goes out of scope only means that it is freed, not overwritten. That is why you are getting the output "5".

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