An Empirical Study of Goto in C Code from GitHub Repositories

Mei Nagappan, Romain Robbes, Yasutaka Kamei, Eric Tanter, Shane McIntosh, Audris Mockus, Ahmed E. Hassan

Go-to statement considered

harmful (1968)

A Case against the GO TO Statement

(1968)

Do Practitioners care?

Do Practitioners care?

https://xkcd.com/292/

Do Practitioners care?

Do Practitioners care?

Do Practitioners care?

Impact of Article on Research Community

Impact of Article on Research Community

Impact of Article on Research Community

Impact of Article

Koenig: Dijkstra provides strong logical evidence for why goto statements can introduce problems in software.

Well informed and often well argued opinions

Shift from opinion to facts

Our Goal: Empirically Examine the use of goto

Shift from opinion to facts

Example

Label

Goto Statement

PRELIMINARY ANALYSIS

Do developers use goto statements in their source code?

Experiment

11,627 Projects Goto Miner

11,627 Projects Goto Miner 3,093

Projects

Despite the popularity of Dijkstra’s case against goto…

11,627 Projects Goto Miner 3,093

Projects

246,657 out of the 2,150,387 files (11.47%)

Extent of use of goto statements by Developers is non-trivial

Two Research Questions

Two Research Questions

RQ1: What are goto statements used for?

Two Research Questions

RQ1: What are goto statements used for?

RQ2: Do developers remove/modify goto statements?

Qualitative Analysis

246,669 C files

384files

Properties and Purposesof Goto

IterativeTagging

Sample Selection

Basic Bean Counting

Num of Goto Statements per Function

Num of Goto Labels per Function

Not many goto statements per function

Num of Goto Statements per Function

Num of Goto Labels per Function

Not many goto statements per function

Very few lines of code in label blocksMedian = 4 LOC

Properties

Context

Why did Dijkstra think goto statements would have ‘disastrous effects’?

‘The go to statement as it stands is just too primitive; it is too much an invitation to make a mess of one’s program.’

Desire to make programs verifiable

Desire to make programs verifiable

3 ways to reach the code under the label “back”

First way: Top down execution

Second way: condition2 == true

Third way: done == false

Difficult to know what has been executed so far.

Hence go to statements must be avoided!

Our Findings

Multiple goto jumps to the same label

This is what Dijkstra feared about!

62%

Single point of entry into label block!

9%

But not all use of goto is like what Dijkstra feared!

Stacking labels at the bottom of functions is prevalent

27%

Spaghetti code is uncommon

6%

Most of the jumps are forward, not backwards

90%

14%

Purposes

Most goto usage is for error handling and cleanup

Error Handling = 80%Cleanup = 40%

Most goto usage is for error handling and cleanup

Error Handling = 80%

Goto and labels used to emulate a try/catch or finally mechanism

Cleanup = 40%

Less intuitive usages such as control-exit and loop-create are less common

10%

Less intuitive usages such as control-exit and loop-create are less common

9%Most usages of goto statements appear to be for

error handling and cleanup

Quantitative Analysis

Mine Commits

6 OSS Projects

Quantitative Analysis

Mine Commits

6 OSS Projects

Link to Bugs

180 Days Of Commit History

Quantitative Analysis

Mine Commits

6 OSS Projects

Link to Bugs

180 Days Of Commit History

Bug Fix Commits

Extract Goto

Almost no Gotos are removed/modified in the post-release phase of a project

clamav-d

evel

ghostp

dlgim

p

openldap

postgresq

lVTK

02468

10121416

RemovedModified

Even fewer Gotos are removed/modified in the post-release bug fixes

clamav-d

evel

ghostp

dlgim

p

openldap

postgresq

lVTK

0

1

2

3

4

5

6

RemovedModified

Even fewer Gotos are removed/modified in the post-release bug fixes

clamav-d

evel

ghostp

dlgim

p

openldap

postgresq

lVTK

0

1

2

3

4

5

6

RemovedModified

Developers did not remove/modify goto statements in the post-release phase.

Summary