Dealing with segmentation faults

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In summary, the conversation discusses how to prompt gdb to show where a segmentation fault occurs in an application and how to find the specific line or lines causing the fault. Common causes of segfaults are also mentioned, such as using uninitialized pointers or variables for pointer arithmetic. The suggestion is made to use gdb to step through the code and put a breakpoint in to find the offending line. It is also recommended to compile the program with debugging information or to run gdb on a core dump for more information.
  • #1
heartless
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Hi everyone,
I'd like to know how to prompt gdb to show me where segmentation fault in an application occurs and why. Any ideas?
 
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  • #2
Segfaults happen when your program tries to write to memory that doesn't belong to it. Common causes are:

1) Using uninitialized pointers.
2) Using uninitialized variables for pointer arithmentic (array indexing).
3) Letting a counter go beyond the boundaries of an array.

You should be able to use gdb to step through your code until you find the offending line.

- Warren
 
  • #3
Yepp, that's what wikipedia says. However do you have any ideas how to find specific line or lines that cause segmentation fault in an application. I tried gdb for "normal" aps, and everything works perfectly, but I can't debug any applications with segmentation faults.

For example:
adrian@ubuntu:~/files/asm$ gdb env_show --quiet
(no debugging symbols found)
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
(gdb) r
Starting program: /home/adrian/files/asm/env_show

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
You can't do that without a process to debug.
(gdb) info registers
The program has no registers now.
(gdb) bt
No stack.
(gdb)

and so on, I'm not very experienced with gdb,
--Thanks,
 
  • #4
Rather than just running your program, put a breakpoint in and step through it line by line.

- Warren
 
  • #5
Also, I presume that you compiled your program with gcc -g to include debugging information in the executable.

- Warren
 
  • #6
Or run gdb on the core dump. Never done it myself, but the core file should have the program state at the time of the segfault.
 

1. What is a segmentation fault?

A segmentation fault is a type of error that occurs when a program tries to access a memory address that it is not allowed to access. This can happen due to various reasons such as trying to access an uninitialized pointer or accessing memory that has already been deallocated.

2. How do I debug a segmentation fault?

The best way to debug a segmentation fault is to use a debugger, such as GDB, to step through your code and find the exact line where the error occurs. You can also use print statements to track the values of variables and pointers as your program executes.

3. Can a segmentation fault be fixed?

Yes, a segmentation fault can be fixed. It usually requires identifying and fixing the source of the error in the code. This can involve properly initializing pointers, checking for null values, and avoiding accessing deallocated memory.

4. How can I prevent segmentation faults?

To prevent segmentation faults, it is important to follow good coding practices such as properly initializing pointers, checking for null values, and avoiding accessing deallocated memory. Using a debugger and thorough testing can also help catch potential errors before they result in a segmentation fault.

5. Can a segmentation fault cause damage to my computer or data?

In most cases, a segmentation fault will not cause any damage to your computer or data. However, if the error occurs in a critical part of the code, it could potentially cause problems. It is always important to regularly save your work and ensure that your code is properly tested to prevent any potential damage.

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