this post was submitted on 21 May 2024
1599 points (98.8% liked)
Programmer Humor
32361 readers
425 users here now
Post funny things about programming here! (Or just rant about your favourite programming language.)
Rules:
- Posts must be relevant to programming, programmers, or computer science.
- No NSFW content.
- Jokes must be in good taste. No hate speech, bigotry, etc.
founded 5 years ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
Now I want to know what int3 does.
https://en.wikipedia.org/wiki/INT_(x86_instruction) (scroll down to INT3)
https://stackoverflow.com/a/61946177
The TL;DR is that it's used by debuggers to set a breakpoint in code.
For example, if you're familiar with gdb, one of the simplest ways to make code stop executing at a particular point in the code is to add a breakpoint there.
Gdb replaces the instruction at the breakpoint with 0xCC, which happens to be the opcode for INT 3
generate interrupt 3. When the CPU encounters the instruction, it generates interrupt 3, following which the kernel's interrupt handler sends a signal (SIGTRAP) to the debugger. Thus, the debugger will know it's meant to start a debugging loop there.
Hey thank you!
Not what I thought it was for sure 😃
How does it work if an instruction gets replaced by the INT3 though?
Excellent question!
Before replacing the instruction with INT 3, the debugger keeps a note of what instruction was at that point in the code. When the CPU encounters INT 3, it hands control to the debugger.
When the debugging operations are done, the debugger replaces the INT 3 with the original instruction and makes the instruction pointer go back one step, thereby ensuring that the original instruction is executed.
Whoo that seems complicated, I mean you akready compile a debug version.
Thanks for the explanation!
The debug version you compile doesn't affect the code; it just stores more information about symbols. The whole shtick about the debugger replacing instructions with INT3 still happens.
You can validate that the code isn't affected yourself by running objdump on two binaries, one compiled with debug symbols and one without. Otherwise if you're lazy (like me 😄):
https://stackoverflow.com/a/8676610
And for completeness: https://gcc.gnu.org/onlinedocs/gcc-14.1.0/gcc/Debugging-Options.html
Thanks, excellent information!
How come debug exes are bigger? Is the nifty stuff tucked on at the end?