E-COM-NET

Question

In this thread, we look at examples of good uses of goto in C or C++. It's inspired by an answer which people voted up because they thought I was joking.

Summary (label changed from original to make intent even clearer):

infinite_loop: // code goes here goto infinite_loop;

Why it's better than the alternatives:

It's specific. goto is the language construct which causes an unconditional branch. Alternatives depend on using structures supporting conditional branches, with a degenerate always-true condition.
The label documents the intent without extra comments.
The reader doesn't have to scan the intervening code for early breaks (although it's still possible for an unprincipled hacker to simulate continue with an early goto).

Rules:

Pretend that the gotophobes didn't win. It's understood that the above can't be used in real code because it goes against established idiom.
Assume that we have all heard of 'Goto considered harmful' and know that goto can be used to write spaghetti code.
If you disagree with an example, criticize it on technical merit alone ('Because people don't like goto' is not a technical reason).

Let's see if we can talk about this like grown ups.

Edit

This question seems finished now. It generated some high quality answers. Thanks to everyone, especially those who took my little loop example seriously. Most skeptics were concerned by the lack of block scope. As @quinmars pointed out in a comment, you can always put braces around the loop body. I note in passing that for(;;) and while(true) don't give you the braces for free either (and omitting them can cause vexing bugs). Anyway, I won't waste any more of your brain power on this trifle - I can live with the harmless and idiomatic for(;;) and while(true) (just as well if I want to keep my job).

Considering the other responses, I see that many people view goto as something you always have to rewrite in another way. Of course you can avoid a goto by introducing a loop, an extra flag, a stack of nested ifs, or whatever, but why not consider whether goto is perhaps the best tool for the job? Put another way, how much ugliness are people prepared to endure to avoid using a built-in language feature for its intended purpose? My take is that even adding a flag is too high a price to pay. I like my variables to represent things in the problem or solution domains. 'Solely to avoid a goto' doesn't cut it.

I'll accept the first answer which gave the C pattern for branching to a cleanup block. IMO, this makes the strongest case for a goto of all the posted answers, certainly if you measure it by the contortions a hater has to go through to avoid it.

I don't get why the gotophobes don't just mandate "#define goto report_to_your_supervisor_for_re_education_through_labour" at the top of the project's include file. If it's always wrong, make it impossible. Otherwise, it's sometimes right...
"can't be used in real code"? I use it in "real" code any time it's the best tool for the job. "Established idiom" is a nice euphemism for "blind dogmatism".
I agree that "goto" can be useful (there are great examples below), but I disgagree with your specific example. A line that says "goto infinite_loop" sounds like it means "go to the part of the code where we're going to start looping forever," as in "initialize(); set_things_up(); goto infinite_loop;" when what you really mean to convey is "begin the next iteration of the loop we're already in," which is completely different. If you're trying to loop, and your language has constructs designed specifically for looping, use those for clarity. while(true) {foo()} is pretty unambiguous.
One major downside to your example is that it is not apparent which ohter places in the code may decide to jump to this label. A "normal" infinite loop (for (;;)) has no surprising entry points.

Mitch Wheat 146k 14 207 299 · Accepted Answer · Apr 23 '11 at 9:04

up vote 55 down vote accepted

Heres one trick I've heard of people using. I've never seen it in the wild though. And it only applies to C because C++ has RAII to do this more idiomatically.

void foo() { if (!doA()) goto exit; if (!doB()) goto cleanupA; if (!doC()) goto cleanupB; // everything succeed return; cleanupB: undoB(); cleanupA: undoA(); exit: return; }

edited Apr 23 '11 at 9:04

Mitch Wheat
146k 14 207 299

answered Oct 29 '08 at 4:08

Greg Rogers
15.8k 6 35 73

5

I knew I shoulda taken that "speed typing" course in high school! Good answer. – Adam Liss Oct 29 '08 at 4:10

6

The extra blocks cause unnecessary indentation that's hard to read. It also doesn't work if one of the conditions is inside a loop. – Adam Rosenfield Oct 29 '08 at 4:42

16

You can see a lot of this kind of code in most low level Unix things (like the linux kernel, for example). In C, that's the best idiom for error recovering IMHO. – David Cournapeau Oct 29 '08 at 7:25

4

As cournape mentioned, the Linux kernel uses this style all the time. – CesarB Oct 29 '08 at 11:00

5

Not only linux kernel take a look in Windows driver samples from microsoft and you will find same pattern. In general this is a C way to handle exceptions and very useful one :). I usually prefer only 1 label and in few cases 2. 3 can be avoided in 99% of cases. – Ilya Oct 29 '08 at 12:37

show 5 more comments

Paul Nathan 20.8k 4 49 123 · Answer 2 · Oct 29 '08 at 4:31

up vote 54 down vote

The classic need for GOTO in C is as follows

for ... for ... if(breakout_condition) goto final; final:

There is no straightforward way to break out of nested loops without a goto.

answered Oct 29 '08 at 4:31

Paul Nathan
20.8k 4 49 123

33

Most often when this need comes up I can use a return instead. But you need to be writing small functions for it to work out that way. – Darius Bacon Oct 29 '08 at 5:50

6

I definitely agree with Darius - refactor it to a function and return instead! – metao Oct 29 '08 at 6:13

7

@metao: Bjarne Stroustrup disagree. On his C++ Programming Language book, this is exactly the example given of a "good use" of goto. – paercebal Oct 29 '08 at 9:45

14

@Adam Rosenfield: The whole problem with goto, highlighted by Dijkstra, is that structured programming offers more comprehensible constructs. Making code harder to read in order to avoid goto proves that the author failed to understand that essay... – Steve Jessop Oct 30 '08 at 0:43

4

@Steve Jessop: Not only have people misunderstood the essay, most of the "no go to" cultists don't understand the historical context in which it was written. – JUST MY correct OPINION Apr 23 '11 at 9:09

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Mitch Wheat 146k 14 207 299 · Answer 3 · Apr 23 '11 at 10:57

up vote 23 down vote

Here's my non-silly example, (from Stevens APITUE) for Unix system calls which may be interrupted by a signal.

restart: if (system_call() == -1) { if (errno == EINTR) goto restart; // handle real errors }

The alternative is a degenerate loop. This version reads like English "if the system call was interrupted by a signal, restart it".

edited Apr 23 '11 at 10:57

Mitch Wheat
146k 14 207 299

answered Oct 29 '08 at 4:07

fizzer
7,255 3 19 37

3

this way is used for example linux scheduler have a goto like this but i would say there are very few cases where backwards goto's are acceptable and in general should be avoided. – Ilya Oct 29 '08 at 12:38

4

@Amarghosh, continue is just a goto in a mask. – jball Jun 3 '10 at 16:31

2

@jball ...hmm.. for the sake of argument, yeah; you can make good readable code with goto and spaghetti code with continue.. Ultimately it depends on the person who writes the code. The point is it is easy to get lost with goto than with continue. And newbies normally use the first hammer they get for every problems. – Amarghosh Jun 4 '10 at 4:06

2

@Amarghosh. Your 'improved' code isn't even equivalent to the original - it loops forever in the success case.– fizzer Jun 4 '10 at 12:05

3

@fizzer oopsie.. it's gonna need two else breaks (one for each ifs) to make it equivalent.. what can I say... goto or not, your code is only as good as you :( – Amarghosh Jun 4 '10 at 12:15

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Mitch Wheat 146k 14 207 299 · Answer 4 · Oct 29 '08 at 4:08

If Duff's device doesn't need a goto, then neither should you! ;)

void dsend(int count) { int n; if (!count) return; n = (count + 7) / 8; switch (count % 8) { case 0: do { puts("case 0"); case 7: puts("case 7"); case 6: puts("case 6"); case 5: puts("case 5"); case 4: puts("case 4"); case 3: puts("case 3"); case 2: puts("case 2"); case 1: puts("case 1"); } while (--n > 0); } }

code above from Wikipedia entry.

This is an example of a logical fallacy also known as the "appeal to authority". Check it out on Wikipedia.

zvrba 12.5k 1 20 44 · Answer 5 · Oct 29 '08 at 7:17

up vote 12 down vote

Knuth has written a paper "Structured programming with GOTO statements", you can get it e.g. fromhere. You'll find many examples there.

answered Oct 29 '08 at 7:17

zvrba
12.5k 1 20 44

This paper looks deep. I will read it though. Thanks – fizzer Oct 29 '08 at 22:09

2

That paper is so out of date, it's not even funny. Knuth's assumptions there simply don't hold any longer. – Konrad Rudolph Jun 29 '09 at 15:17

3

Which assumptions? His examples are as real today for a procedural language as C (he gives them in some pseudo-code) as they were at that time. – zvrba Jun 29 '09 at 17:08

2

im dissappointed you didnt write:: You can GOTO 'here' to get it, the pun to me would have been to unbearable not to make! – RhysW Jan 8 at 13:58

Mitch Wheat 146k 14 207 299 · Answer 6 · Apr 23 '11 at 10:57

I have nothing against gotos in general, but I can think of several reasons why you wouldn't want to use them for a loop like you mentioned:

It does not limit scope hence any temp variables you use inside won't be freed until later.
It does not limit scope hence it could lead to bugs.
It does not limit scope hence you cannot re-use the same variable names later in future code in the same scope.
It does not limit scope hence you have the chance of skipping over a variable declaration.
People are not accustomed to it and it will make your code harder to read.
Nested loops of this type can lead to spaghetti code, normals loops will not lead to spaghetti code.

Points 1-4 are dubious for this example even without the braces. 1 It's an infinite loop. 2 Too vague to address. 3 Trying to hide a variable of the same name in an enclosing scope is poor practice. 4. A backward goto can't skip a declaration.
1-4 as with all infinite loops, you usually have some kind of break condition in the middle. So they are applicable. Re a bakward goto can't skip a declaration... not all gotos are backwards...

ephemient 74.4k 12 103 226 · Answer 7 · Oct 29 '08 at 4:18

Very common.

do_stuff(thingy) { lock(thingy); foo; if (foo failed) { status = -EFOO; goto OUT; } bar; if (bar failed) { status = -EBAR; goto OUT; } do_stuff_to(thingy); OUT: unlock(thingy); return status; }

The only case I ever use goto is for jumping forwards, usually out of blocks, and never into blocks. This avoids abuse of do{}while(0) and other constructs which increase nesting, while still maintaining readable, structured code.

I think this is the typcial C way of error handling. I can not see it replaced nicely, better readable any other way Regards
Why not... do_stuff(thingy) { RAIILockerObject(thingy); ..... /* -->*/ } /* <---*/ :)
@ПетърПетров That's a pattern in C++ that has no analogue in C.

score 7 · Answer 8 · Oct 29 '08 at 4:35

up vote 7 down vote

@fizzer.myopenid.com: your posted code snippet is equivalent to the following:

 while (system_call() == -1) { if (errno != EINTR) { // handle real errors break; } }

I definitely prefer this form.

edited Oct 29 '08 at 4:35

answered Oct 29 '08 at 4:15

Mitch Wheat
146k 14 207 299

1

OK, I know the break statement is just a more acceptable form of goto.. – Mitch Wheat Oct 29 '08 at 4:16

8

To my eyes, it's confusing. It's a loop which you don't expect to enter in the normal execution path, so the logic seems backwards. Matter of opinion, though. – fizzer Oct 29 '08 at 4:23

1

Backwards? It starts, it continues, it falls through. I think this form more obviously states its intentions. – Mitch Wheat Oct 29 '08 at 4:25

7

I'd agree with fizzer here that the goto provides a clearer expectation as to the condition's value. – Marcus Griep Oct 29 '08 at 4:48

3

In which way is this snippet easier to read than fizzer's. – erikkallen Dec 26 '08 at 18:59

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FlySwat 53.1k 34 171 258 · Answer 9 · Oct 29 '08 at 4:01

up vote 6 down vote

Here is an example of a good goto:

// No Code

answered Oct 29 '08 at 4:01

FlySwat
53.1k 34 171 258

3

"NO Goto for you!" – Mitch Wheat Oct 29 '08 at 4:06

er, no gotos? (unsuccessful attempt at funny :d ) – moogs Oct 29 '08 at 4:18

LOL!! – Anonymous Type Dec 22 '10 at 1:00

Adam Liss 23.5k 8 57 98 · Answer 10 · Oct 29 '08 at 4:09

One good place to use a goto is in a procedure that can abort at several points, each of which requires various levels of cleanup. Gotophobes can always replace the gotos with structured code and a series of tests, but I think this is more straightforward because it eliminates excessive indentation:

if (!openDataFile())
  goto quit;

if (!getDataFromFile())
  goto closeFileAndQuit;

if (!allocateSomeResources)
  goto freeResourcesAndQuit;

// Do more work here....

freeResourcesAndQuit:
   // free resources
closeFileAndQuit:
   // close file
quit:
   // quit!

I would replace this with a set of functions: freeResourcesCloseFileQuit, closeFileQuit, and quit.
If you created these 3 extra functions, you'd need to pass pointers/handles to the resources and files to be freed/closed. You'd probably make freeResourcesCloseFileQuit() call closeFileQuit(), which in turn would call quit(). Now you have 4 tightly coupled functions to maintain, and 3 of them will probably be called at most once: from the single function above. If you insist on avoiding goto, IMO, nested if() blocks have less overhead and are easier to read and maintain. What do you gain with 3 extra functions?

Patrick 15.6k 8 33 51 · Answer 11 · Oct 29 '08 at 6:49

Even though I've grown to hate this pattern over time, it's in-grained into COM programming.

#define IfFailGo(x) {hr = (x); if (FAILED(hr)) goto Error} ... HRESULT SomeMethod(IFoo* pFoo) { HRESULT hr = S_OK; IfFailGo( pFoo->PerformAction() ); IfFailGo( pFoo->SomeOtherAction() ); Error: return hr; }

score 1 · Answer 12 · Jan 5 '11 at 19:29

I've seen goto used correctly but the situations are normaly ugly. It is only when the use of goto itself is so much less worse than the original. @Johnathon Holland the poblem is you're version is less clear. people seem to be scared of local variables:

void foo() { bool doAsuccess = doA(); bool doBsuccess = doAsuccess && doB(); bool doCsuccess = doBsuccess && doC(); if (!doCsuccess) { if (doBsuccess) undoB(); if (doAsuccess) undoA(); } }

And I prefer loops like this but some people prefer while(true).

for (;;) { //code goes here }

Never, ever, ever compare a boolean value to true or false. It's already a boolean; just use "doBsuccess = doAsuccess && doB();" and "if (!doCsuccess){}" instead. It's simpler and protects you from clever programmers who write things like "#define true 0" because, well, because they can. It also protects you from programmers who mix int's and bool's, and then assume any non-zero value is true.
Thank you and point taken == true removed. This is closer to my real style anyway. :)

Alexandre C. 28.7k 1 42 107 · Answer 13 · Aug 4 '11 at 9:54

If your intent is to run a (small) service like:

int main(int argc, char *argv[]) { setup_blabla(argc, argv); for (;;) // I prefer this to while(1) since some // compilers (MSVC) warn about the latter { ... } }

then an exotic alternative that I like is to use tail call optimization like that:

int main(int argc, char *argv[]) { setup_blabla(argc, argv); // this sets up globals return service_loop(); } volatile int interrupted; int service_loop() { // Do something // You can even use static variables here. ... ... if (interrupted) { cleanup(); return EXIT_FAILURE; } ... ... return service_loop(); // Restart the loop }

The intent is quite clean this way. No need for while(true) and break statements. Even a background service will have signal handlers which will trigger cleanup code and early exit, so you'll have to break or goto anyway.

The real (only ?) interest is that you save a scope: no nested loop with an extra pair of braces insideservice_loop.

Externalizing the loop implies that you must dispatch (eg. via a switch statement) error codes outside theservice_loop function in order to restart the loop. No such thing here.

Beware however that coding standard nazis will object to using TCO instead of the politically correctfor(;;) or while(1). Send them to hell.

This code is NOT guarranteed to UNWIND the Stack! Some compilers will do a Stack Overflow!

Jim Nelson 901 8 16 · Answer 14 · Oct 29 '08 at 4:11

My gripe about this is that you lose block scoping; any local variables declared between the gotos remains in force if the loop is ever broken out of. (Maybe you're assuming the loop runs forever; I don't think that's what the original question writer was asking, though.)

The problem of scoping is more of an issue with C++, as some objects may be depending on their dtor being called at appropriate times.

For me, the best reason to use goto is during a multi-step initialization process where the it's vital that all inits are backed out of if one fails, a la:

if(!foo_init()) goto bye; if(!bar_init()) goto foo_bye; if(!xyzzy_init()) goto bar_bye; return TRUE; bar_bye: bar_terminate(); foo_bye: foo_terminate(); bye: return FALSE;

user25306 185 2 4 6 · Answer 15 · Oct 29 '08 at 5:53

I don't use goto's myself, however I did work with a person once that would use them in specific cases. If I remember correctly, his rationale was around performance issues - he also had specific rules for how. Always in the same function, and the label was always BELOW the goto statement.

Additional data: note that you cannot use goto to go outside the function, and that in C++, it is forbidden to bypass an object construction through a goto

score -1 · Answer 16

@Greg:

Why not do your example like this:

void foo() { if (doA()) { if (doB()) { if (!doC()) { UndoA(); UndoB(); } } else { UndoA(); } } return; }

【编程思考】如何使用叛逆的goto ?

16 Answers

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