I am working on a linux kernel module.
A struct tcpsp_conn is defined in the header file as follows:
struct tcpsp_conn {
...
struct timer_list timer; /* exp. timer*/
...
};
Then I declare a pointer to the structure and try to assign the function:
struct tcpsp_conn *cp;
cp->timer.function = tcpsp_conn_expire;
tcpsp_conn_expire function is defined in the same way as in the struct timer_list of the kernel:
static void tcpsp_conn_expire(unsigned long data)
I don't understand why am I getting this error:
error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]
cp->timer.function = tcpsp_conn_expire;
It doesn't look to have a problem with types.
Type of your tcpsp_conn_expire function differs from the type of .function field of the timer_list structure.
In the newest kernel (since 4.15) this function-field is declared with struct timer_list * argument instead of unsigned long, as follows:
struct timer_list {
...
void (*function)(struct timer_list *);
...
};
Having such argument, you may obtain the pointer to the struct tcpsp_conn structure, into which the timer is embedded, with macro container_of.
Related
I am working with shared_ptr storing pointers of a C library.
Here an example of such a C library containing the header bar.h:
#pragma once
typedef struct Flupp MyFlupp;
MyFlupp *
create_flupp();
void
del_flupp(MyFlupp * fp);
void
print_flupp(MyFlupp * f);
Here the struct has a forward declaration and is defined in the bar.so.
I am using the bar.so in my C++ code:
#include <memory>
extern "C"{
#include "bar.h"
}
int main()
{
std::shared_ptr<MyFlupp> flupp_ptr(nullptr, del_flupp);
flupp_ptr.reset(create_flupp());
print_flupp(flupp_ptr.get());
return 0;
}
Here I am storing the MyFlupp* in a shared_ptr. On the declaration, MyFlupp* is unknown and set to nullptr. Later I am calling the reset operation to set the valid pointer. But when I am compling the code, I get the following error:
In file included from /usr/include/c++/8/bits/shared_ptr.h:52,
from /usr/include/c++/8/memory:81,
from test_foo.cpp:1:
/usr/include/c++/8/bits/shared_ptr_base.h: In instantiation of ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Yp*) [with _Yp = Flupp; <template-parameter-2-2> = void; _Tp = Flupp; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2]’:
/usr/include/c++/8/bits/shared_ptr_base.h:1293:4: required from ‘std::__shared_ptr<_Tp, _Lp>::_SafeConv<_Yp> std::__shared_ptr<_Tp, _Lp>::reset(_Yp*) [with _Yp = Flupp; _Tp = Flupp; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2; std::__shared_ptr<_Tp, _Lp>::_SafeConv<_Yp> = void]’
test_foo.cpp:10:35: required from here
/usr/include/c++/8/bits/shared_ptr_base.h:1126:19: error: invalid application of ‘sizeof’ to incomplete type ‘Flupp’
static_assert( sizeof(_Yp) > 0, "incomplete type" );
When I am providing the deleter to the reset operation than it is working.
flupp_ptr.reset(create_flupp(), del_flupp);
Can anybody explain me whats going on? I already looked #cppreference but I does not found an answer.
The problem is that the type Flupp has only been forward-declared, but not defined. In the context of the use here, it is considered an incomplete type.
This has certain implications for the use with std::shared_ptr:
std::shared_ptr may be used with an incomplete type T. However, the
constructor from a raw pointer (template<class Y> shared_ptr(Y*)) and
the template<class Y> void reset(Y*) member function may only be
called with a pointer to a complete type (note that std::unique_ptr
may be constructed from a raw pointer to an incomplete type).
Source: cppreference.com
Instead you need to use the respective overloads that accept a pointer and the deleter as arguments.
With unique_ptr this is not necessary, as that one stores the custom deleter as part of the type. But with shared_ptr the deleter is type-erased and only recovered at runtime. This allows you to change the deleter of an existing shared_ptr when calling reset. For this reason you always need to re-state which deleter to use whenever you're calling reset. If no deleter is given, each call to reset will also implicitly reset the deleter to just calling delete on the managed pointer.
So to make it work, just change your reset call to
flupp_ptr.reset(create_flupp(), del_flupp);
So, I have this template class and its specialization.
#include <iostream>
using namespace std;
template<bool> struct CompileTimeChecker{
CompileTimeChecker(...); //constructor, can accept any number of parameters;
};
//specialized template definition
template<> struct CompileTimeChecker<false> {
//default constructor, body empty
};
Case 1:
In the main function I am defining a local class called ErrorA. When I create a temporary of CompileTimeChecker<false> with temporary object of ErrorA fed as an initializer, the compiler is not detecting any error.
int main()
{
class ErrorA {};
CompileTimeChecker<false>(ErrorA()); //Case 1;
CompileTimeChecker<false>(int()); //Case 2;
return 0;
}
Case 2:
Next I feed it with temporary object of type int, and suddenly the compiler recognizes the issue (there is no constructor that takes args in the specialized template CompileTimeChecker<false>)
main.cpp:30:36: error: no matching function for call to ‘CompileTimeChecker::CompileTimeChecker(int)’ CompileTimeChecker<false>(int());
main.cpp:21:23: note: candidate: constexpr CompileTimeChecker::CompileTimeChecker()
template<> struct CompileTimeChecker<false> {
^~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:21:23: note: candidate expects 0 arguments, 1 provided
Why does it not recognize the issue in case 1?
CompileTimeChecker<false>(ErrorA());
does not create a temporary of type CompileTimeChecker<false>, passing a temporary ErrorA() to its constructor. Rather, it declares a function named ErrorA, taking no parameters and returning CompileTimeChecker<false> . See also: most vexing parse.
On the other hand, CompileTimeChecker<false>(int()); cannot be parsed as a declaration, so it does unambiguously create a temporary of type CompileTimeChecker<false>.
The easiest way out is to use braces in place of parens to indicate initialization:
CompileTimeChecker<false>{ErrorA{}};
I am reading android kernel code and I'm facing this kind of data structures ,
static const struct file_operations tracing_fops = {
.open = tracing_open,
.read = seq_read,
.write = tracing_write_stub,
.llseek = tracing_seek,
.release = tracing_release,
};
can someone explain this syntax generally ? right side of equations are functions names and &tracing_fops later is passed as an argument to another function that inits debugfs file system.
The assignment is an example of using Compund Literals. According to C99 Section #6.5.2.5:
A postfix expression that consists of a parenthesized type name
followed by a brace- enclosed list of initializers is a compound
literal. It provides an unnamed object whose value is given by the
initializer list.
In simpler version, according to GCC docs: Compound literals:
A compound literal looks like a cast of a brace-enclosed aggregate
initializer list. Its value is an object of the type specified in the
cast, containing the elements specified in the initializer. Unlike the
result of a cast, a compound literal is an lvalue. ISO C99 and later
support compound literals. As an extension, GCC supports compound
literals also in C90 mode and in C++, although as explained below, the
C++ semantics are somewhat different.
An simple example:
struct foo { int x; int y; };
func() {
struct foo var = { .x = 2, .y = 3 };
...
}
In the question's example, the struct file_operations is defined in include/linux/fs.h and tracing_fops is in kernel/trace/trace.c file in Linux source tree.
struct file_operations {
struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
...
};
The open, read, write are Function Pointers which are pointers that points to a function. After dereferencing the function pointer, it can be used as normal function call. The tracing_fops structure is file_operations type. The values of function pointer members are assigned to the functions in the same trace.c file using compound literals.
With compound literals, we don't have to explicitly specify/assign all members in the structure type because other members are set to zero or null. Structure objects created using compound literals can be passed to functions without depending on member order. The function parameters should be same for both side. For example, the parameters of
int (*open) (struct inode *, struct file *);
is same as
int tracing_open(struct inode *inode, struct file *file);
In object oriented programming, this idea is somewhat similar as Virtual Function Table.
This is simply a struct initialization, using field names to assign values to specific fields only. You can take a look at struct initialization at cppreference which demonstrates these use cases (and even more advanced situations, such as omitting specific field names, etc.)
The Linux kernel sources often make use of structs consisting of sets of function pointers for related operations. These are used to provide distinct implementations of the same interface, akin to what would be accomplished using class inheritance in object-oriented languages. For instance, in C++ this same idea would be implemented using virtual methods and the function pointers would be stored in the class vtable (which means this would be implicit rather than explicit in C++.)
Using this struct in C is similar to how you'd use an object of a class using virtual methods in C++, since you can simply call one of the "methods" using:
int r = fops->open(inode, filp);
The actual code typically tests whether the struct member is set, since the struct initialization will keep the pointers that are not explicitly mentioned set to NULL, making it possible to use this kind of struct to implement optional operations as well.
The main difference being that in C++ you'd have an implicit reference to the object itself (this), while in C you have to pass that as an additional argument in cases where it's needed.
Can anyone please explain how to use and access string in a union inside a structure with the help of unrestricted union?
#include <iostream>
#include <string>
using namespace std;
typedef struct {
int height;
int width;
} Page;
typedef struct {
int test;
union {
Page page;
int intVar;
string stringVar;
} VarUnion;
} VariableDataStruct;
int main()
{
VariableDataStruct structeg;
structeg.VarUnion.stringVar = "Hello";
return 0;
}
Currently getting following errors on compilation:
unionstring2.cc: In function ‘int main()’:
unionstring2.cc:22:24: error: use of deleted function ‘VariableDataStruct::VariableDataStruct()’
VariableDataStruct structeg;
^
unionstring2.cc:11:16: note: ‘VariableDataStruct::VariableDataStruct()’ is implicitly deleted because the default definition would be ill-formed:
typedef struct {
^
unionstring2.cc:11:16: error: use of deleted function ‘VariableDataStruct::::()’
unionstring2.cc:13:19: note: ‘VariableDataStruct::::()’ is implicitly deleted because the default definition would be ill-formed:
union {
^
unionstring2.cc:16:11: error: union member ‘VariableDataStruct::::stringVar’ with non-trivial ‘std::basic_string<_CharT, _Traits, _Alloc>::basic_string() [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]’
string stringVar;
^
unionstring2.cc:11:16: error: use of deleted function ‘VariableDataStruct::::~()’
typedef struct {
^
unionstring2.cc:13:19: note: ‘VariableDataStruct::::~()’ is implicitly deleted because the default definition would be ill-formed:
union {
^
unionstring2.cc:16:11: error: union member ‘VariableDataStruct::::stringVar’ with non-trivial ‘std::basic_string<_CharT, _Traits, _Alloc>::~basic_string() [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]’
string stringVar;
^
unionstring2.cc:22:24: error: use of deleted function ‘VariableDataStruct::~VariableDataStruct()’
VariableDataStruct structeg;
^
unionstring2.cc:18:11: note: ‘VariableDataStruct::~VariableDataStruct()’ is implicitly deleted because the default definition would be ill-formed:
} VariableDataStruct;
^
unionstring2.cc:18:11: error: use of deleted function ‘VariableDataStruct::::~()’
The error you're getting is not about accessing union, it's about not being able to instantiate your struct:
error: use of deleted function ‘VariableDataStruct::VariableDataStruct()’
You need to provide a constructor for your struct that sensibly initializes the union.
Unions with members with non-trivial special member functions (constructor, assignment, destructors) (such as std::string) must define these special functions as well. Since this union does not provide the designation which member is currently in use, those special member functions cannot be defined.
Use std::variant<Page, int, std::string> instead.
The following code is a snippet of a tuple-like class where it is possible to get a reference to a given type in the tuple, or if that type is not found, the provided default value will be returned instead.
If the default value is a lvalue a reference must be returned, if the default value is a rvalue then a rvalue must be returned.
The following code illustrates the problem I'm having:
struct Foo {
Foo(int d) : data(d) {}
template <typename T, typename TT>
const TT get_or_default(TT&& t) const {
return data;
}
template <typename T, typename TT>
TT get_or_default(TT&& t) {
return data;
}
int data;
};
int main(int argc, char* argv[]) {
int i = 6;
const Foo foo1(5);
Foo foo2(5);
// compile error
foo1.get_or_default<int>(i);
// works
foo1.get_or_default<int>(5);
foo2.get_or_default<int>(i) = 4;
foo2.get_or_default<char>('a');
return 0;
}
When compiling this I get the following error:
cxx.cxx:6:20: error: binding of reference to type 'int' to a value of type 'const int' drops qualifiers
return data;
^~~~
cxx.cxx:23:14: note: in instantiation of function template specialization 'Foo::get_or_default<int, int &>' requested here
foo1.get_or_default<int>(i);
^
1 error generated.
There is a special rule for template argument deduction when the function parameter is of type T&& where T is a template parameter. That rule is:
If the function argument is an lvalue of type U, then U& is used in place of U for type deduction in this case.
It's used to allow perfect forwarding. Basically, it means that T&& for a template parameter T is a "universal reference."
In your case, since i is indeed an lvalue, TT is deduced to int&. Applying a const to that is ignored (it would apply to the reference itself, not to the type referred to), so the fucntion instantiated from the template looks something like this:
int& get_or_default(int& t) const {
return data;
}
And since the function is const, data is considered const as well and so it cannot bind to a non-const reference.