[clang][analyzer] Change value of checker option in unix.StdCLibraryFunctions (second try). (#80457)

Default value of checker option `ModelPOSIX` is changed to `true`.
Documentation is updated.

This is a re-apply of commit 7af4e8bcc3
that was reverted because a test failure (this is fixed now).

clang/docs/analyzer/checkers.rst

@@ -1320,10 +1320,23 @@ range of the argument.
 
 **Parameters**
 
-The checker models functions (and emits diagnostics) from the C standard by
-default. The ``ModelPOSIX`` option enables modeling (and emit diagnostics) of
-additional functions that are defined in the POSIX standard. This option is
-disabled by default.
+The ``ModelPOSIX`` option controls if functions from the POSIX standard are
+recognized by the checker.
+
+With ``ModelPOSIX=true``, many POSIX functions are modeled according to the
+`POSIX standard`_. This includes ranges of parameters and possible return
+values. Furthermore the behavior related to ``errno`` in the POSIX case is
+often that ``errno`` is set only if a function call fails, and it becomes
+undefined after a successful function call.
+
+With ``ModelPOSIX=false``, this checker follows the C99 language standard and
+only models the functions that are described there. It is possible that the
+same functions are modeled differently in the two cases because differences in
+the standards. The C standard specifies less aspects of the functions, for
+example exact ``errno`` behavior is often unspecified (and not modeled by the
+checker).
+
+Default value of the option is ``true``.
 
 .. _osx-checkers:
 

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

@@ -578,7 +578,7 @@ def StdCLibraryFunctionsChecker : Checker<"StdCLibraryFunctions">,
                   "ModelPOSIX",
                   "If set to true, the checker models additional functions "
                   "from the POSIX standard.",
-                  "false",
+                  "true",
                   InAlpha>
   ]>,
   WeakDependencies<[CallAndMessageChecker, NonNullParamChecker]>,

clang/test/Analysis/analyzer-config.c

@@ -129,7 +129,7 @@
 // CHECK-NEXT: unix.DynamicMemoryModeling:Optimistic = false
 // CHECK-NEXT: unix.Errno:AllowErrnoReadOutsideConditionExpressions = true
 // CHECK-NEXT: unix.StdCLibraryFunctions:DisplayLoadedSummaries = false
-// CHECK-NEXT: unix.StdCLibraryFunctions:ModelPOSIX = false
+// CHECK-NEXT: unix.StdCLibraryFunctions:ModelPOSIX = true
 // CHECK-NEXT: unroll-loops = false
 // CHECK-NEXT: verbose-report-filename = false
 // CHECK-NEXT: widen-loops = false

clang/test/Analysis/std-c-library-functions.c

@@ -3,6 +3,7 @@
 // RUN:   -analyzer-checker=unix.StdCLibraryFunctions \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-config eagerly-assume=false \
+// RUN:   -analyzer-config unix.StdCLibraryFunctions:ModelPOSIX=false \
 // RUN:   -triple i686-unknown-linux \
 // RUN:   -verify
 
@@ -11,6 +12,7 @@
 // RUN:   -analyzer-checker=unix.StdCLibraryFunctions \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-config eagerly-assume=false \
+// RUN:   -analyzer-config unix.StdCLibraryFunctions:ModelPOSIX=false \
 // RUN:   -triple x86_64-unknown-linux \
 // RUN:   -verify
 
@@ -19,6 +21,7 @@
 // RUN:   -analyzer-checker=unix.StdCLibraryFunctions \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-config eagerly-assume=false \
+// RUN:   -analyzer-config unix.StdCLibraryFunctions:ModelPOSIX=false \
 // RUN:   -triple armv7-a15-linux \
 // RUN:   -verify
 
@@ -27,6 +30,7 @@
 // RUN:   -analyzer-checker=unix.StdCLibraryFunctions \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-config eagerly-assume=false \
+// RUN:   -analyzer-config unix.StdCLibraryFunctions:ModelPOSIX=false \
 // RUN:   -triple thumbv7-a15-linux \
 // RUN:   -verify
 
@@ -36,6 +40,7 @@
 // RUN:   -analyzer-config unix.StdCLibraryFunctions:DisplayLoadedSummaries=true \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-config eagerly-assume=false \
+// RUN:   -analyzer-config unix.StdCLibraryFunctions:ModelPOSIX=false \
 // RUN:   -triple i686-unknown-linux 2>&1 | FileCheck %s
 
 //      CHECK: Loaded summary for: int isalnum(int)