Segmentation fault

File:Windows null ptr dereference.png

Error occurring in computer programs

Template:Infobox software bug

A segmentation fault (often called a segfault) is a specific kind of error caused by accessing memory that “does not belong to you.” It's a flaw in a program that attempts to read or write an illegal memory location. This typically results in a program crash and the generation of a core dump file, which can be used for debugging.

Causes

Segmentation faults are typically caused by:

• Dereferencing null pointers.
• Accessing memory that the program does not have rights to (e.g., buffer overflow).
• Writing to read-only memory.
• Accessing memory after it has been freed.

Symptoms

When a segmentation fault occurs, the operating system usually terminates the offending process and generates a core dump file. This file contains a snapshot of the program's memory at the time of the crash and can be analyzed using a debugger to determine the cause of the fault.

Debugging

Debugging a segmentation fault can be challenging. Tools such as gdb (GNU Debugger) can be used to analyze core dumps and trace the source of the fault. Common debugging steps include:

• Running the program in a debugger to catch the fault as it happens.
• Analyzing the core dump file to determine the state of the program at the time of the crash.
• Using memory checking tools like Valgrind to detect memory errors.

Prevention

Preventing segmentation faults involves careful programming practices, such as:

• Initializing pointers before use.
• Checking pointer values before dereferencing.
• Using memory management functions correctly.
• Employing bounds checking to prevent buffer overflows.

Operating System Behavior

Different operating systems handle segmentation faults in various ways. On Unix-like systems, a segmentation fault typically results in a signal being sent to the process, which can be caught and handled by the program. On Windows, a segmentation fault usually results in an access violation error.

See also

• Buffer overflow
• Core dump
• Memory management
• Pointer (computer programming)
• Segmentation (memory)

References

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