Gdb
References
- 7.2 Example Debugging Session: Segmentation Fault Example (unkownroad.com)
- Debugging with GDB (delorie.com)
- GDB commands (tutorialspoint.com)
- An Interactive Guide to Faster, Less Frustrating Debugging
- Norm Matloff's Debugging Tutorial (also guide to fast editing and gui debuggers DDD, GVD)
- 8 gdb tricks you should know (oracle.com)
GDB front-ends
Reference list:
Ideally front-ends must use the GDB/MI2 interface. There is also the built-in Text User Interface to GDB (C-x C-a: http://davis.lbl.gov/Manuals/GDB/gdb_21.html
Good candidates
Mainly from SO post above:
Vim:
- Use old shell implementation, not the new neovim terminal.
- Read some comment it was not convenient, and would require extra plugin like https://github.com/notEvil/vim-debug.
Neovim:
- Only for LLDB.
GDBInit hacks
- Using colout one may color almost any gdb output.
Old front-ends
These are using the old "annotation" mechanisms:
GDB configuration
GDB reads file ~/.gdbinit at start.
Some references:
Bare minimum configuration
From StackOverflow [1]:
set history save on
set print pretty
set output-radix 16
set height 0
GDB dashboard
GDB dashboard is a modular visual interface for GDB in Python.
To install simply copy .gdbinit as ~/.gdbinit
cp gdb-dashboard/.gdbinit ~/.gdbinit
Alternatively, source it from ~/.gdbinit:
source ~/.gdbinit-dashboard
- Install pygments
Install pygments to get source highlighting
sudo pip install Pygments # Globally pip install Pygments # Locally
If GDB uses python3 (ldd $(which gdb))), you'll need to install with pip3:
sudo pip3 install Pygments # Globally pip3 install Pygments # Locally
To get the list of available styles:
python from pygments.styles import get_all_styles as styles python for s in styles(): print(s)
Alternative styles:
Prepare debug session
- Compile with debug symbols, use option -g:
gcc -g program.c # -g : debug symbols
gcc -g -O0 program.c # ... -O0: disable optimization
- Force core dumps (see bash help ulimit):
ulimit -c unlimited
./a.out
# Segmentation fault (core dumped)
GDB invocation
gdb a.out
gdb a.out core.1234 # If coredump available
GDB commands
Reference:
- GDB manual
- https://beej.us/guide/bggdb/
| Break points and watch points | |
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Set a breakpoint at current line, at given line NUMBER or NUMBER lines after/before current line. |
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Set breakpoint at LOCATION.
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Stop execution when EXPR changes |
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Stop execution when EXPR is accessed |
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list breakpoints |
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Clear breakpoint by LOCATION |
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Delete all breakpoints |
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Clear breakpoint by NUMBER (as listed by i b)
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Disable breakpoint by NUMBER (as listed by i b)
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Save current breakpoints as script FILE. Use source to reload.
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| Execute program | |
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Start (or restart) program. Arguments may include wildcards (*) and redirections (<, <<...) |
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Kill current program. |
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Continue and interrupted program. |
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Step (into) current line, or NUMBER lines. |
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Run to next line (over current line) |
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Execute till returning from current selected frame. |
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Run until temporary breakpoint set at LOCATION. |
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Execute until the program reaches a source line greater than current (very handy). |
| View stack | |
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Print backtrace of all stack frames, or innermost (outermost) COUNT frames if COUNT>0 (COUNT<0) |
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Select frame FRAME and print stack frame |
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Go up a level in the stack (frame calling current frame). |
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Go down a level in the stack (frame called by current frame). |
| View memory | |
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Display EXPR at each prompt (if within scope). |
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Examine memory at ADDRESS as FMT( |
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Print information on local variables / function arguments in the current frame |
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print EXPR. |
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Undisplay expression by NUMBER. |
| View code | |
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List (10 by default) lines of current frame |
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Disassemble a specified section of memory |
| Miscellaneous | |
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Quit gdb. |
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Get help on COMMAND, or search commands related to WORD. |
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Source script FILE. |
| RETURN | Repeat last command. |
Tips
Define a custom label for breakpoint in C/C++
Say we want to set a breakpoint at a specified location in source file, but this position may move over time. The easiest is to use an asm statement to define the label [2]:
#include <stdio.h>
int main () {
void *ret_p = &&ret;
printf("ret: %p\n", ret_p);
goto *ret_p;
return 1;
ret:
asm("RET:")
return 0;
}
This will add a symbol table entry as follows.
gcc -Wl,--export-dynamic t.c -ldl
readelf -s a.out | grep RET
# 41: 0804858a 0 NOTYPE LOCAL DEFAULT 13 RET
Use help to test a command abbreviation
Don't know if f stands for finish or frame? Just use help f:
help f
# Select and print a stack frame.
# ...
Examples
Simple Segmentation Fault Example
(From [3])
Example program segfault.c:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
char *buf;
buf = malloc(1<<31);
fgets(buf, 1024, stdin);
printf("%s\n", buf);
return 1;
}
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Compile and launch gdb:
gcc -g segfault.c
gdb a.out
The debug session run
backtrace
frame 3
print buf
kill
break segfault.c:8
run
print buf
next
print bufFix the bug, then start again, watching now watch buf
# Start again, answer 'y' when asked to start from beginning
run
# Break at watch point, let's _c_ontinue
c
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More GDB stuff
display i # Display i at each stop
display/4i $pc # Display the 4 next instruction at each stop
u # Continue until we reach an higher pc addressUsing gdb-dashboard
da # Print dashboard again
da reg # Remove register
da reg # Add it back
da m w 0x1000 0x10 # Watch memory zone at 0x1000 for 16 bytes
da m w 0x2000 0x10 # Add a second zone at 0x2000