I have the following lines of code:
(where a is int64_t* and i is a simple int counter)
uintptr_t p = *a + (i * 4);
int64_t value = *reinterpret_cast<int64_t *>(p); //***
I have followed the following stack overflow post to get this syntax:
C++ - Get value of a particular memory address
The problem is I keep getting a segmentation fault on the indicated line.
I think this may have something to do with pointer arithmetic but I am not completely sure. Does anyone know why I could be getting a seg fault here?
You probably don't want to dereference a in your first line of code.
Also please keep in mind what §3.10/10 says about aliasing:
If a program attempts to access the stored value of an object through
a glvalue of other than one of the following types the behavior is
undefined:
the dynamic type of the object,
a cv-qualified version of the dynamic type of the object,
a type similar (as defined in 4.4) to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, an aggregate or union type that includes one of the aforementioned types among its elements or nonstatic data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
a char or unsigned char type.
std::byte was added to that list with C++17.
Related
My payload is stored in a std::string xyz (holds binary data), and I need to pass it to a function that takes it as const unsigned int*. How would I convert from std::string to const unsigned int*?
I tried reinterpret_cast<const unsigned int*>(&xyz.front()) but it is not working!
The function prototype is as follows:
void roll(void *pdst, const unsigned int *psrc);
pdst will hold the results.
Don't use std::string to store binary data; that class is specifically designed for working with strings. It feels like there was original C code that was using a char array to store a sequence of bytes and translated that to std::string for C++. In this case, it's not being used as a string, so it doesn't make sense to store it in a std::string.
From there, translating to an unsigned int, well for starters, you can't simply cast it even if you were using a more primitive type such as a char *, as it would violate the rules of strict aliasing resulting in undefined behavior. What you want to do is create a new variable and memcpy the data into this new variable.
Here is the section from the C++14 standard working draft describing compatible types (3.10 p10):
If a program attempts to access the stored value of an object through a glvalue of other than one of the
following types the behavior is undefined:
54
— the dynamic type of the object,
— a cv-qualified version of the dynamic type of the object,
— a type similar (as defined in 4.4) to the dynamic type of the object,
— a type that is the signed or unsigned type corresponding to the dynamic type of the object,
— a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type
of the object,
— an aggregate or union type that includes one of the aforementioned types among its elements or non-
static data members (including, recursively, an element or non-static data member of a subaggregate
or contained union),
— a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
— a
char
or
unsigned char
type.
As you can see, it explicitly allows for accessing any object as a char or unsigned char, but it gives no such allowance to access a char or unsigned char as anything else.
The problem is how you store binary data to std string? If you are simply using the constructor, you could get your binary data by xyz.data().
Let's consider following piece of code:
struct Blob {
double x, y, z;
} blob;
char* s = reinterpret_cast<char*>(&blob);
s[2] = 'A';
Assuming that sizeof(double) is 8, does this code trigger undefined behaviour?
Quoting from N4140 (roughly C++14):
3.9 Types [basic.types]
2 For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes (1.7) making up the object can be copied into an array of char or unsigned char.42 If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value.
42) By using, for example, the library functions (17.6.1.2) std::memcpy or std::memmove.
3 For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes (1.7) making up obj1 are copied
into obj2,43 obj2 shall subsequently hold the same value as obj1. [ Example: ... ]
43) By using, for example, the library functions (17.6.1.2) std::memcpy or std::memmove.
This does, in principle, allow assignment directly to s[2] if you take the position that assignment to s[2] is indirectly required to be equivalent to copying all of some other Blob into an array that just happens to be bytewise identical except for the third byte, and copying it into your Blob: you're not assigning to s[0], s[1], etc. For trivially copyable types including char, that is equivalent to setting them to the exact value they already have, which also has no observable effect.
However, if the only way to get s[2] == 'A' is by memory manipulation, then a valid argument could also be made that what you're copying back into your Blob isn't the underlying bytes that made up any previous Blob. In that case, technically, the behaviour would be undefined by omission.
I do strongly suspect, especially given the "whether or not the object holds a valid value of type T" comment, that it's intended to be allowed.
Chapter 3.10 of the standard seems to allow for that specific case, assuming that "access the stored value" means "read or write", which is unclear.
3.10-10
If a program attempts to access the stored value of an object through
a glvalue of other than one of the following types the behavior is
undefined:
—(10.1) the dynamic type of the object,
—(10.2) a cv-qualified version of the dynamic type of the object,
—(10.3) a type similar (as defined in 4.4) to the dynamic type of the
object,
—(10.4) a type that is the signed or unsigned type corresponding to
the dynamic type of the object,
—(10.5) a type that is the signed or unsigned type corresponding to a
cv-qualified version of the dynamic type of the object,
—(10.6) an aggregate or union type that includes one of the
aforementioned types among its elements or nonstatic data members
(including, recursively, an element or non-static data member of a
subaggregate or contained union),
—(10.7) a type that is a (possibly cv-qualified) base class type of the
dynamic type of the object,
—(10.8) a char or unsigned char type.
In case of pointers, we know that their size is always same irrespective of data type of the variable it is pointing.
Data type is needed when dereferencing the pointer so it knows how much data it should read. So why cant i assign address of variable of double type to a pointer of int type?
why cant it happen like dereferencing a int pointer reads next 4 bytes from variable of double type and print its value?
Many computers have alignment requirements, so (for example) to read a 2-byte value, the address at which it's located must be a multiple of 2 (and likewise, a 4-byte value must be located at an address that's a multiple of 4, and so on). In fact, this alignment requirement is common enough that it's frequently referred to as "natural alignment".
Likewise, some types (e.g., floating point types) impose requirements on the bit sequence that can be read as that type, so if you try to take some arbitrary data and treat it as a double, you might trigger something like a floating point exception.
If you want to do this badly enough, you can use a cast to turn the pointer into the target type (but the results, if any, aren't usually portable).
You are guaranteed that you can convert a pointer to any other type of object to a pointer to unsigned char, and use that to read the bytes that represent the pointee object.
Also, if you primarily want an opaque pointer, without type information attached, you can assign a pointer to some other type to a void *.
Finally: no, not all pointers are actually the same. Pointers to different types can be different sizes (e.g., on the early Cray compilers, a char * was substantially different from an int *).
In case of pointers, we know that their size is always same irrespective of data type of the variable it is pointing.
No, we do not know that.
Chapter and verse for C
6.2.5 Types
...
28 A pointer to void shall have the same representation and alignment requirements as a
pointer to a character type.48) Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and alignment requirements. All
pointers to structure types shall have the same representation and alignment requirements
as each other. All pointers to union types shall have the same representation and
alignment requirements as each other. Pointers to other types need not have the same
representation or alignment requirements.
48) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
Emphasis added.
Chapter and verse for C++
3.9.2 Compound types
...
3 The type of a pointer to void or a pointer to an object type is called an object pointer type. [ Note: A pointer
to void does not have a pointer-to-object type, however, because void is not an object type. — end note ]
The type of a pointer that can designate a function is called a function pointer type. A pointer to objects
of type T is referred to as a “pointer to T.” [Example: a pointer to an object of type int is referred to as
“pointer to int ” and a pointer to an object of class X is called a “pointer to X.” — end example ] Except
for pointers to static members, text referring to “pointers” does not apply to pointers to members. Pointers
to incomplete types are allowed although there are restrictions on what can be done with them (3.11).
A valid value of an object pointer type represents either the address of a byte in memory (1.7) or a null
pointer (4.10). If an object of type T is located at an address A, a pointer of type cv T* whose value is the
address A is said to point to that object, regardless of how the value was obtained. [ Note: For instance,
the address one past the end of an array (5.7) would be considered to point to an unrelated object of the
array’s element type that might be located at that address. There are further restrictions on pointers to
objects with dynamic storage duration; see 3.7.4.3. — end note ] The value representation of pointer types
is implementation-defined. Pointers to layout-compatible types shall have the same value representation and
alignment requirements (3.11). [ Note: Pointers to over-aligned types (3.11) have no special representation,
but their range of valid values is restricted by the extended alignment requirement. This International
Standard specifies only two ways of obtaining such a pointer: taking the address of a valid object with
an over-aligned type, and using one of the runtime pointer alignment functions. An implementation may
provide other means of obtaining a valid pointer value for an over-aligned type. — end note ]
4 A pointer to cv-qualified (3.9.3) or cv-unqualified void can be used to point to objects of unknown type.
Such a pointer shall be able to hold any object pointer. An object of type cv void* shall have the same
representation and alignment requirements as cv char*.
Emphasis added. It is entirely possible to have different sizes and representations for different pointer types. There is no reason to expect a pointer to int to have the same size and representation as a pointer to double, or a pointer to a struct type, or a pointer to a function type. It's true for commodity platforms like x86, but not all the world runs on x86.
This is why you can't assign pointer values of one type to pointer values of another type without an explicit cast (except for converting between void * and other pointer types in C), since a representation change may be required.
Secondly, pointer arithmetic depends on the size of the pointed-to type. Assume you have pointers to a 32-bit int and a 64-bit double:
int *ip;
double *dp;
The expression ip + 1 will return the address of the next integer object (current address plus 4), while the expression dp + 1 will return the address of the next double object (current address plus 8).
If I assign the address of a double to a pointer to int, incrementing that int pointer won't take me to the next double object.
Note: This question has been renamed and reduced to make it more focused and readable. Most of the comments refer to the old text.
According to the standard, objects of different type may not share the same memory location. So this would not be legal:
std::array<short, 4> shorts;
int* i = reinterpret_cast<int*>(shorts.data()); // Not OK
The standard, however, allows an exception to this rule: any object may be accessed through a pointer to char or unsigned char:
int i = 0;
char * c = reinterpret_cast<char*>(&i); // OK
However, it is not clear to me whether this is also allowed the other way around. For example:
char * c = read_socket(...);
unsigned * u = reinterpret_cast<unsigned*>(c); // huh?
Some of your code is questionable due to the pointer conversions involved. Keep in mind that in those instances reinterpret_cast<T*>(e) has the semantics of static_cast<T*>(static_cast<void*>(e)) because the types that are involved are standard-layout. (I would in fact recommend that you always use static_cast via cv void* when dealing with storage.)
A close reading of the Standard suggests that during a pointer conversion to or from T* it is assumed that there really is an actual object T* involved -- which is hard to fulfill in some of your snippet, even when 'cheating' thanks to the triviality of types involved (more on this later). That would be besides the point however because...
Aliasing is not about pointer conversions. This is the C++11 text that outlines the rules that are commonly referred to as 'strict aliasing' rules, from 3.10 Lvalues and rvalues [basic.lval]:
10 If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:
the dynamic type of the object,
a cv-qualified version of the dynamic type of the object,
a type similar (as defined in 4.4) to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
a char or unsigned char type.
(This is paragraph 15 of the same clause and subclause in C++03, with some minor changes in the text with e.g. 'lvalue' being used instead of 'glvalue' since the latter is a C++11 notion.)
In the light of those rules, let's assume that an implementation provides us with magic_cast<T*>(p) which 'somehow' converts a pointer to another pointer type. Normally this would be reinterpret_cast, which yields unspecified results in some cases, but as I've explained before this is not so for pointers to standard-layout types. Then it's plainly true that all of your snippets are correct (substituting reinterpret_cast with magic_cast), because no glvalues are involved whatsoever with the results of magic_cast.
Here is a snippet that appears to incorrectly use magic_cast, but which I will argue is correct:
// assume constexpr max
constexpr auto alignment = max(alignof(int), alignof(short));
alignas(alignment) char c[sizeof(int)];
// I'm assuming here that the OP really meant to use &c and not c
// this is, however, inconsequential
auto p = magic_cast<int*>(&c);
*p = 42;
*magic_cast<short*>(p) = 42;
To justify my reasoning, assume this superficially different snippet:
// alignment same as before
alignas(alignment) char c[sizeof(int)];
auto p = magic_cast<int*>(&c);
// end lifetime of c
c.~decltype(c)();
// reuse storage to construct new int object
new (&c) int;
*p = 42;
auto q = magic_cast<short*>(p);
// end lifetime of int object
p->~decltype(0)();
// reuse storage again
new (p) short;
*q = 42;
This snippet is carefully constructed. In particular, in new (&c) int; I'm allowed to use &c even though c was destroyed due to the rules laid out in paragraph 5 of 3.8 Object lifetime [basic.life]. Paragraph 6 of same gives very similar rules to references to storage, and paragraph 7 explains what happens to variables, pointers and references that used to refer to an object once its storage is reused -- I will refer collectively to those as 3.8/5-7.
In this instance &c is (implicitly) converted to void*, which is one of the correct use of a pointer to storage that has not been yet reused. Similarly p is obtained from &c before the new int is constructed. Its definition could perhaps be moved to after the destruction of c, depending on how deep the implementation magic is, but certainly not after the int construction: paragraph 7 would apply and this is not one of the allowed situations. The construction of the short object also relies on p becoming a pointer to storage.
Now, because int and short are trivial types, I don't have to use the explicit calls to destructors. I don't need the explicit calls to the constructors, either (that is to say, the calls to the usual, Standard placement new declared in <new>). From 3.8 Object lifetime [basic.life]:
1 [...] The lifetime of an object of type T begins when:
storage with the proper alignment and size for type T is obtained, and
if the object has non-trivial initialization, its initialization is complete.
The lifetime of an object of type T ends when:
if T is a class type with a non-trivial destructor (12.4), the destructor call starts, or
the storage which the object occupies is reused or released.
This means that I can rewrite the code such that, after folding the intermediate variable q, I end up with the original snippet.
Do note that p cannot be folded away. That is to say, the following is defintively incorrect:
alignas(alignment) char c[sizeof(int)];
*magic_cast<int*>(&c) = 42;
*magic_cast<short*>(&c) = 42;
If we assume that an int object is (trivially) constructed with the second line, then that must mean &c becomes a pointer to storage that has been reused. Thus the third line is incorrect -- although due to 3.8/5-7 and not due to aliasing rules strictly speaking.
If we don't assume that, then the second line is a violation of aliasing rules: we're reading what is actually a char c[sizeof(int)] object through a glvalue of type int, which is not one of the allowed exception. By comparison, *magic_cast<unsigned char>(&c) = 42; would be fine (we would assume a short object is trivially constructed on the third line).
Just like Alf, I would also recommend that you explicitly make use of the Standard placement new when using storage. Skipping destruction for trivial types is fine, but when encountering *some_magic_pointer = foo; you're very much likely facing either a violation of 3.8/5-7 (no matter how magically that pointer was obtained) or of the aliasing rules. This means storing the result of the new expression, too, since you most likely can't reuse the magic pointer once your object is constructed -- due to 3.8/5-7 again.
Reading the bytes of an object (this means using char or unsigned char) is fine however, and you don't even to use reinterpret_cast or anything magic at all. static_cast via cv void* is arguably fine for the job (although I do feel like the Standard could use some better wording there).
This too:
// valid: char -> type
alignas(int) char c[sizeof(int)];
int * i = reinterpret_cast<int*>(c);
That is not correct. The aliasing rules state under which circumstances it is legal/illegal to access an object through an lvalue of a different type. There is an specific rule that says that you can access any object through a pointer of type char or unsigned char, so the first case is correct. That is, A => B does not necessarily mean B => A. You can access an int through a pointer to char, but you cannot access a char through a pointer to int.
For the benefit of Alf:
If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:
the dynamic type of the object,
a cv-qualified version of the dynamic type of the object,
a type similar (as defined in 4.4) to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
an aggregate or union type that includes one of the aforementioned types among its elements or non- static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
a char or unsigned char type.
Regarding the validity of …
alignas(int) char c[sizeof(int)];
int * i = reinterpret_cast<int*>(c);
The reinterpret_cast itself is OK or not, in the sense of producing a useful pointer value, depending on the compiler. And in this example the result isn't used, in particular, the character array isn't accessed. So there is not much more that can be said about the example as-is: it just depends.
But let's consider an extended version that does touch on the aliasing rules:
void foo( char* );
alignas(int) char c[sizeof( int )];
foo( c );
int* p = reinterpret_cast<int*>( c );
cout << *p << endl;
And let's only consider the case where the compiler guarantees a useful pointer value, one that would place the pointee in the same bytes of memory (the reason that this depends on the compiler is that the standard, in §5.2.10/7, only guarantees it for pointer conversions where the types are alignment-compatible, and otherwise leave it as "unspecified" (but then, the whole of §5.2.10 is somewhat inconsistent with §9.2/18).
Now, one interpretation of the standard's §3.10/10, the so called "strict aliasing" clause (but note that the standard does not ever use the term "strict aliasing"),
If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:
the dynamic type of the object,
a cv-qualified version of the dynamic type of the object,
a type similar (as defined in 4.4) to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
an aggregate or union type that includes one of the aforementioned types among its elements or non- static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
a char or unsigned char type.
is that, as it itself says, concerns the dynamic type of the object residing in the c bytes.
With that interpretation, the read operation on *p is OK if foo has placed an int object there, and otherwise not. So in this case, a char array is accessed via an int* pointer. And nobody is in any doubt that the other way is valid: even though foo may have placed an int object in those bytes, you can freely access that object as a sequence of char values, by the last dash of §3.10/10.
So with this (usual) interpretation, after foo has placed an int there, we can access it as char objects, so at least one char object exists within the memory region named c; and we can access it as int, so at least that one int exists there also; and so David’s assertion in another answer that char objects cannot be accessed as int, is incompatible with this usual interpretation.
David's assertion is also incompatible with the most common use of placement new.
Regarding what other possible interpretations there are, that perhaps could be compatible with David's assertion, well, I can't think of any that make sense.
So in conclusion, as far as the Holy Standard is concerned, merely casting oneself a T* pointer to the array is practically useful or not depending on the compiler, and accessing the pointed to could-be-value is valid or not depending on what's present. In particular, think of a trap representation of int: you would not want that blowing up on you, if the bitpattern happened to be that. So to be safe you have to know what's in there, the bits, and as the call to foo above illustrates the compiler can in general not know that, like, the g++ compiler's strict alignment-based optimizer can in general not know that…
I had a discussion with someone on IRC and this question turned up. We are allowed by the Standard to change an object of type int by a char lvalue.
int a;
char *b = (char*) &a;
*b = 0;
Would we be allowed to do this in the opposite direction, if we know that the alignment is fine?
The issue I'm seeing is that the aliasing rule does not cover the simple case of the following, if one considers the aliasing rule as a non-symmetric relation
int a;
a = 0;
The reason is, that each object contains a sequence of sizeof(obj) unsigned char objects (called the "object representation"). If we change the int, we will change some or all of those objects. However, the aliasing rule only states we are allowed to change a int by an char or unsigned char, but not the other way around. Another example
int a[1];
int *ra = a;
*ra = 0;
Only one direction is described by 3.10/15 ("An aggregate or union type that includes..."), but this time we need the other way around ("A type that is the element or non-static data member type of an aggregate...").
Is the other direction implied? This question also applies to C.
The aliasing rule simply states that there's one "effective type" (C99 6.5.7, plus footnote 73) for any given object in memory, and any accesses to such an object go through one of:
A type compatible with the effective type (qualifiers such as const and restrict, as well signed/unsigned-ness may vary)
A struct or union containing one such type
A character type
The effective type isn't specified in advanced, of course - it's just a construct that is used to specify aliasing. But the intent is simply that you don't access the same object with two different non-character types.
So the answer is, yes, you can indeed go the other direction.
The standard (C99 6.3.2.3 §7) defines pointer casts like this as "just fine", the casted pointer will point at the same address. (Unless the CPU has alignment which makes the cast impossible, then it is undefined behavior.)
That is, the actual cast in itself is fine. What will happen if you start to manipulate the data... now that's another implementation-defined story.
Here's from the standard:
"A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned (57) for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer.
When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object. Successive increments of the result, up to the size of the object, yield pointers
to the remaining bytes of the object."
"57) In general, the concept ‘‘correctly aligned’’ is transitive: if a pointer to type A is correctly aligned for a pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is
correctly aligned for a pointer to type C."
I think I'm a bit confused by the question, but the 2nd and 3rd examples are accessing the int through an lvalue that has the object's type (int in the examples).
C++ 3.10/15 states as it's first item that it's OK to access the object through an lvalue that has the the type of "the dynamic type of the object".
What am I misunderstanding in the question?