Compilation warning not present (GCC and g++) - c++

1 #include <stdio.h>
2 #include <stdlib.h>
3
4 int main(int argc, char* argv[])
5 {
6 int bret = 1;
7 bret - 2;
8
9 printf("bret=%d",bret);
10 return 0;
11 }
In line:7, there was no left hand operator to reveice the value, still compiler was not generating any warning, GCC and g++ both. Is there anyintended purpore behind this?
[ADDED/EDIT]
As per comment I shall get warning only after using following flags: -Wall -Wextra
[debd#test]$gcc -Wall -Wextra test2.c
test2.c: In function 'main':
test2.c:7: warning: statement with no effect
test2.c:4: warning: unused parameter 'argc'
test2.c:4: warning: unused parameter 'argv'
[debd#test]$

As far as the language is concerned, there's no error - a statement is not required to have a side effect.
However, since a statement that does nothing is almost certainly a mistake, most compilers will warn about it. Yours will, but only if you enable that warning via the command line arguments.
You can enable just that warning with -Wunused-value, but I suggest you enable a decent set of warnings (including this one) with -Wall -Wextra.
As you found, this will also give a warning that the function parameters are unused. Since this is main, you can easily fix it by changing the signature to not have any parameters:
int main()
More generally, to avoid the warning if you need to ignore parameters, C++ allows you not to name them:
int main(int, char**)
and both languages allow you to explicitly use but ignore the value
(void)argc;
(void)argv;

bret - 2;
is an expression statement and it is has no side-effect.
C does not requires any warning for this statement. The compiler is free to add or not an informative warning to say the statement has no effect. The compiler can optimize out the statement.

Related

Calling a function with uninitalized struct variable as a parameter doesn't produce a warning

I have the following code in c++:
#include <iostream>
typedef struct Pair{
int x;
int y;
}Pair;
void dumFun(Pair p){}
int main() {
Pair p;
if (0){
p = {1,2};
}
dumFun(p);
return 0;
}
When I compiled the code, I expected to get a warning for the line with dumFun(p) since I'm calling a function with an uninitialized variable.
What I actually want is that my Makefile will give me warning for uninitialized scalar variable issues that I see with the tool Coverity.
Tried to use flag -Wall and I thought it shows warnings for unused variables usage as this - but it doesn't.
Is there any flag to use on a Makefile that will show me warning for the line I wrote above?
You did not tell the compiler, but at least in Clang there is no warning that would catch this. -Weverything shows all possible warnings, and you can use it to find the specific parameter needed to trigger each warning. Demonstration with -Weverything shows no warnings.
I did not find a suitable parameter in GCC either.

error: jump to label 'foo' crosses initialization of 'bar'

The following C++ example fails to compile with gcc or clang, but only generates a warning with ICC, and nothing at all with MSVC:
int main(int argc, char *argv[])
{
if (argc < 2)
goto clean_up;
#if 1 // FAIL
int i = 0;
#elif 0 // workaround - OK
{
int i = 0;
}
#else // workaround - OK
int i;
i = 0;
#endif
clean_up:
return 0;
}
g++:
init.cpp:13: error: jump to label ‘clean_up’
init.cpp:4: error: from here
init.cpp:7: error: crosses initialization of ‘int i’
clang++:
init.cpp:4:9: error: cannot jump from this goto statement to its label
goto clean_up;
^
init.cpp:7:9: note: jump bypasses variable initialization
int i = 0;
^
ICC:
init.cpp(4): warning #589: transfer of control bypasses initialization of:
variable "i" (declared at line 7)
goto clean_up;
^
I understand the cause of the error, and for a simple example such as this it is fairly easy to work around (I've included a couple of possible workarounds in the example above), but I'm working on a large cross-platform legacy code base that is peppered with error-handling macros which use a similar goto construct. Other developers working with MSVC or ICC keep introducing inline initialisations which subsequently result in errors for gcc or clang builds (and of course they just ignore the warnings they get with MSVC/ICC).
So I need to find a way to either (a) make such cases result in errors on ICC/MSVC or (b) reduce them to warnings with gcc/clang. I tried -fpermissive with gcc but that doesn't seem to help.
For extra credit I'm also curious as to the rationale behind this error for simple scalar initialisation - I can see why jumping over a constructor might be problematic, but initialising an int as in the above example doesn't seem like it could ever be an issue, and simply splitting the definition+initialisation into a definition + assignment makes the error go away?
The MSVC flag for treating a warning as en error is /we n where n is the number of the warning.
For example, /we4326 flags warning number C4326 as an error.
See https://msdn.microsoft.com/en-us/library/thxezb7y.aspx for details.

How to trace out why gcc and g++ produces different code

Is it possible to see what is going on behind gcc and g++ compilation process?
I have the following program:
#include <stdio.h>
#include <unistd.h>
size_t sym1 = 100;
size_t *addr = &sym1;
size_t *arr = (size_t*)((size_t)&arr + (size_t)&addr);
int main (int argc, char **argv)
{
(void) argc;
(void) argv;
printf("libtest: addr of main(): %p\n", &main);
printf("libtest: addr of arr: %p\n", &arr);
while(1);
return 0;
}
Why is it possible to produce the binary without error with g++ while there is an error using gcc?
I'm looking for a method to trace what makes them behave differently.
# gcc test.c -o test_app
test.c:7:1: error: initializer element is not constant
# g++ test.c -o test_app
I think the reason can be in fact that gcc uses cc1 as a compiler and g++ uses cc1plus.
Is there a way to make more precise output of what actually has been done?
I've tried to use -v flag but the output is quite similar. Are there different flags passed to linker?
What is the easiest way to compare two compilation procedures and find the difference in them?
In this case, gcc produces nothing because your program is not valid C. As the compiler explains, the initializer element (expression used to initialize the global variable arr) is not constant.
C requires initialization expressions to be compile-time constants, so that the contents of local variables can be placed in the data segment of the executable. This cannot be done for arr because the addresses of variables involved are not known until link time and their sum cannot be trivially filled in by the dynamic linker, as is the case for addr1. C++ allows this, so g++ generates initialization code that evaluates the non-constant expressions and stores them in global variables. This code is executed before invocation of main().
Executables cc1 and cc1plus are internal details of the implementation of the compiler, and as such irrelevant to the observed behavior. The relevant fact is that gcc expects valid C code as its input, and g++ expects valid C++ code. The code you provided is valid C++, but not valid C, which is why g++ compiles it and gcc doesn't.
There is a slightly more interesting question lurking here. Consider the following test cases:
#include <stdint.h>
#if TEST==1
void *p=(void *)(unsigned short)&p;
#elif TEST==2
void *p=(void *)(uintptr_t)&p;
#elif TEST==3
void *p=(void *)(1*(uintptr_t)&p);
#elif TEST==4
void *p=(void *)(2*(uintptr_t)&p);
#endif
gcc (even with the very conservative flags -ansi -pedantic-errors) rejects test 1 but accepts test 2, and accepts test 3 but rejects test 4.
From this I conclude that some operations that are easily optimized away (like casting to an object of the same size, or multiplying by 1) get eliminated before the check for whether the initializer is a constant expression.
So gcc might be accepting a few things that it should reject according to the C standard. But when you make them slightly more complicated (like adding the result of a cast to the result of another cast - what useful value can possibly result from adding two addresses anyway?) it notices the problem and rejects the expression.

Overkilling "crosses initialization of variable" error in C++?

I noticed that g++ complain a bit too strictly about crossed initialization and I wonder why these false-positive errors could not be removed just by looking at the SSA form of the program while compiling.
Let me give a very simple example:
#include <cstdlib>
int main ()
{
goto end;
int i = 0; // unused variable declaration
end:
return EXIT_SUCCESS;
}
When compiled with g++ -Wall -Wextra -o example1 example1.cc (g++ 4.8.1), the compiler gives the following error message:
example1.cc: In function ‘int main()’:
example1.cc:10:2: error: jump to label ‘end’ [-fpermissive]
end:
^
example1.cc:6:8: error: from here [-fpermissive]
goto end;
^
example1.cc:8:7: error: crosses initialization of ‘int i’
int i = 0;
^
example1.cc:8:7: warning: unused variable ‘i’ [-Wunused-variable]
So, it will raise an error where there is actually no risk because the variable is unused (the compiler obviously has both information and cannot combine it to deduce that the error is a false-positive).
More strange, I hoped that LLVM was more efficient at analyzing a program. So, I tried clang++ (LLVM) on this simple example with clang++ -Wall -Wextra -o example1 example1.cc (clang++ 3.4). And, I got about the same error message:
example1.cc:8:7: warning: unused variable 'i' [-Wunused-variable]
int i = 0;
^
example1.cc:6:3: error: goto into protected scope
goto end;
^
example1.cc:8:7: note: jump bypasses variable initialization
int i = 0;
^
1 warning and 1 error generated.
So, I am pretty sure that I am missing something important here, a problem that make the detection of this false-positive harder than I though. But, I do not know what is it. Or maybe, the C++ specification specifically says that it must be like this.
If somebody has an idea, feel free to share !
Edit: I also compiled the exact same code in C (gcc or clang), and it went fine just with the warning about i being an unused variable. So, it reinforce the fact that this is more likely linked to the specification of C++ and not a problem detecting this issue at compile time.
There is nothing wrong with the compilers. Your code is ill-formed according to the standard.
In your particular case, the requirement of the standard may not be necessary and the jump could be allowed and the compiler could create valid code. However, this is only because the initialisation of the variable int i has no side effects.
You can make your code valid by simply enclosing the jumped section in its own scope:
#include <cstdlib>
int main ()
{
goto end;
{
int i = 0; // unused variable declaration
}
end:
// cannot use i here, as it's not defined.
return EXIT_SUCCESS;
}
This is disallowed because potentially you'd call destructors for objects that aren't properly constructed. Admittedly, int doesn't have constructor or destructor, but it's making it "fair" for all types of objects. And technically, something at label end: could be using i, and by making the rule strict, it prevents the machine having to check every single code-path (which becomes a "halting problem").

Is there any static checking tool to check buggy code like this

I wrote the some buggy code like this:
#include "stdafx.h"
#include <string>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
string some_file = "afdfadf";
if(true)
{
string some_file = "/"+ some_file;
}
return 0;
}
It will throw an exception when calling std::operator+.
I guess this is because in the if statement the second some_file is an uninitialized string.
Is there any static checking tool that can help find this kind of bug?
I just tried, clang can help find the bug:
[~]$ clang bug.cpp
bug.cpp:11:29: warning: variable 'some_file' is uninitialized when used within
its own initialization [-Wuninitialized]
string some_file = "/"+ some_file;
~~~~~~~~~ ^~~~~~~~~
GCC has a warning for that case:
$ g++ t.cc -Wshadow
t.cc: In function ‘int main(int, char**)’:
t.cc:11:16: warning: declaration of ‘some_file’ shadows a previous local [-Wshadow]
t.cc:7:12: warning: shadowed declaration is here [-Wshadow]
Compilers can warn you about using a variable in its own initialization.
In GCC and CLANG, you can use -Winit-self
I am not sure about MSVC, but compiling with /W4 might give you a warning about those, too.
I was quit happy with using pclint. It will find this type of errors but it might take some time to configure it when used with an existing, larger code base.