c - NULL pointer comparison to zero segfaults -

-

so, have piece of c code:

226     if (!pair) 227         return; 228     if (!pair->index) 229         free(pair->index);

i running through non-null 'pair' pointer has null (0) member 'index'. works wonderfully, 1 might expect. on other hand, this

226     if (!pair) 227         return; 228     if (pair->index!=null) 229         free(pair->index);

generates segmentation fault (on line 228, if is). seems weird, since 2 should identical, right? (the second makes more sense me first, that's why used in first place)

i fine using negative expression works, want understand why second segfaults. ideas? :)

(i building gcc (debian 4.7.2-5) 4.7.2 )

thanks!

first thing note, standard c has null check built free ought not check again yourself.

in first snippet, line if (!pair->index) free(pair->index); benign due typo: free called if pair->index null, , free pass on i've said. have errant ! in if statement. program unlikely crash there. (technically might if pair->index uninitialised since use of uninitialised variable undefined behaviour in c).

there no problem in second snippet present, unless pair->index not pointing memory given prior call malloc, calloc etc. if sure own memory @ pair->index problem due heap corruption or undefined behaviour construct elsewhere in program.


Comments

Post a Comment

Popular posts from this blog

c++ - QTextObjectInterface with Qml TextEdit (QQuickTextEdit) -

-
i registred handler simple qtextobjectinterface , draws 10x10 red rectangle. when used qtextedit in normal qwidget app, worked. when used qquicktextedit (textedit qml component) in qt quick app, doesn't worked (nothing drawed, rectangle in textedit reserved, because when change cursor position, notice there something, empty space, nothing drawed. qtextobjectinterface intrinsicsize method called (that explains why see there empty space 10x10), drawobject method isn't. i did research , found problem here: qquicktextedit.cpp qt 5.3.0 sources (line 1821) qsgnode *qquicktextedit::updatepaintnode(qsgnode *oldnode, updatepaintnodedata *updatepaintnodedata) { . . . if (textframe->firstposition() > textframe->lastposition() && textframe->frameformat().position() != qtextframeformat::inflow) { updatenodetransform(node, d->document->documentlayout()->frameboundingrect(textframe).topleft()); c...
Read more

javascript - angular ng-required radio button not toggling required off in firefox 33, OK in chrome -

-
i trying toggle required attribute on radio button using angular show html5 popup. i cannot following code function correctly in firefox 33. function correctly in chrome. required property being toggled via checkbox value, in firefox, when set false radio button still acts required , shows html5 required popup. see plnkr below example doesn't work me in ff 33. <!doctype html> <html> <head> <script data-require="angular.js@*" data-semver="1.3.1" src="//code.angularjs.org/1.3.1/angular.js"></script> <link href="style.css" rel="stylesheet" /> <script src="script.js"></script> </head> <body ng-app="myapp"> <script> angular.module('myapp',[]) .controller('mycontroller',['$scope',function($scope) { $scope.foorequired=true; $scope.submitform = function() { $scope.submitmessage = ...
Read more

xcode - Swift Playground - Files are not readable -

-
Image
files not readable in swift playground . how make files readable? same code runs on xcode terminal app, fails on swift playground. demo code below. import foundation println("hello, world!") var fname:string = "/users/holyfield/desktop/com.apple.iconcomposer.plist" var fm:nsfilemanager = nsfilemanager.defaultmanager() if(fm.fileexistsatpath(fname)){ println("file exists") if(fm.isreadablefileatpath(fname)){ println("file readable") var fd:nsdata? = nsdata(contentsoffile: fname) println(fd?.length) let pl = nsdictionary(contentsoffile: fname) println(pl?.count) println(pl?.allkeys) }else{ println("file not readable") } } else{ println("file not exists") } sample images: i have thank nate cook first quick response , solid answer. just case share answer post, title misleading. see nate's answer here: https://stackoverf...
Read more