Segmentation Faults

„ Otherwise known as segfaults.

„ Occur when you try to access memory that you don't have permission to.

„ Results in one of the least-informative error messages when using gcc.

„ It is often more useful to be told the line on which the segfault occurs.

„ GDB is a tool which you can use to debug C code, which tells you what line the segfault occurred on. GDB

„ GDB allows you to do many things:

„ Pause your code at any time.

„ Print the contents of any existing variables at that point in the code.

„ Step through your code executing each line individually before pausing again.

„ Find out what line of your code caused that annoying segmentation fault. (Note: the line that caused the segmentation fault, is often not the line which has the error). Using GDB

1.Compile using the -g flag:

• % gcc -Wall -Werror -g -o binaryFile myFile.c

2.Start gdb:

• % gdb binaryFile

3.To run your program:

• (gdb) run

4.To quit:

• (gdb) quit

• Note the change in prompt symbol when gdb is running. GDB Useful Commands

„ To stop a program during execution:

„ Break

„ To view contents of variables at current point:

„ Print

„ To keep going:

„ Continue n

„ Step n - goes into functions

„ Next n - steps over functions

„ To view the stack (useful at seg faults)‏

„ Where Other Useful Commands

„ To see the code around the line you're currently at:

„ List GDB Examples

„ llsegfault.c

„ cause: derefrencing null pointer

„ segfault line num

„ print i curr

„ stackoverflow.c

„ cause: running out of memory by recursive non- ending function calls

„ where

„ backtraceExample.c

„ where DDD - graphical GDB

„ DDD has the same commands as gdb, but is graphical.

„ One of the main advantages of this is the ability to visually represent data structures. Viewing Data Structures with DDD

„ Compile with -g flag.

„ Run ddd:

„ % ddd binaryFile

„ Set a breakpoint at the point that you want to view the data structure:

„ break OR

„ drag the breakpoint image onto the line you want to break at

„ Run the program:

„ (gdb) run Viewing Data Structures with DDD (cntd)‏

„ When the breakpoint is reached:

„ If you can't see the place where data structures would be drawn, select View->Data Window.

„ Right click in the data window and select 'New Display'.

„ Enter the variable name that you want to draw a picture of and press enter.

„ Wherever you see ... it means you can expand the data structure. Do this by double clicking on the ... . DDD Examples

„ aircraft.c (Lab 6)‏ Segfaults: a deeper view

„ Another handy tool is: Electric Fence.

„ First, some background as to why it is handy. SegFaults (ctnd)‏

„ Ask for a piece of memory of the right size:

„ malloc(sizeof(int)); SegFaults (ctnd)‏

„ The memory is allocated. SegFaults (ctnd)‏

„ When only a small piece of memory is required, instead of providing only that small piece, the operating system gives the user access to a whole page of memory.

„ Further mallocs will return other sections of this same page. SegFaults (ctnd)‏

„ A segfault occurs when the user tries to access the red section of memory.

„ Eg, if you malloc an array, and go 1 or 2 indexes off the end, you may never get a segfault. SegFaults (ctnd)‏

„ But you don't know that it will never segfault. Electric Fence

„ If you try to access memory that's on the same page in memory, as something you've malloced, then no segfault will occur.

„ But, if you've accessed memory that you haven't malloced, you want to know sooner rather than later (when it wreaks havoc on your program).

„ Electric Fence is a program which causes segfaults to happen whenever you access memory in the heap that you haven't malloced. Using Electric Fence

„ Compile with -g and -lefence flags:

„ % gcc -lefence -g -o binaryFile myFile.c

„ Run the program within gdb:

„ % gdb binaryFile

„ (gdb) run Valgrind

„ When you want to check if you've freed everything you've malloced.

„ Using Valgrind:

„ Compile your program with -g flag.

„ Pass your file into valgrind:

„ % valgrind --leak-check=full ./binaryFile

„ The output will summerise any memory leaks and violations. This includes memory blocks that have been malloced, but not freed.

„ The flag --leak-check=full means that it tells you where you malloced something that you forgot to free. Valgrind Demos

„ valgrindDemo.c