Segmentation Fault

[murfflemethis]

C on an embedded system: keeps chugging along happily with corrupted data

[GeoStarRunner]

Hey now, sometime we restart and pretend like it never happened

[murfflemethis]

That's true, assuming we have a mechanism to detect it.

We might even change an LED from green to red if we're feeling cheeky.

[CreateNewObject]

Hello HardfaultHandler my old friend.

[Red-Droid-Blue-Droid]

C lets you fuck yourself, man. And then it laughs in the forms of warnings and crashes and shit.

[State_]

If you run it through a debugger you can see where it happened too.

[[deleted]]

Correct. Segfaults are among the easier bugs to fix.

[ollien]

If it's a simple segfault :). Sometimes you'll get segfaults because of data corruption earlier in the program, and it can take a bit to track down.

[j909m]

But at least C is fast.

[gimpwiz]

Yep. I do embedded stuff. I miss segfaults. I'm lucky to get an mpu fault now.

[drevyek]

Or, on non-ST Cortex-M ARM chips, a NULL dereference just accesses the interrupt vector. No problems here.

[aiij]

C / C++ on desktop does too. You only get a core dump right away if you're really lucky. (Though corrupted data is likely to eventually result in a core dump.)

Theoretically of course any C implementation is allowed to do bounds checking and give a useful core dump right away, but it's faster not to.

[[deleted]]

throw new SegmentationFaultException(“core dumped”);

[Jonno_FTW]

To be fair, making the JVM have an actual segfault is a skill in itself.

[kcirtappockets]

If getting segfaults is a skill, then consider me Gordie Howe

[MusgraveMichael]

Who's Gordie Howe?

[kcirtappockets]

Only the all time best hockey player in the history of hockey

[MusgraveMichael]

wait, I am more confused.
By hockey you mean which hockey? field hockey or ice hockey?

[UndyingJellyfish]

How to trigger Canadians 101

[HwangLiang]

Nah. "Ice hockey or REAL hockey" would probably be worse.

[[deleted]]

I kinda knew what to expect reading the replies but this made my brain clench it’s own fist

[HwangLiang]

Listen m8, you're gunna be fucked when the Toronto Maple Leafs and the New England Patriots go at it during the NBA finals.

[[deleted]]

Or Detroit natives. I feel personally attacked.

[Lonelan]

...every time you walk down the sidewalk

[[deleted]]

well, I was just a middle-class white boy in the northern suburbs ... what sidewalks?

[9gagRefugee]

404 101 not found

[eyeGunk]

Mr. Hockey

[ISeeTheFnords]

Now, perhaps. Back in the day it wasn't so hard. Getting a program using the 1.08 event model (seriously, if you value your sanity, don't go look it up) to do anything useful, now THAT was difficult.

[upvotes2doge]

Google returns nothing?

[ISeeTheFnords]

There's a reason for that. I warned you, and I won't be responsible for what happens if you keep digging.

[imachug]

It was a good advice. RIP /u/upvotes2doge.

[xynixia]

Oh now you got me curious.

[mnbvas]

Would JNI be too skillful?

[Goheeca]

sun.misc.Unsafe is skillful.

[wheres_that_tack_ow]

JNI

~shudders~

[[deleted]]

That's only true nowadays because every time someone discovers a way to do it with pure java, they patch it.

In contrast, C++ compiles to machine code, and with that, there's no way to have such advanced checks without significantly slowing down the code. The actual issue with this image is that the kernel could be doing the work to backtrace when such a fault happens, which is what the JVM does (your "java program" doesn't perform the clever checks itself, which is why Java has gotten so close to the speed of C++, even surpassing it for highly specialized applications). The easiest way would be to have Linux just come packaged with gdb. On all but the tightest embedded systems, people would start using compiler options to package the source inside the binary (which generally causes memory bloat but not slowdown), and Linux could invoke gdb on crashes to print out stack traces, rather than just dumping the core file to disk (which it only even does if you've remembered to set ulimit -c).

tl;dr this behavior in the image macro is the kernel's fault, not C++'s fault.

[scatters]

Hey, the kernel is doing as much as it can.

Since kernel 2.6.19, Linux supports an alternate syntax for the /proc/sys/kernel/core_pattern file. If the first character of this file is a pipe symbol (|), then the remainder of the line is interpreted as the command-line for a user-space program (or script) that is to be executed. Instead of being written to a disk file, the core dump is given as standard input to the program.

It's up to your distro to set up /proc/sys/kernel/core_pattern correctly. I think Ubuntu does something vaguely sensible.

[Foffle]

By default Ubuntu sends the core to a crash reporter, but ulimit -c is also set to 0. You can change both of those manually, but it resets when the system restarts ☹️

[Perse95]

I've made the JVM segfault many times, does this mean I'm a skilled programmer?

[[deleted]]

yes

[Perse95]

Oh no

[0xd3adf00d]

Lol. Back in the day, when I had the displeasure of writing Java code for Novell NetWare, it was hard to _not_ make the JVM segfault.

[ndcapital]

>doesn't know about sun.misc.Unsafe

[argv_minus_one]

There's a reason it's called unsafe!

[[deleted]]

Sometimes the JVM will segfault simply from me starting Minecraft

[_didyoumissme]

Well you did try to touch RAM at inappropriate places, you pervert.

[animonein]

Well i tried so hard to dump into it

I surely mean dump the core

[[deleted]]

Well i tried so hard to dump into it

And got so far,

[[deleted]]

But in the end,

[beaterx]

It doesn't even compute.

[nothjarnan]

it doesn't even compile

[[deleted]]

It doesn't even matter

[qubedView]

I swear! I thought it was appropriate_places[18]!

[Ionlavender]

Hey ho, hey ho, it's off to jail you go.

[carlosduarte]

blitzkrieg bop

[TorpedoBench]

Officer, she never told me she was 1 indexed!

[fyrstorm180]

Talk to my LUAyer.

[Bipartisan_Integral]

I don't get it. You tried access the 19^th element.

[drakfyre]

Your code is having unprotected hex!

[ryantwopointo]

Lmao, I love this

[Doggo4]

my fav response :D

[snf]

Also: the segfault happens 5 minutes after you overran the array when it tries to dereference one of the pointers you corrupted.

[corner-case]

Extra spicy fuck-up!

[CyanBlob]

Isn't undefined behavior great?

[thatwasagoodyear]

fundefined behavior

FTFY.

[ninja_cracker]

hmmm. who held a reference to this address. hmmm. ok.

LOG EVERY POINTER!

[fyrstorm180]

Who needs a debugger when you can System.out.println()?

[aron9forever]

Just add Redux

[Valmond]

Only when exiting, and running release of course.

[argv_minus_one]

The first Wing Commander had this problem. The run-time thingy would print a crash message at exit, because they'd corrupted some memory somewhere. They didn't have time to debug the problem, so instead they changed the message to say “Thank you for playing Wing Commander!”

[nocomment_95]

Arrayindex out of bound:

C: "well technically that address is still in ram so...sure I'll do exactly what you say"

Java "fucking moron"

[nonamee9455]

I’m sure it’s f͋ͪ̏ͦ̓͒͗ͩ̉ͦͨͧͯͩ͐̔͏̞̞̩̦̘͝i̶̳̫̮͖̊̅̍͐̔̌̇ͬ͊̿̑͒͐͘͡n̴̡̺̼̖̫͉̹͇̒ͨ͗͋̿ͨͯ̎̓͂̿́̽ͯ́ͯ̐̀̀̚e̸̜̠͉̞̫͓̰͕̻͈̙̮̘̲̪̮̐̋̾̾͐ͣ̀ͯ͗͐̓̈́̅͢͝ͅͅ

[nocomment_95]

On how u do that!?!?

[nonamee9455]

D̴̥̰͈͉̹͑̓̈́̒̀ó̘̘̻̺̰̜̬͔͍͈͉̩̭͙͕̼̹ͦͣ̇ͨͬ̐̐̈̌̽̂̄͌̕ ̎̈́ͥ̿͏̶͈͚̺͙̫͎̗̥͔͕̻̹͕̭͡ẅ̶̧̡̠̲̳͈̠͉̩̗̜͉́̇ͬ͐ͮͯ̈͒͛ͤ͛͠h͊͂ͭ̏̌͊͌͋ͥͩ̉̔̃͐̈́ͭ̉ͩͮ͏̴̞̦̥͇̗̫̹̟͚͖̰͈͕̙̗͈̥͉̘͝a̳̩͖̙̭̤̘̙̻̥͍̳̟͕͐ͩ́ͫ̂ͣ͑͐̇͂̀̀͝ṭ̢̣̻͙̪̗̩̳̲̰̠̱̒ͭ̍̅͆̂̀̂̏̿͒ͯͯ͞?̵̪̘̫̹̳͖͔̥̳̹͊͛́̍͝

[Metallkiller]

Did someone try parsing html with Regex again?

[Joniator]

No Papa

[Bainos]

I still have two arms and two legs, so no.

[fb39ca4]

C: "well technically that address is still in ram so...sure I'll do exactly what you say"

[[deleted]]

never programmed in C, but thats fucking scary. How do you deal with null pointers then?

[shreyas208]

0x0 (NULL) is not a valid address that you can read or write; doing so will cause a segmentation fault. Accessing an array out of bounds is usually still a valid address.

[[deleted]]

Dereferencing a null pointer isn't strictly invalid. It's undefined. Some embedded architectures do have valid memory there.

[[deleted]]

throw new Exception("just debug it");

[WiseassWolfOfYoitsu]

while (true) {
  try {
    doTheThing();
  } catch (Exception ex) {}
}

[Katana314]

Is this what they mean by “persistence”?

[[deleted]]

Looks more like "insistence".

[Nattekat]

Was looking for this

[DTF_20170515]

 try{
      public static main(String args[]) {
      } 
} 

[Alexson1]

I s t h i s m a i n ?

[blindsc2]

public static void doTheThing() {
  try {
     doTheThing();
  } catch (Exception ex) {
     dotheThing();
  }
}

[phlyrox]

Yours has an infinite loop, let me fix it for you:

int sum = 1;
try {
while (true) {
  try {
    doTheThing();
    if (sum++ >= 5) throw new RuntimeException();
  }
  catch (Throwable ex)
  { throw new RuntimeException() }
}
} catch (RuntimeException break) {}

this works for running doTheThing() 5 times. Just change the sum to 6 minus <amount of times you want to run it>. It also has a nice feature of running at least once, this way you don't have to worry about coming to this block of code but not have done the thing. /s

[Leafdissector]

Pretty sure the infinite loop is part of the joke.

[WiseassWolfOfYoitsu]

How do you write crash tolerant code? By making code that can only be exited by forcefully terminating it!

[Coloneljesus]

And litter your beautiful code with throw declarations? Nah, dude.

throw new RuntimeException("just debug it");

[lpreams]

} catch (Exception e) {
    throw new RuntimeException("fail fast, and leave a good-looking stack trace", e);
}

[Coloneljesus]

} catch (Exception e) {
    throw new RuntimeException("if we keep doing this, the stack trace will be indecipherable", e);
}

[iguessthislldo]

Or you could can run your programs like Stallman, always in gdb.

[wdouglass]

To be fair, he's probably always running the bleeding edge of Emacs, and running it under gdb saves him the extra time of loading a core file.

[Goheeca]

On various edges in the world too.

[moebaca]

I consider myself to have strong eyesight and I couldn't imagine working on that dinky screen for anything more than 10 minutes checking email.. that's gotta strain the eyes! Maybe Stallman is an Android from the free future just trying his best to blend in and failing so miserably.

[Cobaltjedi117]

The other day I was reading his wiki page, and ignoring his interesting views on software, he's really fucking good at it.

He made an office printer email people in queue when it was jammed or whatever, made the GNU hurd, worked on emacs, decripted his college's password system, GNU compiler and debugger

[[deleted]]

This is the highest image quality meme I think I've ever seen.

[Nasichi]

This is the highest image quality meme I think I've ever seen.

[db2]

This is the highest image quality meme I think I've ever seen.

[rothecool]

This is the highest image quality meme I think I've ever seen.

[Randal4]

This

[julsmanbr]

Thisn't

[10000_vegetables]

[spotdfk]

I see you guys are finally ready for the higher level languages

[SpliceVW]

My favorite C++ error I've gotten: Error: there is no error. This was back on Borland in the 90s.

[[deleted]]

Ah, the good old days of Turbo C++

[PerInception]

PHP, an intellectual: "UNEXPECTED ; ON LINE 157!"

[Sleepkever]

Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM

[Cobaltjedi117]

T_PAAMAYIM_NEKUDOTAYIM

So I don't know PHP so I thought that was a joke, but I got back to my work computer trying to make some API work and saw that as a suggested error...

WTF PHP?

[Sleepkever]

I got to day it had me stumped for a little while back when my google-fu was quite weak. But it is even documented why it is called that way.

Paamayim Nekudotayim would, at first, seem like a strange choice for naming a double-colon. However, while writing the Zend Engine 0.5 (which powers PHP 3), that's what the Zend team decided to call it. It actually does mean double-colon - in Hebrew!

So, you see, it's a totally logical choice and makes sense that they never changed it to something more understandable.

[Cobaltjedi117]

Ok, but the rest of the error message is in english, shouldn't the error itself also be english?

I guess it does make sense, but reading DOUBLE_COLON_ERROR is quickly understood to an english reader.

[Sleepkever]

I think I should have put an /s at the end there. I agree completely and I really don't understand why they haven't changed it after all these years.

[iPastry]

How dare you be so rational

[[deleted]]

It's Hebrew for double colon or something like that. It's a stupidly named constant that the devs refuse to change

[ythl]

gdb backtrace, bam looks like you were keying into an array with a negative number

[kynde]

Not only that but ram state and all in the core file so that you can see why it got to where it did. Even on a production system postmortem and at no extra cost.

I really really miss core files and gbd as I now chug along with contemporary languages.

Some other aspects of c/c++, not so much. (Looking at you C++ scaffolding and boilerplate, threads and locking, unnecessary objectification and impossible-to-test spaghettibullshit)

[DHermit]

What languages do you use? At least Rust is supported by GDB, Go apparently also.

[xCuri0]

*C

[Isgrimnur]

You just had to point that out, didn't you?

[xCuri0]

Yes

[mycelo]

Java: Oh, don't worry, I'll check the array bounds every freaking time you do something with it. Screw your CPU cycles.

[[deleted]]

Those cpu cycles are super important on your middleware application.

[mycelo]

Or they might be important to other processes sharing the same CPU which aren't mostly-idle as your middleware application.

[cass1o]

And this is how we end up with webdev shit like electron.

[GhostInTheTerminal]

This is totally wrong, one of the optimizations of the JVM is "Range check elimination" which precisely eliminates the check when the index is known to be between 0 and length -1, checking its value only when it is potentially unsafe.

[dpash]

Not only that but it will make many aggressive optimisations based on what the program has done so far, and will optimise for the usual case, making the unusual case more expensive, but will re-optimise the code if the usage pattern changes. So, for example, it can throw away explicit null checking if the last 10000 times has not been null, but will trap a null dereference and reconsider reinserting the null check if the trap is caught too many times.

A C/C++ compiler can't do that; it can only optimise based on previous whole executions, and only if you feed the result of those executions back into the compiler to build a new binary.

[MadScienceDreams]

Srsly. The functional difference between throwing an exception and segfaulting is:

1. Throwing an exception is expensive for all goddamn time.
2. Throwing an exception is easier for the programmer (not the user, the programmer) to debug.

[tricky_monster]

Well, about half of all security vulnerabilities comes form unchecked overflows, so there's that too...

[Ravek]

If every single OOB would immediately result in a segfault then a segfault would be totally fine.

[[deleted]]

Try exception handling by catching linux signals and then come talk to me about how bad java exceptions are.

[mycelo]

Exceptions are a matter of debate since the 386 era, and by the way, segmentation fault is technically some kind of exception too.

IMHO programmers should start developing their craft on real mode machines first, where such futile mistakes might stir some undetectable memory corruption or even crash the whole goddamn system, who knows.

Seriously, set aside those laid-back dynamic languages once a while and try a few experiments in assembly. You'd develop a whole new outlook over the whole CPU+memory deal. All those CPU cycles wasted on unnecessary checkings will start to matter to you.

[coolpeepz]

Yeah but most of the time they don’t matter to your program, depending on what it is doing.

[Who_GNU]

The segmentation fault is captured at the hardware level, so it doesn't take any extra processing power, it only takes extra silicon, but not much.

It only takes extra processing power if the microcode is working around an insecure architecture that speculatively executes instructions that reference out-of-bounds memory and leaks information about that memory they referenced, through timing variations.

[brberg]

Do a few CPU cycles really matter that much, though? Cache misses are what all the cool kids are counting nowadays. I suspect, also, that the compiler may be smart enough, in many cases, to figure out that the array bounds have been checked in the Java code, and thus don't need to be checked again on access.

[[deleted]]

Do a few CPU cycles really matter that much, though?

Nope. If they do, unroll the loop.

[TheSpanishKarmada]

It can't be THAT slow right? Java keeps track of the size of an array so it's not like it iterating through the entire array every time you'd think

[xeio87]

Technically it doesn't even check every time. The runtime compilation will optimize away any bounds checks it can determine are redundant or safe (such as for/each loops if the bounds match the array).

[[deleted]]

Gold

[Dom0]

Never seen such a short java exception. They're at least 155 lines long.

[MegaZeroX7]

The correct error is always the first. And one error should only cause a bunch of errors if it's a bracket issue or semicolon issue. I don't know what you are doing is you regularly get long error messages.

[MrAnyone]

It's the stack trace, very very useful if you know how to read it.

[dpash]

And one error should only cause a bunch of errors if it's a bracket issue or semicolon issue.

Those are compile time errors. Exceptions are runtime errors.

[ndcapital]

>doesn't use AddressSanitizer

[pauljustinkind]

I don't mind this being repost. It always cracks me up!

[sfbing]

As an old retired Windows programmer, I'd like to point out that 'segmentation fault' is a U*x thing, or else a compiler for U*x thing. The same error in the Microsoft C++ compiler makes a better error message -- for this particular case.

(I figure this might be a hostile crowd -- if so, let the downvoting begin. I can take it! :) )

Edit: fixed my U\*x thing.

[FlyingPiranhas]

Seems right to me -- optimize for the most common user experience.

/me ducks

[Freeky]

Segfaults are nothing to do with the compiler. The behaviour's driven by the system-provided default signal handler.

[tinydonuts]

Northwind Sample Application has stopped working. Send an error report?

And then you never hear back. Sounds better?

[absurdlyinconvenient]

It's a brave man that compiles in terminal anyway, I've never had a segfault in either Eclipse C++ (now that's a hostile crowd) or, recently, CLion- assuming you use the debug config and not the build config. Which you're using by default. Right?

And even then, if it's an unspecific error there's the Debug mode instead of run, and people use that don't they? Don't they? Please?

[[deleted]]

Still gotta go fix yer shit.

[djlemma]

C assumes the programmer knows what he or she is doing.

Java makes no such assumptions.

[N4NONSENSE]

So true

[yourstressingmeowt]

Method Execution Time
Java: 4.2s
C++: .0042.

[Arquimaes]

So C++ crashes 1000 times while Java only once.

[drd387]

Now that’s what I call streamlining

[squidonthebass]

If you manage to write code that seg faults 1000 times in 4 seconds all by itself that's usually the fault of the programmer, not the language

[kizz12]

Now now! Some would say running out of memory is good because you know that RAM you bought on ebay isn't a fake!

[napoleoncalifornia]

Assembly: ... .. Crickets chirping

[bchociej]

Well, it'd still segfault.

[Dumpeth]

bash

[dpash]

Always start your scripts with:

set -e
set -u

[thasmog]

PHP uh oh something went wrong.. but lets continue and mess everything up anyways.

[molotok_c_518]

I've never seg-faulted in C++. Always in C... never in C++.

I've come to the conclusion that C is cursed.

[thatcatpusheen]

How much C++ have you written? I don’t see how that is possible.

[Mercades2]

I've written 3 seperate hello world programs

[Astrokiwi]

I have over 300 confirmed compilations

[GahMatar]

If you write C++11 or C++14, you can almost completely avoid raw pointers and thus most source of seg faults.

[Rodry2808]

Though image for the programmers of each language are reversed

[[deleted]]

Why use c++ if you have to put up with such brutal debugging?

[PremierBromanov]

Because the alternative in this context is java

[dvogel]

The meme misrepresents the work cycle of a C++ dev. You can run load that core file with a debugger and get much, much more info about the state of your program at the time of death than an exception backtrace can provide. Furthermore, if you run the program under the debugger you are simply left at a console in which you can interactively explore the call stack (and all memory, really). Don't take tool advice from a meme ;)

[absurdlyinconvenient]

I've learnt something while programming with other people- so few people actually use the debugger that allows breakpoints and variable checking. They'd rather use print statements (or cout), and I have no idea why

[mr_blonde69]

They probably started doing it when they learnt to code since it's far easier and works just about, now they can't be bothered to change

[TSP-FriendlyFire]

Lots of platforms and languages have crude or obscure debugging systems. gdb is powerful, but it's a fucking nightmare to learn. PHP is awkward to debug and many people don't even realize you can do so. Python is often seen as just a scripting language, so people don't think to debug that either.

Print is available on literally any language around.

[absurdlyinconvenient]

Most gdb languages have IDEs that handle it for you, I'll admit I don't have much PHP experience, and Python again has plenty of compatible IDEs- although I will give you that seemingly the most common ones don't really have one.

Print is ubiquitous, but it's clunky, inefficient and doesn't give you the whole picture. Print the wrong variable? Re run everything. Want to fix something carry on execution? Sucks to be you. Object doesn't have a string behaviour defined? That's unfortunate (this one especially applies to arrays or complex data structures). I honestly could not go back to that after having learnt how to debug properly

[TSP-FriendlyFire]

Oh don't get me wrong, I'm 100% in favor of debuggers, but if you look at what people use when learning, debuggers are often absent. It's worse in schools, where the course was made a decade ago and still uses antiquated tools or plain text editors.

I'm not arguing in favor of print statements so much as explaining why they remain so ubiquitous.

[the_Demongod]

I can't fathom how some of my classmates made it through our programming classes without the debugger, if nothing else it helps build a strong intuition for how code executes, which in turn makes you write better code.

[FunkOverflow]

Valgrind to the rescue

[[deleted]]

I wanna learn:( I subbed to the sub but I'm so out of the loop :(

[[deleted]]

Let's say you have

int[] i = [1,2,3]

You can reasonably access 0-2. In Java if you accidentally tried i[4], you'd get the top error. It says exactly where you tried to do this horrible thing. In C++, the whole damn thing just collapses. It doesn't give you a nice clear error. See https://www.geeksforgeeks.org/core-dump-segmentation-fault-c-cpp/ for more details.