Debugging with GDB

Prerequisites

• Compile with debug symbols. To do this, add the -g flag when you compile. For example, clang -g -o program.o program.c. For convenience, add a target to your makefile, i.e.:

  debug: CFLAGS += -g
  debug: all
  

• Launch GDB using ./gdb --args followed by the your normal method of execution. For example, for a program named program and one flag –flag, run gdb --args ./program --flags.

• Once inside GDB, the command run will run your program.

Finding Segmentation Faults

First, run your program using GDB. When your program crashes, you should see the exact line on which your program failed.

For more information, use the backtrace command. The stack trace should show the path your program took during its execution. For example, if your program crashes in the standard library’s strlen function, there won’t be much information there. Running backtrace will show you the arguments passed to the function and which function called strlen.

Example

$ gdb --args ./executable
Reading symbols from ./executable...

GDB has read the executable. The (gdb) prompt means anything typed is sent to GDB, not to the shell.

(gdb) run
Starting program: /path/to/executable

Program received signal SIGSEGV, Segmentation fault.
0x000055555555512c in my_function (array=0x0) at ./segfault.c:4
4       return array[0];

The program received a segmentation fault in the function my_function. The function has one argument, array=0x0 (i.e., NULL).This is the source of the segmentation fault, since the code is attempting to access the 0th element of NULL memory.

(gdb) backtrace
#0  0x000055555555512c in my_function (array=0x0) at ./segfault.c:4
#1  0x0000555555555150 in main () at ./segfault.c:9
(gdb)

The function my_function was called from the function main.