Segmentation Faults Otherwise known as segfaults. Occur when you - - PowerPoint PPT Presentation
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
Otherwise known as segfaults. Occur when you try to access memory that you
Results in one of the least-informative error
It is often more useful to be told the line on
GDB is a tool which you can use to debug C
GDB allows you to do many things:
Pause your code at any time. Print the contents of any existing variables at that
Step through your code executing each line
Find out what line of your code caused that
To stop a program during execution:
Break <line number or function name>
To view contents of variables at current point:
Print <variable name>
To keep going:
Continue n Step n - goes into functions Next n - steps over functions
To view the stack (useful at seg faults)
Where
To see the code around the line you're currently
List
llsegfault.c
cause: derefrencing null pointer segfault line num print i curr
stackoverflow.c
cause: running out of memory by recursive non-
where
backtraceExample.c
where
DDD has the same commands as gdb, but is
One of the main advantages of this is the ability
Compile with -g flag. Run ddd:
% ddd binaryFile
Set a breakpoint at the point that you want to
break <line number> OR drag the breakpoint image onto the line you want to
Run the program:
(gdb) run
When the breakpoint is reached:
If you can't see the place where data structures
Right click in the data window and select 'New
Enter the variable name that you want to draw a
Wherever you see ... it means you can expand
aircraft.c (Lab 6)
Another handy tool is: Electric Fence. First, some background as to why it is handy.
Ask for a piece of memory of
malloc(sizeof(int));
The memory is allocated.
When only a small piece of
Further mallocs will return other
A segfault occurs when the
Eg, if you malloc an array, and
But you don't know that it will
If you try to access memory that's on the same
But, if you've accessed memory that you
Electric Fence is a program which causes
Compile with -g and -lefence flags:
% gcc -lefence -g -o binaryFile myFile.c
Run the program within gdb:
% gdb binaryFile (gdb) run
When you want to check if you've freed
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
The flag --leak-check=full means that it tells you
valgrindDemo.c