GDB Tutorial: Debugging a Segfault

To follow this guide, you'll need to use the segfault.c file located in the guides/segfault/support directory.

Compile and run the source code from the skeleton (if you are not using the Makefile, make sure to compile with the -g flag). In short, the program takes a number n, allocates a vector of size n, and initializes it with the first n numbers from the Fibonacci sequence. However, after running the program, you see: Segmentation fault (core dumped).

Start GDB with the executable:

gdb ./segfault

Once you have started GDB, all interaction happens through the GDB prompt. Run the program using the run command. What do you notice? GDB hangs at the input reads.

Set a breakpoint at main using the break main command. You will see the message in the prompt:

Breakpoint 1 at 0x7d3: file seg.c, line 21 /* The memory address should not be the same */

Next, we will step through the instructions one by one. To do this, use the next or n command (watch the GDB cursor to see the current instruction and repeat the process). You will notice that GDB hangs at scanf, so input a value for n and continue stepping through. If you have entered a large value for n and want to skip the iteration, use the continue command. Eventually, you will reach the line v[423433] = 3;, and GDB will display:

Program received signal SIGSEGV, Segmentation fault

Inspect the memory at v[423433] using x &v[423433] and you will receive the message:

Cannot access memory at address 0x5555558f3e94 /* The memory address should not be the same */

What happened? We accessed a memory area with restricted access.