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Write better* C++
with help from your friend Clang
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René Milk
23rd October 2013
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Contents
What is Clang?
Dynamic analysis (sanitizers)
Tools
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Static analysis
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LLVM1
LLVM (formerly Low Level Virtual Machine) is a compiler infrastructure written in C++; it is
designed for compile-time, link-time, run-time, and "idle-time" optimization of programs
written in arbitrary programming languages.
Clang is a compiler front end for the C, C++, Objective-C and Objective-C++ programming
languages. It uses LLVM as its back end and has been part of its releases since LLVM 2.6.
1
2
https://en.wikipedia.org/wiki/LLVM
https://en.wikipedia.org/wiki/Clang
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Clang2
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I
fast compiles with low memory usage 3
I
detailed “fix-it” notes for warnings and errors
I
build on top of libclang, API allows for extensive tooling
I
fully4 supports c++11 since version 3.3
3
4
http://clang.llvm.org/features.html#performance
http://clang.llvm.org/cxx_status.html
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Clang features
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code with fewer bugs
I
shorter compile times
I
consistent formatting
I
not faster execution
I
not code quality in terms of style, complexity, etc.
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for this talk we’ll take “better” as referring to:
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static code (program) analysis5
5
https://en.wikipedia.org/wiki/Static_program_analysis
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Static program analysis is the analysis of computer software that is performed without
actually executing programs (analysis performed on executing programs is known as
dynamic analysis).
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Why you should compile at high warning levels
I
typos
I
unintentional operator usage
I
potential data over/underflows
I
dead code
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Catch code segments that are valid C++, but (probably) not what you meant to write.
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error free build 6= error free code
/// \file warnings_00.cc
#include <cstddef>
#include <string>
int main(int argn, char** argv)
{
auto name = argv[0];
int* i = 0;
10
unsigned long long bigstuff = 8 << 32;
if ( name == "foo" )
char* name = "bar";
else if ( name = NULL )
bigstuff = -1;
15
for ( ; bigstuff < 0; i++);
bigstuff--;
return bigstuff**i;
}
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how many warnings did that generate?
default
“most"
6
13
”most“ warnings settings
clang++ -g -std=c++11 -Wsign-conversion -O0 -g -Wall -Wextra -Wc++0x-compat
-Winline -pedantic -Wredundant-decls -Wloop-analysis -Wstring-compare -Wshadow
-Wunused-variable -Wundef -Wnon-virtual-dtor -Wdocumentation -Wshorten-64-to-32
-Wused-but-marked-unused -Wdisabled-macro-expansion -Wparentheses
-Wsometimes-uninitialized -Wconditional-uninitialized -Wfloat-equal -Wswitch-enum
-Warray-bounds -Wcovered-switch-default -Wunreachable-code
-Wnon-literal-null-conversion -Wtautological-compare
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Table : clang version 3.4-1 exp1
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sign-conversion / tautological-compare
/// \file warnings_01.cc
#include <cstddef>
int main(int, char**)
{
std::size_t bigstuff = -1;
return bigstuff < 0;
}
warnings_01.cc:5:28: warning: implicit conversion changes signedness: ’int’ to
’std::size_t’ (aka ’unsigned long’) [-Wsign-conversion]
std::size_t bigstuff = -1;
~~~~~~~~
^~
warnings_01.cc:6:21: warning: comparison of unsigned expression < 0 is always
false [-Wtautological-compare]
return bigstuff < 0;
~~~~~~~~ ^ ~
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parentheses
warnings_02.cc:4:14: warning: using the result of an assignment as
a condition without parentheses [-Wparentheses]
if (argn = 1)
~~~~~^~~
warnings_02.cc:4:14: note: place parentheses around the assignment to silence this warning
if (argn = 1)
^
(
)
warnings_02.cc:4:14: note: use ’==’ to turn this assignment into an equality comparison
if (argn = 1)
^
==
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/// \file warnings_02.cc
int main(int argn, char**)
{
if (argn = 1)
return -1;
}
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loop-analysis / empty-body
/// \file warnings_03.cc
warnings_03.cc:6:10: warning: variable ’argn’ used in loop condition
not modified in loop body [-Wloop-analysis]
for(;argn > 0;);
^~~~
warnings_03.cc:6:20: warning: for loop has empty body [-Wempty-body]
for(;argn > 0;);
^
warnings_03.cc:6:20: note: put the semicolon on a separate line to silence this warning
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#include <iostream>
int main(int argn, char** argv)
{
for(;argn > 0;);
std::cout << argv[--argn] << std::endl;
}
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switch / return-type
/// \file warnings_04.cc
enum Things { Dog, Frog };
int foo(Things thing) {
switch (thing) {
case Dog: return 9001;
}
}
10
int main(int, char**) { return 0;}
warnings_04.cc:6:13: warning: enumeration value ’Frog’ not handled in switch [-Wswitch]
switch (thing) {
^
warnings_04.cc:9:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
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covered-switch-default
/// \file warnings_05.cc
enum Things { Dog, Frog };
int bar(Things thing) {
int ret;
switch (thing) {
case Dog: case Frog: ret = 0;
default: ret = -1;
10
}
return ret;
}
int main(int, char**) { return 0;}
warnings_05.cc:9:9: warning: default label in switch which covers
all enumeration values [-Wcovered-switch-default]
default: ret = -1;
^
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array-bounds
/// \file warnings_06.cc
const int ones[] = {1};
int main() {
const auto bar = ones[1];
return bar;
}
warnings_06.cc:4:22: warning: array index 1 is past the end of the array
(which contains 1 element) [-Warray-bounds]
const auto bar = ones[1];
^
~
warnings_06.cc:1:1: note: array ’ones’ declared here
const int ones[] = {1};
^
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the analyzer plugin
#include <cstddef>
#include <string>
inline int the_meaning_of_life_and_everything() { return 42; }
int main(int, char**)
{
const int* i = nullptr;
auto j = the_meaning_of_life_and_everything();
j = sizeof(int);
10
return j+*i;
}
static_00.cc:8:10: warning: Value stored to ’j’ during its initialization is never read
auto j = the_meaning_of_life_and_everything();
^
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
static_00.cc:10:14: warning: Dereference of null pointer (loaded from variable ’i’)
return j+*i;
^~
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Address Sanitizer - out-of-bounds checks, use-after-free,...
I
Thread Sanitizer - finds data races
I
Memory Sanitizer - detects uninitialized reads
I
(soon?) Undefined Behavior Sanitizer - ???
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available sanitizers
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inserts instrumentation code into the binary
I
“replays” memory access in shadow memory
I
runs algorithms to find patterns of unwanted behaviour
I
incurs runtime and memory penalty when activated
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What is a sanitizer?
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memory sanitizer - unitialized read
==14624== WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x7fdb3ee1b9e6 in main instrumentation_01.cc:6
#1 0x7fdb3d9a3de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260
#2 0x7fdb3ee1b89c in _start (01+0x6589c)
SUMMARY: MemorySanitizer: use-of-uninitialized-value instrumentation_01.cc:6 main
Exiting
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int main(int argc, char**)
{
int* i = new int[666];
if (i[0])
return -1;
}
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address sanitizer - heap-use-after-free I
10
15
20
25
struct Player {
const YYZ& song_;
Player(const YYZ& song) : song_(song) {}
int length() const { return song_.length(); }
};
int main(int, char**)
{
unique_ptr<Player> player(nullptr);
{
YYZ* yyz = new YYZ;
player = unique_ptr<Player>(new Player(*yyz));
delete yyz;
}
auto le = player->length();
std::cout << le;
return le;
}
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#include <memory>
#include <iostream>
using namespace std;
struct YYZ {
int length () const { return (4-int(radio_)*60)+25; }
bool radio_;
YYZ(bool radio = false) : radio_(radio){}
};
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=================================================================
==12704==ERROR: A6ddressSanitizer: heap-use-after-free on address 0x60200000eff0
at pc 0x473841 bp 0x7fff778bdcc0 sp 0x7fff778bdcb8
READ of size 1 at 0x60200000eff0 thread T0
#0 0x473840 in YYZ::length() const instrumentation_02.cc:5
#1 0x472f78 in main instrumentation_02.cc:23
#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260
#3 0x472dac in _start (02+0x472dac)
0x60200000eff0 is located 0 bytes inside of 1-byte region [0x60200000eff0,0x60200000eff1)
freed by thread T0 here:
#0 0x45eb29 in operator delete(void*) (02+0x45eb29)
#1 0x472f68 in main instrumentation_02.cc:21
#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260
previously allocated by thread T0 here:
#0 0x45e829 in operator new(unsigned long) (02+0x45e829)
#1 0x472f12 in main instrumentation_02.cc:19
#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260
SUMMARY: AddressSanitizer: heap-use-after-free instrumentation_02.cc:5 YYZ::length() const
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heap-use-after-free II
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Shadow bytes around the buggy address:
0x0c047fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c047fff9df0: fa fa fa fa fa fa fa fa fa fa 00 fa fa fa[fd]fa
0x0c047fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff9e40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable:
00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone:
fa
Heap right redzone:
fb
Freed heap region:
fd
Stack left redzone:
f1
Stack mid redzone:
f2
Stack right redzone:
f3
Stack partial redzone: f4
Stack after return:
f5
Stack use after scope: f8
Global redzone:
f9
Global init order:
f6
Poisoned by user:
f7
ASan internal:
fe
==12704==ABORTING
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heap-use-after-free III
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clang-format
I
clang-modernize
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include-what-you-use (iwyu)
I
cldoc
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Tools built with (lib)Clang
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/// \file format.cc
#include <cstddef>
#include <string>
int main(int argn,char** argv){auto name=argv[0];int* i=0;unsigned long
long bigstuff=8<<32;if(name=="foo")char* name="bar";else if(name=NULL)bigstuff=
-1;for(;bigstuff<0;i++);bigstuff--;return bigstuff**i;}
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Example clang-format (before)
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Example clang-format (after)
/// \file format.cc
#include <cstddef>
#include <string>
int main(int argn, char **argv) {
5
auto name = argv[0];
int *i = 0;
unsigned long long bigstuff = 8 << 32;
if (name == "foo")
char *name = "bar";
10
else if (name = NULL)
bigstuff = -1;
for (; bigstuff < 0; i++)
;
bigstuff--;
15
return bigstuff * *i;
}
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clang-format-3.4 -style=LLVM format.cc > format_after.cc
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Example clang-modernize (before)
/// \file modernize.cc
#include <vector>
template <class T>
void increment(std::vector<T>& vec) {
for (typename std::vector<T>::iterator it = vec.begin();
it != vec.end(); ++it)
{
++(*it);
10
}
}
int main() {std::vector<int> a(2,4); increment(a);}
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Example clang-modernize (after)
clang-modernize -format-style=LLVM modernize.cc
5
template <class T>
void increment(std::vector<T>& vec) {
for (auto &elem : vec) {
++(elem);
}
}
10
int main() {std::vector<int> a(2,4); increment(a);}
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/// \file modernize.cc
#include <vector>
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Example iwyu (input)
/// \file iwyu.cc
10
<vector>
<assert.h>
<iterator>
<memory>
<sstream>
<iostream>
int main()
{
using namespace std;
cout << abs(long(-1));
}
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#include
#include
#include
#include
#include
#include
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Example iwyu (output)
-std=c++11 iwyu.cc
iwyu.cc should add these lines:
#include <cstdlib>
5
// for abs
iwyu.cc should remove these lines:
- #include <assert.h> // lines 2-2
- #include <iterator> // lines 3-3
- #include <memory> // lines 4-4
- #include <sstream> // lines 5-5
- #include <vector> // lines 1-1
10
The full include-list for iwyu.cc:
#include <cstdlib>
#include <iostream>
---
// for abs
// for cout, ostream
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iwyu
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Example cldoc
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http://jessevdk.github.io/cldoc/example/
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Thank you for
your attention.
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