reference, declarationdefinition
definition → references, declarations, derived classes, virtual overrides
reference to multiple definitions → definitions
unreferenced
    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
   24
   25
   26
   27
   28
   29
   30
   31
   32
   33
   34
   35
   36
   37
   38
   39
   40
   41
   42
   43
   44
   45
   46
   47
   48
   49
   50
   51
   52
   53
   54
   55
   56
   57
   58
   59
   60
   61
   62
   63
   64
   65
   66
   67
   68
   69
   70
   71
   72
   73
   74
   75
   76
   77
   78
   79
   80
   81
   82
   83
   84
   85
   86
   87
   88
   89
   90
   91
   92
   93
   94
   95
   96
   97
   98
   99
  100
  101
  102
  103
  104
  105
  106
  107
  108
  109
  110
  111
  112
  113
  114
  115
  116
  117
  118
  119
  120
  121
  122
  123
  124
  125
  126
  127
  128
  129
  130
  131
  132
  133
  134
  135
  136
  137
  138
  139
  140
  141
  142
  143
  144
  145
  146
  147
  148
  149
  150
  151
  152
  153
  154
  155
  156
  157
  158
  159
  160
  161
  162
  163
  164
  165
  166
  167
  168
  169
  170
  171
  172
  173
  174
  175
  176
  177
  178
  179
  180
  181
  182
  183
  184
  185
  186
  187
  188
  189
  190
  191
  192
  193
  194
  195
  196
  197
  198
  199
  200
  201
  202
  203
  204
  205
  206
  207
  208
  209
  210
  211
  212
  213
  214
  215
  216
  217
  218
  219
  220
  221
  222
  223
  224
  225
  226
  227
  228
  229
  230
  231
  232
  233
  234
  235
  236
  237
  238
  239
  240
  241
  242
  243
  244
  245
  246
  247
  248
  249
  250
  251
  252
  253
  254
  255
  256
  257
  258
  259
  260
  261
  262
  263
  264
  265
  266
  267
  268
  269
  270
  271
  272
  273
  274
  275
  276
  277
  278
  279
  280
  281
  282
  283
  284
  285
  286
  287
  288
  289
  290
  291
  292
  293
  294
  295
  296
  297
  298
  299
  300
  301
  302
  303
  304
  305
  306
  307
  308
  309
  310
  311
  312
  313
  314
  315
  316
  317
  318
  319
  320
  321
  322
  323
  324
  325
  326
  327
  328
  329
  330
  331
  332
  333
  334
  335
  336
  337
  338
  339
  340
  341
  342
  343
  344
  345
  346
  347
  348
  349
  350
  351
  352
  353
  354
  355
  356
  357
  358
  359
  360
  361
  362
  363
  364
  365
  366
  367
  368
  369
  370
  371
  372
  373
  374
  375
  376
  377
  378
  379
  380
  381
  382
  383
  384
  385
  386
  387
  388
  389
  390
  391
  392
  393
  394
  395
  396
  397
  398
  399
  400
  401
  402
  403
  404
  405
  406
  407
  408
  409
  410
  411
  412
  413
  414
  415
  416
  417
  418
  419
  420
  421
  422
  423
  424
  425
  426
  427
  428
  429
  430
  431
  432
  433
  434
  435
  436
  437
  438
  439
  440
  441
  442
  443
  444
  445
  446
  447
  448
  449
  450
  451
  452
  453
  454
  455
  456
  457
  458
  459
  460
  461
  462
  463
  464
  465
  466
  467
  468
  469
  470
  471
  472
  473
  474
  475
  476
  477
  478
  479
  480
  481
  482
  483
  484
  485
  486
  487
  488
  489
  490
  491
  492
  493
  494
  495
  496
  497
  498
  499
  500
  501
  502
  503
  504
  505
  506
  507
  508
  509
  510
  511
  512
  513
  514
  515
  516
  517
  518
  519
  520
  521
  522
  523
  524
  525
  526
  527
  528
  529
  530
  531
  532
  533
  534
  535
  536
  537
  538
  539
  540
  541
  542
  543
  544
  545
  546
  547
  548
  549
  550
  551
  552
  553
  554
  555
  556
  557
  558
  559
  560
  561
  562
  563
  564
  565
  566
  567
  568
  569
  570
  571
  572
  573
  574
  575
  576
  577
  578
  579
  580
  581
  582
  583
  584
  585
  586
  587
  588
  589
  590
  591
  592
  593
  594
  595
  596
  597
  598
  599
  600
  601
  602
  603
  604
  605
  606
  607
  608
  609
  610
  611
  612
  613
  614
  615
  616
  617
  618
  619
  620
  621
  622
  623
  624
  625
  626
  627
  628
  629
  630
  631
  632
  633
  634
  635
  636
  637
  638
  639
  640
  641
  642
  643
  644
  645
  646
  647
  648
  649
  650
  651
  652
  653
  654
  655
  656
  657
  658
  659
  660
  661
  662
  663
  664
  665
  666
  667
  668
  669
  670
  671
  672
  673
  674
  675
  676
  677
  678
  679
  680
  681
  682
  683
  684
  685
  686
  687
  688
  689
  690
  691
  692
  693
  694
  695
  696
  697
  698
  699
  700
  701
  702
  703
  704
  705
  706
  707
  708
  709
  710
  711
  712
  713
  714
  715
  716
  717
  718
  719
  720
  721
  722
  723
  724
  725
  726
  727
  728
  729
  730
  731
  732
  733
  734
  735
  736
  737
  738
  739
  740
  741
  742
  743
  744
  745
  746
  747
  748
  749
  750
  751
  752
  753
  754
  755
  756
  757
  758
  759
  760
  761
  762
  763
  764
  765
  766
  767
  768
  769
  770
  771
  772
  773
  774
  775
  776
  777
  778
  779
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
          "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
  <title>Source Annotations</title>
  <link type="text/css" rel="stylesheet" href="menu.css">
  <link type="text/css" rel="stylesheet" href="content.css">
  <script type="text/javascript" src="scripts/menu.js"></script>
</head>
<body>

<div id="page">
<!--#include virtual="menu.html.incl"-->

<div id="content">

<h1>Source Annotations</h1>

<p>The Clang frontend supports several source-level annotations in the form of
<a href="https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html">GCC-style
attributes</a> and pragmas that can help make using the Clang Static Analyzer
more useful. These annotations can both help suppress false positives as well as
enhance the analyzer's ability to find bugs.</p>

<p>This page gives a practical overview of such annotations. For more technical
specifics regarding Clang-specific annotations please see the Clang's list of <a
href="https://clang.llvm.org/docs/LanguageExtensions.html">language
extensions</a>. Details of &quot;standard&quot; GCC attributes (that Clang also
supports) can be found in the <a href="https://gcc.gnu.org/onlinedocs/gcc/">GCC
manual</a>, with the majority of the relevant attributes being in the section on
<a href="https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html">function
attributes</a>.</p>

<p>Note that attributes that are labeled <b>Clang-specific</b> are not
recognized by GCC. Their use can be conditioned using preprocessor macros
(examples included on this page).</p>

<h4>Specific Topics</h4>

<ul>
<li><a href="#generic">Annotations to Enhance Generic Checks</a>
  <ul>
    <li><a href="#null_checking"><span>Null Pointer Checking</span></a>
    <ul>
      <li><a href="#attr_nonnull"><span>Attribute 'nonnull'</span></a></li>
    </ul>
    </li>
  </ul>
</li>
<li><a href="#macosx">Mac OS X API Annotations</a>
  <ul>
    <li><a href="#cocoa_mem">Cocoa &amp; Core Foundation Memory Management Annotations</a>
    <ul>
      <li><a href="#attr_ns_returns_retained">Attribute 'ns_returns_retained'</a></li>
      <li><a href="#attr_ns_returns_not_retained">Attribute 'ns_returns_not_retained'</a></li>
      <li><a href="#attr_cf_returns_retained">Attribute 'cf_returns_retained'</a></li>
      <li><a href="#attr_cf_returns_not_retained">Attribute 'cf_returns_not_retained'</a></li>
      <li><a href="#attr_ns_consumed">Attribute 'ns_consumed'</a></li>
      <li><a href="#attr_cf_consumed">Attribute 'cf_consumed'</a></li>
      <li><a href="#attr_ns_consumes_self">Attribute 'ns_consumes_self'</a></li>
    </ul>
    </li>
    <li><a href="#osobject_mem">Libkern Memory Management Annotations</a>
      <ul>
        <li><a href="#attr_os_returns_retained">Attribute 'os_returns_retained'</a></li>
        <li><a href="#attr_os_returns_not_retained">Attribute 'os_returns_not_retained'</a></li>
        <li><a href="#attr_os_consumed">Attribute 'os_consumed'</a></li>
        <li><a href="#attr_os_consumes_this">Attribute 'os_consumes_this'</a></li>
        <li><a href="#os_out_parameters">Out Parameters</a></li>
      </ul>

    </li>
  </ul>
</li>
<li><a href="#custom_assertions">Custom Assertion Handlers</a>
  <ul>
    <li><a href="#attr_noreturn">Attribute 'noreturn'</a></li>
    <li><a href="#attr_analyzer_noreturn">Attribute 'analyzer_noreturn'</a></li>
  </ul>
  </li>
</ul>

<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<h2 id="generic">Annotations to Enhance Generic Checks</h2>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->

<h3 id="null_checking">Null Pointer Checking</h3>

<h4 id="attr_nonnull">Attribute 'nonnull'</h4>

<p>The analyzer recognizes the GCC attribute 'nonnull', which indicates that a
function expects that a given function parameter is not a null pointer. Specific
details of the syntax of using the 'nonnull' attribute can be found in <a
href="https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-nonnull-function-attribute">GCC's
documentation</a>.</p>

<p>Both the Clang compiler and GCC will flag warnings for simple cases where a
null pointer is directly being passed to a function with a 'nonnull' parameter
(e.g., as a constant). The analyzer extends this checking by using its deeper
symbolic analysis to track what pointer values are potentially null and then
flag warnings when they are passed in a function call via a 'nonnull'
parameter.</p>

<p><b>Example</b></p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
int bar(int*p, int q, int *r) __attribute__((nonnull(1,3)));

int foo(int *p, int *q) {
   return !p ? bar(q, 2, p)
             : bar(p, 2, q);
}
</pre>

<p>Running <tt>scan-build</tt> over this source produces the following
output:</p>

<img src="images/example_attribute_nonnull.png" alt="example attribute nonnull">

<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<h2 id="macosx">Mac OS X API Annotations</h2>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->

<h3 id="cocoa_mem">Cocoa &amp; Core Foundation Memory Management
Annotations</h3>

<!--
<p>As described in <a href="/available_checks.html#retain_release">Available
Checks</a>,
-->
<p>The analyzer supports the proper management of retain counts for
both Cocoa and Core Foundation objects. This checking is largely based on
enforcing Cocoa and Core Foundation naming conventions for Objective-C methods
(Cocoa) and C functions (Core Foundation). Not strictly following these
conventions can cause the analyzer to miss bugs or flag false positives.</p>

<p>One can educate the analyzer (and others who read your code) about methods or
functions that deviate from the Cocoa and Core Foundation conventions using the
attributes described here. However, you should consider using proper naming
conventions or the <a
href="https://clang.llvm.org/docs/LanguageExtensions.html#the-objc-method-family-attribute"><tt>objc_method_family</tt></a>
attribute, if applicable.</p>

<h4 id="attr_ns_returns_retained">Attribute 'ns_returns_retained'
(Clang-specific)</h4>

<p>The GCC-style (Clang-specific) attribute 'ns_returns_retained' allows one to
annotate an Objective-C method or C function as returning a retained Cocoa
object that the caller is responsible for releasing (via sending a
<tt>release</tt> message to the object). The Foundation framework defines a
macro <b><tt>NS_RETURNS_RETAINED</tt></b> that is functionally equivalent to the
one shown below.</p>

<p><b>Placing on Objective-C methods</b>: For Objective-C methods, this
annotation essentially tells the analyzer to treat the method as if its name
begins with &quot;alloc&quot; or &quot;new&quot; or contains the word
&quot;copy&quot;.</p>

<p><b>Placing on C functions</b>: For C functions returning Cocoa objects, the
analyzer typically does not make any assumptions about whether or not the object
is returned retained. Explicitly adding the 'ns_returns_retained' attribute to C
functions allows the analyzer to perform extra checking.</p>

<p><b>Example</b></p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
#import &lt;Foundation/Foundation.h&gt;

#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef NS_RETURNS_RETAINED
#if __has_feature(attribute_ns_returns_retained)
<span class="code_highlight">#define NS_RETURNS_RETAINED __attribute__((ns_returns_retained))</span>
#else
#define NS_RETURNS_RETAINED
#endif
#endif

@interface MyClass : NSObject {}
- (NSString*) returnsRetained <span class="code_highlight">NS_RETURNS_RETAINED</span>;
- (NSString*) alsoReturnsRetained;
@end

@implementation MyClass
- (NSString*) returnsRetained {
  return [[NSString alloc] initWithCString:"no leak here"];
}
- (NSString*) alsoReturnsRetained {
  return [[NSString alloc] initWithCString:"flag a leak"];
}
@end
</pre>

<p>Running <tt>scan-build</tt> on this source file produces the following output:</p>

<img src="images/example_ns_returns_retained.png" alt="example returns retained">

<h4 id="attr_ns_returns_not_retained">Attribute 'ns_returns_not_retained'
(Clang-specific)</h4>

<p>The 'ns_returns_not_retained' attribute is the complement of '<a
href="#attr_ns_returns_retained">ns_returns_retained</a>'. Where a function or
method may appear to obey the Cocoa conventions and return a retained Cocoa
object, this attribute can be used to indicate that the object reference
returned should not be considered as an &quot;owning&quot; reference being
returned to the caller. The Foundation framework defines a
macro <b><tt>NS_RETURNS_NOT_RETAINED</tt></b> that is functionally equivalent to
the one shown below.</p>

<p>Usage is identical to <a
href="#attr_ns_returns_retained">ns_returns_retained</a>.  When using the
attribute, be sure to declare it within the proper macro that checks for
its availability, as it is not available in earlier versions of the analyzer:</p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef NS_RETURNS_NOT_RETAINED
#if __has_feature(attribute_ns_returns_not_retained)
<span class="code_highlight">#define NS_RETURNS_NOT_RETAINED __attribute__((ns_returns_not_retained))</span>
#else
#define NS_RETURNS_NOT_RETAINED
#endif
#endif
</pre>

<h4 id="attr_cf_returns_retained">Attribute 'cf_returns_retained'
(Clang-specific)</h4>

<p>The GCC-style (Clang-specific) attribute 'cf_returns_retained' allows one to
annotate an Objective-C method or C function as returning a retained Core
Foundation object that the caller is responsible for releasing. The
CoreFoundation framework defines a macro <b><tt>CF_RETURNS_RETAINED</tt></b>
that is functionally equivalent to the one shown below.</p>

<p><b>Placing on Objective-C methods</b>: With respect to Objective-C methods.,
this attribute is identical in its behavior and usage to 'ns_returns_retained'
except for the distinction of returning a Core Foundation object instead of a
Cocoa object.

This distinction is important for the following reason:
as Core Foundation is a C API,
the analyzer cannot always tell that a pointer return value refers to a
Core Foundation object.
In contrast, it is
trivial for the analyzer to recognize if a pointer refers to a Cocoa object
(given the Objective-C type system).

<p><b>Placing on C functions</b>: When placing the attribute
'cf_returns_retained' on the declarations of C functions, the analyzer
interprets the function as:</p>

<ol>
  <li>Returning a Core Foundation Object</li>
  <li>Treating the function as if it its name
contained the keywords &quot;create&quot; or &quot;copy&quot;. This means the
returned object as a +1 retain count that must be released by the caller, either
by sending a <tt>release</tt> message (via toll-free bridging to an Objective-C
object pointer), or calling <tt>CFRelease</tt> or a similar function.</li>
</ol>

<p><b>Example</b></p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
$ cat test.m
#import &lt;Cocoa/Cocoa.h&gt;

#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef CF_RETURNS_RETAINED
#if __has_feature(attribute_cf_returns_retained)
<span class="code_highlight">#define CF_RETURNS_RETAINED __attribute__((cf_returns_retained))</span>
#else
#define CF_RETURNS_RETAINED
#endif
#endif

@interface MyClass : NSObject {}
- (NSDate*) returnsCFRetained <span class="code_highlight">CF_RETURNS_RETAINED</span>;
- (NSDate*) alsoReturnsRetained;
- (NSDate*) returnsNSRetained <span class="code_highlight">NS_RETURNS_RETAINED</span>;
@end

<span class="code_highlight">CF_RETURNS_RETAINED</span>
CFDateRef returnsRetainedCFDate()  {
  return CFDateCreate(0, CFAbsoluteTimeGetCurrent());
}

@implementation MyClass
- (NSDate*) returnsCFRetained {
  return (NSDate*) returnsRetainedCFDate(); <b><i>// No leak.</i></b>
}

- (NSDate*) alsoReturnsRetained {
  return (NSDate*) returnsRetainedCFDate(); <b><i>// Always report a leak.</i></b>
}

- (NSDate*) returnsNSRetained {
  return (NSDate*) returnsRetainedCFDate(); <b><i>// Report a leak when using GC.</i></b>
}
@end
</pre>

<p>Running <tt>scan-build</tt> on this example produces the following output:</p>

<img src="images/example_cf_returns_retained.png" alt="example returns retained">

<h4 id="attr_cf_returns_not_retained">Attribute 'cf_returns_not_retained'
(Clang-specific)</h4>

<p>The 'cf_returns_not_retained' attribute is the complement of '<a
href="#attr_cf_returns_retained">cf_returns_retained</a>'. Where a function or
method may appear to obey the Core Foundation or Cocoa conventions and return
a retained Core Foundation object, this attribute can be used to indicate that
the object reference returned should not be considered as an
&quot;owning&quot; reference being returned to the caller. The
CoreFoundation framework defines a macro <b><tt>CF_RETURNS_NOT_RETAINED</tt></b>
that is functionally equivalent to the one shown below.</p>

<p>Usage is identical to <a
href="#attr_cf_returns_retained">cf_returns_retained</a>.  When using the
attribute, be sure to declare it within the proper macro that checks for
its availability, as it is not available in earlier versions of the analyzer:</p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef CF_RETURNS_NOT_RETAINED
#if __has_feature(attribute_cf_returns_not_retained)
<span class="code_highlight">#define CF_RETURNS_NOT_RETAINED __attribute__((cf_returns_not_retained))</span>
#else
#define CF_RETURNS_NOT_RETAINED
#endif
#endif
</pre>

<h4 id="attr_ns_consumed">Attribute 'ns_consumed'
(Clang-specific)</h4>

<p>The 'ns_consumed' attribute can be placed on a specific parameter in either
the declaration of a function or an Objective-C method. It indicates to the
static analyzer that a <tt>release</tt> message is implicitly sent to the
parameter upon completion of the call to the given function or method. The
Foundation framework defines a macro <b><tt>NS_RELEASES_ARGUMENT</tt></b> that
is functionally equivalent to the <tt>NS_CONSUMED</tt> macro shown below.</p>

<p><b>Example</b></p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef NS_CONSUMED
#if __has_feature(attribute_ns_consumed)
<span class="code_highlight">#define NS_CONSUMED __attribute__((ns_consumed))</span>
#else
#define NS_CONSUMED
#endif
#endif

void consume_ns(id <span class="code_highlight">NS_CONSUMED</span> x);

void test() {
  id x = [[NSObject alloc] init];
  consume_ns(x); <b><i>// No leak!</i></b>
}

@interface Foo : NSObject
+ (void) releaseArg:(id) <span class="code_highlight">NS_CONSUMED</span> x;
+ (void) releaseSecondArg:(id)x second:(id) <span class="code_highlight">NS_CONSUMED</span> y;
@end

void test_method() {
  id x = [[NSObject alloc] init];
  [Foo releaseArg:x]; <b><i>// No leak!</i></b>
}

void test_method2() {
  id a = [[NSObject alloc] init];
  id b = [[NSObject alloc] init];
  [Foo releaseSecondArg:a second:b]; <b><i>// 'a' is leaked, but 'b' is released.</i></b>
}
</pre>

<h4 id="attr_cf_consumed">Attribute 'cf_consumed'
(Clang-specific)</h4>

<p>The 'cf_consumed' attribute is practically identical to <a
href="#attr_ns_consumed">ns_consumed</a>. The attribute can be placed on a
specific parameter in either the declaration of a function or an Objective-C
method. It indicates to the static analyzer that the object reference is
implicitly passed to a call to <tt>CFRelease</tt> upon completion of the call
to the given function or method. The CoreFoundation framework defines a macro
<b><tt>CF_RELEASES_ARGUMENT</tt></b> that is functionally equivalent to the
<tt>CF_CONSUMED</tt> macro shown below.</p>

<p>Operationally this attribute is nearly identical to 'ns_consumed'.</p>

<p><b>Example</b></p>

<pre class="code_example">
<span class="command">$ cat test.m</span>
#ifndef __has_feature      // Optional.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef CF_CONSUMED
#if __has_feature(attribute_cf_consumed)
<span class="code_highlight">#define CF_CONSUMED __attribute__((cf_consumed))</span>
#else
#define CF_CONSUMED
#endif
#endif

void consume_cf(id <span class="code_highlight">CF_CONSUMED</span> x);
void consume_CFDate(CFDateRef <span class="code_highlight">CF_CONSUMED</span> x);

void test() {
  id x = [[NSObject alloc] init];
  consume_cf(x); <b><i>// No leak!</i></b>
}

void test2() {
  CFDateRef date = CFDateCreate(0, CFAbsoluteTimeGetCurrent());
  consume_CFDate(date); <b><i>// No leak, including under GC!</i></b>

}

@interface Foo : NSObject
+ (void) releaseArg:(CFDateRef) <span class="code_highlight">CF_CONSUMED</span> x;
@end

void test_method() {
  CFDateRef date = CFDateCreate(0, CFAbsoluteTimeGetCurrent());
  [Foo releaseArg:date]; <b><i>// No leak!</i></b>
}
</pre>

<h4 id="attr_ns_consumes_self">Attribute 'ns_consumes_self'
(Clang-specific)</h4>

<p>The 'ns_consumes_self' attribute can be placed only on an Objective-C method
declaration. It indicates that the receiver of the message is
&quot;consumed&quot; (a single reference count decremented) after the message
is sent. This matches the semantics of all &quot;init&quot; methods.</p>

<p>One use of this attribute is declare your own init-like methods that do not
follow the standard Cocoa naming conventions.</p>

<p><b>Example</b></p>

<pre class="code_example">
#ifndef __has_feature
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif

#ifndef NS_CONSUMES_SELF
#if __has_feature((attribute_ns_consumes_self))
<span class="code_highlight">#define NS_CONSUMES_SELF __attribute__((ns_consumes_self))</span>
#else
#define NS_CONSUMES_SELF
#endif
#endif

@interface MyClass : NSObject
- initWith:(MyClass *)x;
- nonstandardInitWith:(MyClass *)x <span class="code_highlight">NS_CONSUMES_SELF</span> NS_RETURNS_RETAINED;
@end
</pre>

<p>In this example, <tt>-nonstandardInitWith:</tt> has the same ownership
semantics as the init method <tt>-initWith:</tt>. The static analyzer will
observe that the method consumes the receiver, and then returns an object with
a +1 retain count.</p>

<p>The Foundation framework defines a macro <b><tt>NS_REPLACES_RECEIVER</tt></b>
which is functionally equivalent to the combination of <tt>NS_CONSUMES_SELF</tt>
and <tt>NS_RETURNS_RETAINED</tt> shown above.</p>

<h3 id="osobject_mem">Libkern Memory Management Annotations</h3>

<p><a
  href="https://developer.apple.com/documentation/kernel/osobject?language=objc">Libkern</a>
requires developers to inherit all heap allocated objects from <tt>OSObject</tt>
and to perform manual reference counting.
The reference counting model is very similar to MRR (manual retain-release) mode in
<a href="https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html">Objective-C</a>
or to CoreFoundation reference counting.
Freshly-allocated objects start with a reference count of 1,
and calls to <tt>retain</tt> increment it,
while calls to <tt>release</tt> decrement it.
The object is deallocated whenever its reference count reaches zero.</p>

<p>Manually incrementing and decrementing reference counts is error-prone:
over-retains lead to leaks, and over-releases lead to uses-after-free.
The analyzer can help the programmer to check for unbalanced
retain/release calls.</p>

<p>The reference count checking is based on the principle of
<em>locality</em>: it should be possible to establish correctness
(lack of leaks/uses after free) by looking at each function body,
and the declarations (not the definitions) of all the functions it interacts
with.</p>

<p>In order to support such reasoning, it should be possible to <em>summarize</em>
the behavior of each function, with respect to reference count
of its returned values and attributes.</p>

<p>By default, the following summaries are assumed:</p>
<ul>
  <li>All functions starting with <tt>get</tt> or <tt>Get</tt>,
    unless they are returning subclasses of <tt>OSIterator</tt>,
  are assumed to be returning at +0.
  That is, the caller has no reference
  count <em>obligations</em> with respect to the reference count of the returned object
  and should leave it untouched.
  </li>

  <li>
    All other functions are assumed to return at +1.
    That is, the caller has an <em>obligation</em> to release such objects.
  </li>

  <li>
    Functions are assumed not to change the reference count of their parameters,
    including the implicit <tt>this</tt> parameter.
  </li>
</ul>

<p>These summaries can be overriden with the following
<a href="https://clang.llvm.org/docs/AttributeReference.html#os-returns-not-retained">attributes</a>:</p>

<h4 id="attr_os_returns_retained">Attribute 'os_returns_retained'</h4>

<p>The <tt>os_returns_retained</tt> attribute (accessed through the macro <tt>
LIBKERN_RETURNS_RETAINED</tt>) plays a role identical to <a
href="#attr_ns_returns_retained">ns_returns_retained</a> for functions
returning <tt>OSObject</tt> subclasses.
The attribute indicates that it is a callers responsibility to release the
returned object.
</p>


<h4 id="attr_os_returns_not_retained">Attribute 'os_returns_not_retained'</h4>

<p>The <tt>os_returns_not_retained</tt> attribute (accessed through the macro <tt>
LIBKERN_RETURNS_NOT_RETAINED</tt>) plays a role identical to <a
href="#attr_ns_returns_not_retained">ns_returns_not_retained</a> for functions
returning <tt>OSObject</tt> subclasses.
The attribute indicates that the caller should not change the retain
count of the returned object.
</p>

<h5>Example</h5>

<pre class="code_example">
class MyClass {
  OSObject *f;
  LIBKERN_RETURNS_NOT_RETAINED OSObject *myFieldGetter();
}


// Note that the annotation only has to be applied to the function declaration.
OSObject * MyClass::myFieldGetter() {
  return f;
}
</pre>

<h4 id="attr_os_consumed">Attribute 'os_consumed'</h4>

<p>Similarly to <a href="#attr_ns_consumed">ns_consumed</a> attribute,
<tt>os_consumed</tt> (accessed through <tt>LIBKERN_CONSUMED</tt>) attribute,
applied to a parameter,
indicates that the call to the function <em>consumes</em> the parameter:
the callee should either release it or store it and release it in the destructor,
while the caller should assume one is subtracted from the reference count
after the call.</p>

<pre class="code_example">
IOReturn addToList(LIBKERN_CONSUMED IOPMinformee *newInformee);
</pre>

<h4 id="attr_os_consumes_this">Attribute 'os_consumes_this'</h4>

<p>Similarly to <a href="#attr_ns_consumes_self">ns_consumes_self</a>,
the <tt>os_consumes_self</tt> attribute indicates that the method call
<em>consumes</em> the implicit <tt>this</tt> argument: the caller
should assume one was subtracted from the reference count of the object
after the call, and the callee has on obligation to either
release the argument, or store it and eventually release it in the
destructor.</p>

<pre class="code_example">
void addThisToList(OSArray *givenList) LIBKERN_CONSUMES_THIS;
</pre>

<h4 id="os_out_parameters">Out Parameters</h4>

A function can also return an object to a caller by a means of an out parameter
(a pointer-to-OSObject-pointer is passed, and a callee writes a pointer to an
object into an argument).
Currently the analyzer does not track unannotated out
parameters by default, but with annotations we distinguish four separate cases:

<p><b>1. Non-retained out parameters</b>, identified using
    <tt>LIBKERN_RETURNS_NOT_RETAINED</tt> applied to parameters, e.g.:</p>

<pre class="code_example">
void getterViaOutParam(LIBKERN_RETURNS_NOT_RETAINED OSObject **obj)
</pre>

<p>Such functions write a non-retained object into an out parameter, and the
caller has no further obligations.</p>

<p><b>2. Retained out parameters</b>,
identified using <tt>LIBKERN_RETURNS_RETAINED</tt>:</p>
<pre class="code_example">
void getterViaOutParam(LIBKERN_RETURNS_NOT_RETAINED OSObject **obj)
</pre>
<p>
In such cases a retained object is written into an out parameter, which the caller has then to release in order to avoid a leak.
</p>

<p>These two cases are simple - but in practice a functions returning an out-parameter usually also return a return code, and then an out parameter may or may not be written, which conditionally depends on the exit code, e.g.:</p>

<pre class="code_example">
bool maybeCreateObject(LIBKERN_RETURNS_RETAINED OSObject **obj);
</pre>

<p>For such functions, the usual semantics is that an object is written into on "success", and not written into on "failure".<p>

<p>For <tt>LIBKERN_RETURNS_RETAINED</tt> we assume the following definition of
success:</p>

<p>For functions returning <tt>OSReturn</tt> or <tt>IOReturn</tt>
(any typedef to <tt>kern_return_t</tt>) success is defined as having an output of zero (<tt>kIOReturnSuccess</tt> is zero).
For all others, success is non-zero (e.g. non-nullptr for pointers)</p>

<p><b>3. Retained out parameters on zero return</b>
The annotation <tt>LIBKERN_RETURNS_RETAINED_ON_ZERO</tt> states
that a retained object is written into if and only if the function returns a zero value:</p>

<pre class="code_example">
bool OSUnserializeXML(void *data, LIBKERN_RETURNS_RETAINED_ON_ZERO OSString **errString);
</pre>

<p>Then the caller has to release an object if the function has returned zero.</p>

<p><b>4. Retained out parameters on non-zero return</b>
Similarly, <tt>LIBKERN_RETURNS_RETAINED_ON_NONZERO</tt> specifies that a
retained object is written into the parameter if and only if the function has
returned a non-zero value.</p>

<p>Note that for non-retained out parameters conditionals do not matter, as the
caller has no obligations regardless of whether an object is written into or
not.</p>

<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<h2 id="custom_assertions">Custom Assertion Handlers</h2>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->

<p>The analyzer exploits code assertions by pruning off paths where the
assertion condition is false. The idea is capture any program invariants
specified in the assertion that the developer may know but is not immediately
apparent in the code itself. In this way assertions make implicit assumptions
explicit in the code, which not only makes the analyzer more accurate when
finding bugs, but can help others better able to understand your code as well.
It can also help remove certain kinds of analyzer false positives by pruning off
false paths.</p>

<p>In order to exploit assertions, however, the analyzer must understand when it
encounters an &quot;assertion handler.&quot; Typically assertions are
implemented with a macro, with the macro performing a check for the assertion
condition and, when the check fails, calling an assertion handler.  For example, consider the following code
fragment:</p>

<pre class="code_example">
void foo(int *p) {
  assert(p != NULL);
}
</pre>

<p>When this code is preprocessed on Mac OS X it expands to the following:</p>

<pre class="code_example">
void foo(int *p) {
  (__builtin_expect(!(p != NULL), 0) ? __assert_rtn(__func__, "t.c", 4, "p != NULL") : (void)0);
}
</pre>

<p>In this example, the assertion handler is <tt>__assert_rtn</tt>. When called,
most assertion handlers typically print an error and terminate the program. The
analyzer can exploit such semantics by ending the analysis of a path once it
hits a call to an assertion handler.</p>

<p>The trick, however, is that the analyzer needs to know that a called function
is an assertion handler; otherwise the analyzer might assume the function call
returns and it will continue analyzing the path where the assertion condition
failed. This can lead to false positives, as the assertion condition usually
implies a safety condition (e.g., a pointer is not null) prior to performing
some action that depends on that condition (e.g., dereferencing a pointer).</p>

<p>The analyzer knows about several well-known assertion handlers, but can
automatically infer if a function should be treated as an assertion handler if
it is annotated with the 'noreturn' attribute or the (Clang-specific)
'analyzer_noreturn' attribute. Note that, currently, clang does not support
these attributes on Objective-C methods and C++ methods.</p>

<h4 id="attr_noreturn">Attribute 'noreturn'</h4>

<p>The 'noreturn' attribute is a GCC-attribute that can be placed on the
declarations of functions. It means exactly what its name implies: a function
with a 'noreturn' attribute should never return.</p>

<p>Specific details of the syntax of using the 'noreturn' attribute can be found
in <a
href="https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute">GCC's
documentation</a>.</p>

<p>Not only does the analyzer exploit this information when pruning false paths,
but the compiler also takes it seriously and will generate different code (and
possibly better optimized) under the assumption that the function does not
return.</p>

<p><b>Example</b></p>

<p>On Mac OS X, the function prototype for <tt>__assert_rtn</tt> (declared in
<tt>assert.h</tt>) is specifically annotated with the 'noreturn' attribute:</p>

<pre class="code_example">
void __assert_rtn(const char *, const char *, int, const char *) <span class="code_highlight">__attribute__((__noreturn__))</span>;
</pre>

<h4 id="attr_analyzer_noreturn">Attribute 'analyzer_noreturn' (Clang-specific)</h4>

<p>The Clang-specific 'analyzer_noreturn' attribute is almost identical to
'noreturn' except that it is ignored by the compiler for the purposes of code
generation.</p>

<p>This attribute is useful for annotating assertion handlers that actually
<em>can</em> return, but for the purpose of using the analyzer we want to
pretend that such functions do not return.</p>

<p>Because this attribute is Clang-specific, its use should be conditioned with
the use of preprocessor macros.</p>

<p><b>Example</b>

<pre class="code_example">
#ifndef CLANG_ANALYZER_NORETURN
#if __has_feature(attribute_analyzer_noreturn)
<span class="code_highlight">#define CLANG_ANALYZER_NORETURN __attribute__((analyzer_noreturn))</span>
#else
#define CLANG_ANALYZER_NORETURN
#endif
#endif

void my_assert_rtn(const char *, const char *, int, const char *) <span class="code_highlight">CLANG_ANALYZER_NORETURN</span>;
</pre>

</div>
</div>
</body>
</html>