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It only takes a minute to sign up. I just found out something really quite extraordinary. While looking through Stackoverflow, I came across a question about removing goto from a php function. PHP doesn't have goto I thought and looked it up on php. It turns out I was sort of right. PHP introducted goto in version 5.

PHP didn't start out with goto it actually introduced it into the language in ! Why on earth with all the horror stories we have from 40 years of bad programs written with goto in other languages would php actually decide to introduce it? The php. What possible reason from a technical point of view could there be for introducing a the goto feature, which I nievely thought had been vanquished from modern computer programming languages? There are two kinds of programmers: Those who for whatever reason never, ever use goto, and go to any lengths to avoid it, and those who use goto in one of the very rare situations where it is the best solution.

Someone said here in a comment that goto could be misused and programmers would find ways to misuse it - but those people are not programmers. It makes sense to add "goto" to a language that doesn't have it if there is evidence that there are situations where it improves the code, and if it is not too difficult to implement. So I would assume that there was such evidence. The goto in itself isn't "evil", it is just easy to misuse. For example the Linux kernel uses goto often, in a disciplined and safe way. Most algorithms and forms of business logic can fit nicely into the control structures that are built into modern languages, in large measure because such structures were designed to fit the needs of common applications.

Some applications, however, need to implement algorithms or business logic that do not fit any such control structures. Programs are generally most readable if the general design of the program matches the design of the algorithm or business logic implemented thereby. Most algorithms and forms of business logic will either fit naturally into common control structures, or can easily be adjusted to do so. Most structured programming languages provide a more complex iteration construct that allows a counter or array index to be incremented or decremented with each iteration.

This construct is functionally equivalent to a do-while loop, but provides a clearer intent of the controlling entity the counter or index of the loop iterations:. To fully support the for-loop , a language should handle negative increment values as well. The form of the for-loop shown above iterates the body of the loop zero or more times.

Other for-loop variants test the control variable a. Other variants of the for-loop allow more than one loop counter or loop index to be specified. A common programming problem is handling loop-and-a-half constructs, i. The following code uses a break statement to do this:. Some languages provide special syntax specifically for terminating a loop in the middle of its body:. Both forms replace the use of an explicit goto, as shown in the following code:.

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A related programming problem is coding a loop to execute a portion of its body and then skip the rest if some condition occurs, forcing the next iteration of the loop. Some languages provide a continue statement for this:. Several structured programming languages do not provide goto statements at all, including Modula-2, Modula-3, Oberon, Eiffel, and Java, on the assumption that the other flow control mechanisms they do provide are sufficient for all programming tasks and thus goto statements should never be needed.

This is not always a good assumption to make when designing a programming language. Language designers cannot anticipate all possible programming scenarios, and providing an "escape mechanism" out of the normal control structures gives the programmer the ability to program around the syntactic limitations imposed by the language when the need arises. Some examples of inadequate flow control constructs are discussed in more detail below. It is also worth noting that there are programming languages that do not provide structured flow control constructs at all. Many functional programming languages, such as LISP, Scheme, and Prolog, do not provide traditional structured flow control constructs beyond if-then-else or only provide very simple forms of them.

Iteration in such languages is generally accomplished using some form of recursion, and these languages are in fact tailored specifically for recursive tasks and data structures. But what such languages lack in structured flow statements, they generally make up for by providing powerful dynamic programming mechanisms and extremely flexible non-homogeneous data types.

Good programming language design dictates that a language should provide a sufficiently complete and powerful set of flow control constructs in order to make it relatively easy to write efficient code for any programming task. The set of flow control statements and clauses provided should be powerful and flexible enough so that a programmer can express his ideas clearly and succintly without having to resort to the use of extraneous control variables or to rearrange his code unnaturally just to get around the syntactical restrictions of the language.

The following sections discuss two major programming problems that traditionally have been solved by using goto statements. These problems are presented in the light of current programming techniques, with an eye to seeing if the programming languages currently available are sufficiently advanced to handle them without using goto. Dijkstra's call for the complete elimination of goto statements is fine in theory, but would it work in practice? The control flow statements described above are sufficient for most programming logic, but there are programming situations that require more powerful constructs.

A common use for goto statements is to exit early from within loops, especially if the exit must break out from within two or more levels of nested loops. In order to exit a loop early without using such mechanisms, a trade-off must be made in which an additional flag boolean variable to signal completion of the loop is used.

Introduction

This incurs the overhead of an extra variable and an extra test of that variable at the top of each loop. For more complicated exits from loops, namely breaking out of a loop iteration that is nested within two or more levels of loops, more extensive support is required of the language.

Background

Some languages, such as Java, provide the capability of breaking out of loops that are prefixed with label names. The alternative is to use an extra variable and extra if statements to avoid the use of gotos, as in Example L Another common use for goto statements is in the handling of exceptions , or what Dijkstra called abortion clauses. Some languages notably the more recent object-oriented languages provide exception handling mechanisms for dealing with synchronous error conditions, while older languages do not. The code below is a C function that utilizes goto statements fairly effectively for error handling and recovery.

Since C does not not have any kind of exception handling mechanism, well-crafted goto statements provide a reasonable substitute. Consider a procedure that performs four operations: Allocate a control object. Save a copy of a specified file name. Open the named file. Read a header block from the opened file. Any of the four operations can fail, which causes the whole function to fail.


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After each failure, however, resources must be deallocated. Thus a failure at point E-1 requires corresponding clean-up code at point H-1, and likewise for failures at E-2 and E These clean-up operations are performed in the reverse order in which their corresponding allocation operations are performed. This type of use of goto statements is generally accepted as a "correct" use of goto.

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Specifically, using gotos for error handling is generally considered acceptable programming style, at least for languages like C that do not provide exception handling control structures. One must be careful, however, to make it obvious that the goto statements are for error handling, e. The problem with this style of error handling is that we end up with a lot of duplicated clean-up code.

Note that, as before, the clean-up operations are performed in the reverse order that their corresponding allocation operations are performed. This style of error handling comes close to being as clean and succint as the style used in Example E However, it requires an additional error indicator variable and extra conditional if statements. Note that the amount of work required to clean up after a failure is exactly the same as in Example E Moreover, the explicit use of a try-catch statement makes it obvious that the code is for error handling and recovery.

Moving the operations at H-2 and H-3 into the destructor allows us to write a simpler catch clause to handle failures. While the try-catch solution is cleaner than using goto s, it has the disadvantage of requiring more overhead for managing the try and catch clauses, which can be fairly expensive. Some languages, such as Java, provide a finally clause as part of the try-catch statement to provide a way to specify actions that must be taken regardless of whether or not an exception occurs.

Other languages, such as Eiffel, provide a retry statement as part of exception handling to allow a procedure body to be executed again after an exception is caught, presumably after some corrective actions are performed. It is obvious that loop escape and exception handler statements make for more readable and more efficient code. However, they do not corrupt or obfuscate the program execution state. Examples T-2 and N-1 demonstrate that Dijkstra's maxim can be achieved provided that the programming language provides a reasonable set of control structures that can serve in place of simple goto statements.

Examples E-1 and N-2 demonstrate the corollary to this, that if a programming language does not provide reasonably powerful flow control structures, there are programming problems that can be solved reasonably well only by resorting to the use of goto statements. Some structured programming languages do not provide goto statements at all. Languages such as Smalltalk, Eiffel, and Java provide control statements for early and nested loop exits and exception handling, so goto is not really needed.

Other languages such as Modula-2 and Oberon also do not provide goto, but appear to lack enough flow control constructs to make it convenient to write early loop exits and exception handling code; it would seem that such languages were linguistic experiments that took Dijkstra's maxim too far and failed. Dijkstra's belief that unstructured goto statements are detrimental to good programming is still true. This is the Tao of goto: knowing when to use it for good and when not to use it for evil. In parting, I can't resist giving one last example of goto statements.


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Biography of Edsger W. Dijkstra born May , died Aug Wikipedia: en. Biography of C. Tony Hoare Wikipedia: en. Biography of Niklaus Wirth Wikipedia: en. Goto programming statement Wikipedia: en. Formal Verification Wikipedia: en. Program Derivation Wikipedia: en. Hoare Logic Wikipedia: en. Programming Language Newsgroups groups. Annotations by David R. References: Wirth, Niklaus, and Hoare C. ACM 9 June , I prefer to use nested if s, which I think are more readable. When this would lead to a too deep nesting, I'd opt either to decompose my function in smaller parts, or use an boolean indicator in cascade.

Today's optimizing compilers are clever enough to generate almost the same code than the same code with goto. Java's popularity proves that goto statement can be avoided in a modern language. Important Disclaimer: in view of the fierce discussion in the comments, I want to clarify that I don't pretend that the goto statement is the only cause of this bug.

I don't pretend that without goto there would be no bug. I just want to show that a goto can be involved in a serious bug. I don't know how many serious bugs are related to goto in the history of programming: details are often not communicated. The statement that led to this bug was a wrong goto statement. I can't neither confirm any of them: all these arguments are probable hypotheses and interpretation. Nobody really knows. The only objective fact is that a duplicated goto led to exit the function prematurely. Looking at the code, the only other single statement that could have caused the same effect would have been a return.

The History, Controversy, and Evolution of the Goto Statement [pdf] | Hacker News

Indeed curly braces around the conditional block could have prevented the bug: it would have led either to a syntax error at compilation and hence a correction or to a redundant harmless goto. By the way, GCC 6 would be able to spot these errors thanks to its optional warning to detect inconsistent indentation. But in first place, all these gotos could have been avoided with more structured code. So goto is at least indirectly a cause of this bug.

There are at least two different ways that could have avoided it:. Instead of testing lots of conditions for error sequentially, and each time sending to a fail label in case of problem, one could have opted for executing the cryptographic operations in an if -statement that would do it only if there was no wrong pre-condition:. This approach is based on the fact that almost all the statements here call some function to set an err error code, and execute the rest of the code only if err was 0 i.

A nice safe and readable alternative is:. Here, there is not a single goto: no risk to jump to quickly to the failure exit point. This construct is more compact. In fact, the assembler generated by an optimizing compiler is almost equivalent to the original code with gotos: The optimizer detects very well the chain of conditions and generate code, which at the first non null return value jumps to the end online proof.

You could even envisage a consistency check at the end of the function that could during the testing phase identify mismatches between the ok flag and the error code. As said: I don't pretend that alternative constructs would have avoided any bug. I just want to say that they could have made the bug more difficult to occur. The famous Dijkstra article was written at a time when some programming languages were actually capable of creating subroutines having multiple entry and exit points.

In other words, you could literally jump into the middle of a function, and jump out at any place within the function, without actually calling that function or returning from it in the conventional way. That's still true of assembly language. Nobody ever argues that such an approach is superior to the structured method of writing software that we now use.

In most modern programming languages, functions are very specifically defined with one entry and one exit point. The entry point is where you specify the parameters to the function and call it, and the exit point is where you return the resulting value and continue execution at the instruction following the original function call. Within that function, you ought to be able to do whatever you wish, within reason. If putting a goto or two in the function makes it clearer or improves your speed, why not?

The whole point of a function is to sequester a bit of clearly-defined functionality, so that you don't have to think about how it works internally anymore. Once it's written, you just use it. And yes, you can have multiple return statements inside a function; there's still always one place in a proper function from which you return the back side of the function, basically.

That's not at all the same thing as jumping out of a function with a goto before it has a chance to properly return. So it's not really about using goto's. It's about avoiding their abuse. Everyone agrees that you can make a terribly convoluted program using gotos, but you can do the same by abusing functions as well it's just a lot easier to abuse gotos. For what it's worth, ever since I graduated from line-number-style BASIC programs to structured programming using Pascal and curly-brace languages, I've never had to use a goto.

The only time I've been tempted to use one is to do an early exit from a nested loop in languages that don't support multi-level early exit from loops , but I can usually find another way that is cleaner. I used to use goto statements when writing BASIC programs as a child as a simple way to get for and while loops Commodore 64 BASIC didn't have while-loops, and I was too immature to learn the proper syntax and usage of for-loops.

My code was frequently trivial, but any loop bugs could be immediately attributed to my usage of goto. I now use primarily Python, a high level programming language that has determined it has no need for goto. When Edsger Dijkstra declared "Goto considered harmful" in he did not give a handful of examples where related bugs could be blamed on the goto, rather, he declared that goto was unnecessary for higher level languages and that it should be avoided in favor of what we consider normal control flow today: loops and conditionals.

His words:. The unbridled use of the go to has an immediate consequence that it becomes terribly hard to find a meaningful set of coordinates in which to describe the process progress. He probably had mountains of examples of bugs from every time he debugged code with goto in it. But his paper was a generalized position statement backed up by proof that goto was unnecessary for higher level languages.

The generalized bug is that you may have no ability to statically analyze the code under question. Code is much harder to statically analyze with goto statements, especially if you jump back in your control flow which I used to do or to some unrelated section of code. Code can and was written this way. It was done to highly optimize for very scarce computing resources and thus the architectures of the machines.

There was a blackjack program that a maintainer found to be quite elegantly optimized but also impossible for him to "fix" due to the nature of the code. It was programmed in machine code which is heavily reliant on gotos - so I think this story quite relevant. This is the best canonical example I can think of. However, the C source of CPython the most common and reference Python implementation uses goto statements to great effect. They are used to bypass irrelevant control flow inside functions to get to the end of functions, making the functions more efficient without losing readability.

This respects one of the ideals of structured programming - to have a single exit point for functions. When the wigwam was current architectural art, their builders could undoubtedly give you sound practical advice regarding construction of wigwams, how to let smoke escape, and so on. Fortunately, today's builders can probably forget most of that advice. When the stagecoach was current transportational art, their drivers could undoubtedly give you sound practical advice regarding horses of stagecoaches, how to defend against highwaymen, and so on.

Fortunately, today's motorists can forget most of that advice, too. When punched cards were current programming art, their practitioners could likewise give you sound practical advice regarding organization of cards, how to number statements, and so on. I am not sure that that advice is very relevant today. Are you old enough even to know the phrase "to number statements"?

You don't need to know it, but if you don't, then you aren't familiar with the historical context in which the warning against goto was principally relevant. The warning against goto is just not very relevant today. When you do think of it, you probably have a reason, so go ahead. Answer: sure, it can be abused, but its abuse is a pretty tiny problem in software engineering compared to far, far more common mistakes such as using a variable where a constant will serve, or such as cut-and-paste programming otherwise known as neglect to refactor.

I doubt that you're in much danger.

goto.slides.pdf - Structured Programming with goto...

Unless you are using longjmp or otherwise transferring control to faraway code, if you think to use a goto or you would just like to try it for fun, go ahead. You'll be fine. You may notice the lack of recent horror stories in which goto plays the villain. Most or all of these stories seem to be 30 or 40 years old. You stand on solid ground if you regard those stories as mostly obsolete. To add one thing to the other excellent answers, with goto statements it can be difficult to tell exactly how you got to any given place in the program.

You can know that an exception occurred at some specific line but if there are goto 's in the code there is no way to tell what statements executed to result in this exception causing state without searching the whole program. There is no call stack and no visual flow. It is possible that there is a statement lines away which put you in a bad state executed a goto to the line which raised an exception. Programming correct code is hard. Writing correct programs is hard, determining if programs are correct is hard, proving programs correct is hard.

It does not help make checking for correctness easier, while its alternatives often do. There are styles of programming where goto is the appropriate solution. There are even styles of programming where its evil twin, comefrom, is the appropriate solution. In both of these cases, extreme care has to be done to ensure you are using it in an understood pattern, and lots of manual checking that you aren't doing something difficult to ensure correctness with it.

As an example, there is a feature of some languages called coroutines. Coroutines are threadless threads; execution state without a thread to run it on. You can ask them to execute, and they can run part of themselves and then suspend themselves, handing back flow control. There are cases where coroutines are so useful that writing them manually and carefully is worth it. The goto gets hidden behind a higher level abstraction, like while or for or if or switch. Those higher level abstractions are easier to prove correct and check.

If the language was missing some of them like some modern languages are missing coroutines , either limpling along with a pattern that doesn't fit the problem, or using goto, becomes your alternatives. Getting a computer to vaguely do what you want it to do in common cases is easy. Writing reliably robust code is hard. Goto helps the first far more than it helps the second; hence "goto considered harmful", as it is a sign of superficially "working" code with deep and hard to track down bugs. With sufficient effort you can still make code with "goto" reliably robust, so holding the rule of as absolute is wrong; but as a rule of thumb, it is a good one.

But the control flow is absolutely not clear here, and code is mixed together in ways that make it very unclear what's going on.