How to Programming with Pointers

c,pointers,compiler-warnings,suppress-warnings,keil

None of them are "undefined behavior". They are just different pointer declarations: int const *ptr; // pointer to const int int *const *ptr; // pointer to const pointer to int int **const ptr; // const pointer to pointer to int int const **ptr; // pointer to pointer to const int...

Pointer parameter was set but never used warning

It is not an irrelevant warning because the assignment has no effect. You could completely remove ptr_1 from the code without changing its behaviour. Your code is equivalent to this: void foo(char *) { char *ptr_2; bar(ptr_2); } In other words, the function parameter isn't used for anything. If your...

Can I sort an array of pointers in order to remove duplicates?

c,pointers,undefined-behavior

If you have intptr_t (which is an integer type), then you can cast your void* pointers to that, and then you can compare arbitrary values. intptr_t is not a required type, but it is pretty common in practice. Subtracting one intptr_t from another might still overflow, so it is not...

Pointer Referring to an Array

c++,pointers,memory-address

You haven't actually declared any storage for the array, nor is arr actually pointing to the first element of the array (dereferencing is undefined behavior). You are missing something like: int solve[2]; int * arr = solve; or int solve[2]; int * arr = &solve[0]; Both ways will assign the...

`&` in function parameter list

It declares the parameter as a reference. The parameter becomes in/out rather than C++/C's normal in only. This updates the structure passed to the function. void fun1 (ptr_mystruct &val) { val = somevalue ; } This updates a copy of the parameter in fun2 so the caller never sees the...

converted string failed to contain correct values of float

c,pointers,char

The entire float_to_string can be reduced to (as suggested by BLUEPIXY): void float_to_str(char *str, float f, char size) { sprintf(str, "%.*f", size, f); } However, if you are interested in the bug in your code, it is in the for loop: for( i = 0; i < size; ++i )...

combining 2 strings in another string alphabetically

c,string,pointers,recursion

It seems that you mean the following #include <stdio.h> #include <string.h> char * combine_strings( char *result, const char *s1, const char *s2 ) { if ( *s1 == '\0' && *s2 == '\0' ) { *result = '\0'; return result; } else if ( *s1 == '\0' ) { *result++...

Functions with pointer arguments in C++

c++,function,pointers,constructor,ampersand

Item i2(i1); This works because it is not calling your user-defined constructor which takes a pointer, it is calling the implicitly generated copy-constructor which has the signature: Item (const Item &); If you don't want this to be valid, you can delete it (requires C++11): Item (const Item &) =...

compiler's detail of this pointer, virtual function and multiple-inheritance

c++,pointers,multiple-inheritance,virtual-functions,this-pointer

Bjarne wrote: "Naturally, B::bf() expects a B* (to become its this pointer)." The compiler transforms the call into: bf__F1B((B*)((char*)pc+delta(B)), 2); Why here we need a B* pointer to be the this? Consider B in isolation: the compiler needs to be able to compile code ala B::bf(B* this). It doesn't...

How to compare pointer and integer? [duplicate]

c,pointers,char

"a" is not a character, it is a string literal. 'a' is a character literal, which is what you are looking for here. Also note that in your comparison *s == "a" it is actually the "a" which is the pointer, and *s which is the integer... The * dereferences...

Using [] on pointers in C?

c,pointers,indexing

In C (and also C++), array[n] is equivalent to *(array+n). They mean exactly the same thing and produce exactly the same result.

function returns pointer to array

c++,arrays,pointers,rvalue

#include <iostream> using namespace std; double* fct_returns_ptr(double *, int); // function prototype int main(void) { double test_array[] = { 3.0, 10.0, 1.5, 15.0 }; // test value int len = (sizeof test_array)/(sizeof test_array[0]); //double* ptr_result = new double(0.0); //[len] pointer to result double* ptr_result = new double[ len ] ;...

Passing iterator's element to a function: wrong type of pointer

c++,pointers,stl,iterator

Preferred option: change isPrime to take a long (and pass *it to it). Secondary option: pass &*it instead of it. Your original code doesn't work because it is an iterator (which is a class) whereas the function expected long int * and there is no implicit conversion from iterator to...

passing arguments via reference and pointer C++

c++,function,pointers,reference

Yes. There's no copy anywhere as you're either using pointers or references when you “pass” the variable from one place to another, so you're actually using the same variable everywhere. In g(), you return a reference to z, then in f() you keep this reference to z. Moreover, when you...

Function pointer to singleton class instance function

c++,oop,pointers,singleton

Simply create a normal class without static methods, ditch the singleton pattern aside, and create an instance. The burden of singleton pattern usually outweigh any benefit....

function pointer is not a function or function pointer

c++,function,pointers

funcy is a pointer to a member function, so you need to call it on an instance of the class, like this: (this->*funcy)(items,i); ...

Difference between the use of pointers for array and single data point?

c,pointers,function-pointers

First, we should understand that pointers and arrays are handled the same in C or C++, at least once we've moved past the definition. Let's make sure we understand a few things. From the example you have described above, the following are equivalent (after ptr=var). var[0], *var, ptr[0], ptr. All...

Is a pointer to function pointer convertible to a void pointer and vice versa?

§6.3.2.3/p1: A pointer to void may be converted to or from a pointer to any object type. §6.2.5/p20: A pointer type may be derived from a function type or an object type, called the referenced type. [...] A pointer type is a complete object type. "pointer to function" is an...

Pointer Array in C? [duplicate]

c,pointers,segmentation-fault,scanf

Any time you see an expression like a[b] in C, you can mentally think that *(a + b) is happening. So, it's just "the contents of the element before the one p is pointing at right now". Since p is at t + 2, p[-1] refers to t[2 + (-1)]...

Program immediately terminates with segfault when using scanf

In your code, stack* st; st->top=-1; you're using st uninitialized which in turn invokes undefined behaviour. You need to allocate memory to st before using it. Try something like stack* st = malloc(sizeof*st); //also, check for malloc success That said, Please see why not to cast the return value of...

Getting a Expected identifier or '(' issue in Xcode 6.1

c,pointers,hashmap,xcode6,expected-exception

Change: typedef struct HashMap { uint8_t KEY; uint8_t VAL; } *map; with: typedef struct HashMap { uint8_t KEY; uint8_t VAL; } HashMap ; HashMap *map; ...

Are pointers dereferenced by default in golang struct methods?

pointers,go

Yes, the pointer to the struct is automatically dereferenced. From the spec on selectors: The following rules apply to selectors: For a value x of type T or *T where T is not a pointer or interface type, x.f denotes the field or method at the shallowest depth in T...

Initializing, constructing and converting struct to byte array causes misalignment

c,arrays,pointers,struct,memcpy

If you transfer data to another (different) architecture, do not just pass a structure as a blob. That is the way to hell: endianess, alignment, padding bytes, etc. all can (and likely will) cause trouble. Better serialize the struct in a conforming way, possily using some interpreted control stream so...

Parameters to use in a referenced function c++

c++,pointers,reference

Your code makes no sense, why are you passing someStruct twice? For the reference part, you should have something like: void names(someStruct &s) { // <<<< Pass struct once as a reference cout << "First Name: " << "\n"; cin >> s.firstname; cout << "Last Name: " << "\n"; cin...

(Dangling?) Reference returned from function does not “work”

c++,pointers,reference,unique-ptr

The issue is here: auto pos = entity.addComponent<inc::CPosition>(); ^^^^^ addComponent() returns a reference, and everything in that function is fine (no dangling reference issue as far as I can tell). But auto does not deduce a reference type unless you tell it to - so you're simply making a copy...

C/C++: What's faster: a for loop, or incrementing a pointer

c++,c,performance,loops,pointers

Theoretically, the answer to your question is the former, simpler code. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or accessing a volatile...

Why cant set pointers in functions

c++,function,pointers

You haven't assigned any memory for your pointer to point to. That means that dereferencing the pointer (*pointer) causes undefined behavior (like a crash in your case). You need to initialize the pointer before dereferencing it: int* pointer = new int; (And afterwards, delete pointer; to not leak the memory....

Print addresses of all local variables in C

debugging,pointers,gcc,gdb,memory-address

There is no built-in command to do this. There is an open feature request in gdb bugzilla to have a way to show the meaning of all the known slots in the current stack frame, but nobody has ever implemented this. This can be done with a bit of gdb...

Functions and pointers in C

c,function,pointers

That's not the correct way to a pointer from a function. Also r is local variable whose lifetime is limited to the function. Allocate memory using malloc and return it instead: int* randn(int n, int l){ int *r=malloc(n * sizeof *r); if(!r) { */ error */} int i; for (...

Does realloc() invalidate all pointers?

c,pointers,dynamic-memory-allocation,behavior,realloc

Yes, ptr2 is unaffected by realloc(), it has no connection to realloc() call whatsoever(as per the current code). However, FWIW, as per the man page of realloc(), (emphasis mine) The realloc() function returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and...

Is it safe to compare to pointer of std::vector to check equality?

c++,pointers,c++11,vector

std::vector<int> vec; vector<int>* pVec = &vec; vec.reserve(10000); assert(pVec == &vec); is safe. std::vector<int> vec(1,0); int* p = &vec[0]; // some operation here assert(p == &vec[0]); is not safe. The first block is safe since the address vec will not change even when its contents change. The second block is not...

Taking address of temporary

c++,pointers,visual-studio-2012

You mentioned that you are new to C++, it is not safe to return local variables or the addresses of local variables. You have to allocate space for your a Node in order to safely return it and in C++ that is done with new. Objects allocated with new will...

Visual Studio C++ access violation when modifying a pointer passed into a function

When you do this: Common = "Hello World!"; you are making the pointer Common point at a literal C-style string (and incidentally leaking the original char that you allocated via new previously). It is not valid to try to modify such a literal, so when you pass this to FindCommonStr...

pointer to pointer dynamic array in C++

The valid range of indices of an array with N elements is [0, N-1]. Thus instead of for example this loop for (int i=1; i <= n; i++) ^^^^ ^^^^^^ you have to write for ( int i = 0; i < n; i++ ) As you used operator new...

Questions on C pointers

c++,pointers,pass-by-reference,undefined-behavior,const-iterator

printf("%d ", *(Data+m)); printf("%d ", Data[m]); // how come these two print statements display the same thing? Because they mean the same thing; Data[m] and m[Data] would both be equivalent to *(Data+m) here, they all refer to the same element. // Curiously the function still works when passing the...

Passing buffers in C with MISRA compliance

c,arrays,pointers,misra

John Bode has already given a reply about how (in effect) your second code fragment addresses the MISRA guideline. I'll address your question "Is there a way to have a pointer to a cell in an array and increment it that will satisfy MISRA?" The short answer is "no". Rule...

passing a pointer through a function c

c,function,pointers

while(log = 1) while(log = 2) Wrong. In c, the = operator is for assignment, not comparison. You are setting log equal to 1 and 2 there. You want the == operator for comparison: while(log == 1) Next: scanf("%s", &p); Wrong. %s is for arrays of char ending in zero...

Error while trying to update array element

c,arrays,pointers,struct,typedef

In function CM_Init, you are setting cmPacket.Data to point to a local array: uint8_t emptyBuffer[CM_MAX_DATA_SIZE] = {0x00}; cmPacket.Data = emptyBuffer; Accessing this memory address outside the scope of the function yields undefined behavior....

Can assignment from a const_iterator dereference cause undefined behaviour?

The code is perfectly fine. You're not stripping away any constness (there's no way to do that implicitly in C++). *it gives you a const intp &. You're copying the pointer referred to by that reference into arg. Copying from something does not strip constness away. The assignment to arg...

Size of a pointer in C [duplicate]

c,pointers,sizeof

sizeof(void*) generally yields the size of pointers on your architecture, although there may be architectures where pointers to specific types are smaller - since void * is guaranteed to allow a full round-trip to any data pointer (C99 §6.3.2.3 ¶1), it should be surely bigger or equal to all other...

Program not compiling after casting pointer

c,pointers,casting,compiler-errors,void-pointers

This line: ptr = (struct sockaddr_in *) res->ai_addr; does not do what you think it does; it doesn't automagically make ptr be of type struct sockaddr_in *, so you're still trying to dereference a void pointer. What you need to do is printf("Port number: %u\n", ntohs(((struct sockaddr_in *) res->ai_addr)->sin_port)); ...

Access violation on try to fill dynamic array (large number of items)

c,arrays,pointers,malloc,dynamic-memory-allocation

"Access violation writing location 0x00000000" is explained by the manual http://man7.org/linux/man-pages/man3/malloc.3.html#RETURN_VALUE On error, these functions return NULL. Or if you prefer http://www.cplusplus.com/reference/cstdlib/malloc/. Return Value On success, a pointer to the memory block allocated by the function. The type of this pointer is always void*, which can be cast to the...

How exactly do pointers in Fortran differ from C/C++ pointers?

c++,c,pointers,fortran

As Mark says, pointers to functions and subroutines certainly do exist in Fortran. The differences are: In C, pointers are just an address whereas in Fortran, a pointer can have additional information such as array bounds and strides, which is why an explicit interface is required when declaring a pointer...

Porting C endianness & pointers black magic to Swift

c,swift,pointers,endianness

VALUE is a pointer to 32 bytes (char[32]). The pointer is cast to UInt16 pointer. That means the first two bytes of VALUE are being interpreted as UInt16 (2 bytes). * will dereference the pointer. We get the two bytes of VALUE as a 16-bit number. However it has...

Is there a way to restrict a pointer from being modified by specific functions?

Since you were looking for a solution either in C/C++, I created a layer of abstraction in C++ to protect the void** const arr as follows. Please check does it meet your requirement. class array{ public: void setArray(void** key) { arr=key; } void** getArray() { return &(*arr); } private: void**...

assigning from a pointer value

c++,pointers,parameter-passing,void-pointers

No, they're completely different. Say the value of tmp is 1. Their code will give *pf the value of whatever floating point number has the same binary representation as the integer 1. Your code would give it the floating point value 1.0!

void pointer in function parameter

It still compiles fine & crashes at runtime. Why? What is the reason? Because void* is a technique for removing all type-safety and type checking. Should I not get the compile time error? By using void* instead of the correct pointer type int*, you are expressly telling the compiler...

allocating space to list pointers using malloc not working

You did not create an array of std::list objects when you used malloc. All malloc does is allocate memory from the heap -- no objects are created. Thus attempting to use your std::list's as if they are created correctly will result in undefined behavior. You should use a container such...

Private members of objects in an array not-modifiable

c++,pointers,syntax,reference,variable-assignment

Look at the signature of your 'getVip' method: Vip Store::getVip(int index) { ... } ^^^ You are returning the 'Vip' object by value, which means you are returning a COPY of the object. Calling any method on that copy will result in a modification of that copy, not of the...

copy data from txt file to char pointer in c++

c++,string,pointers

BTW, if you are just reading the whole file, you can write something like this: #include <string> #include <fstream> #include <streambuf> std::ifstream t("timestamp.txt"); std::string str((std::istreambuf_iterator<char>(t)), std::istreambuf_iterator<char>()); (If you care about the efficiency of the code above, there is a more efficient version here: Read whole ASCII file into C++ std::string)...

C++ - Why does 2 local references to the same object stay in sync?

string & ref1 = *it; string & ref2 = *it; Okay, so you create two references to the same string. ++it; // After this line, both ref1 && ref2 evaluates to "a" Right, because they are both references to the same string. This is the way you created them. ref2...

C++ operator overloading for wrapper class

c++,pointers,operator-overloading,wrapper

This is a conversion operator. For example, you could use it to convert a Pointer<T> to a real T* and, additionally, use it everywhere a T* is expected: Pointer<float> p(new float); const float* p2 = p; In this case, the operator is only defined for the conversion to const raw...

modifying double pointer to an integer array

c,arrays,pointers,double-pointer

You should not do that because in this case there will be a memory leak because you already allocated memory for pointer array and the assignment will overwrite the value stored in the pointer. Write the function simpler void function(int **arr) { int *tmp = *arr; tmp[0]=1; tmp[1]=2; tmp[2]=3; tmp[3]=4;...

Does C language specify any implicit initialization for void pointers only?

c,pointers,initialization,language-lawyer,void-pointers

I think, the C11 standard is quite clear in this regard. Referring chapter 6.7.9, paragraph 10, If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. Now, an indeterminate value is , well, indeterminate (which you're referring to as garbage and / or NULL...

Add Array Program Not Working

c,pointers,pointer-arithmetic

In your second loop in AddArray you want d < b instead of c < d. The second loop never executes as is because d starts at zero and c will always be greater than zero after the first loop.

Pointer arithmetic; Must do it twice initally

This is a really cool bug to run into, especially for someone who is just beginning to learn C. Understanding this seemingly strange behaviour will improve your understanding of how your programs work quite a bit. Your code does not work properly because int j and a[0] do not have...

Swift 2 - UnsafeMutablePointer to object

swift,pointers,swift2

This should work: pass the object pointer as an opaque unmanaged pointer to the callback: context.info = UnsafeMutablePointer(Unmanaged.passUnretained(myObject).toOpaque()) SCNetworkReachabilitySetCallback(reachability, callback, &context) and retrieve in the callback via: func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutablePointer<Void>) { let myObject = Unmanaged<MyObject>.fromOpaque(COpaquePointer(info)).takeUnretainedValue() } Of course this assumes that some strong...

How to pass pointer to struct to a function?

c,function,pointers,struct

Inside your doStuff() function, myCoords is already a pointer. You can simply pass that pointer itself to calculateCoords() and calculateMotorSpeeds() function. Also, with a function prototype like void calculateCoords(struct coords* myCoords) calling calculateCoords(&myCoords); from doStuff() is wrong, as it is passing struct coords**. Solution: Keep your function signature for calculateCoords()...

binary tree code does not work properly

c,pointers,operators,address-operator

It looks like you never assign any value to root. Remember, C passes arguments by value, so when you call enter(root, word); from scan(), enter() can't change the value of root. It changes its local copy of it, though, but that is not enough. The same problem occurs when you...

Why can't I use `&&a` in C?

Why can't I use &&a Because & gives you the address of a varibale and &a is not a variable. The C 11 Draft specifies the following: 6.5.3.2 Address and indirection operators Constraints 1 The operand of the unary & operator shall be either a function designator, the result...

Although maps are always reference types, what if they're returned from a non-pointer receiver?

pointers,dictionary,go

Why won't you just check it? emp := ExampleMapHolder{make(map[string]int)} m := emp.TheMap() m["a"] = 1 fmt.Println(emp) // Prints {map[a:1]} Playground: http://play.golang.org/p/jGZqFr97_y...

Passing Pointers through a Function Then Getting Giberish

c++,function,pointers

x is the address of arr and arr is a stack variable so you cannot pass it as a return value. If you want check to return a pointer to an array you need to allocate it with new: arr = new int[r]. Note that you will need to eventually...

Allocate a new bool array pointer

c,string,pointers,char-pointer

If you want to dynamically allocate an array of booleans, you need to do bool *arr = new bool[10]; You have to specify the array size. The syntax for static allocation is bool arr[10]; ...

String manipulation code using char ** pointer gives unexpected result

The purpose was to add prefix and suffix to every word in the string so I need the string before the word , the word , and after the word. ex: prefix_I_suffix have done form here comes. I prefix_have_suffix have done form here comes. You're doing a few things...

Properly casting a `void*` to an integer in C++

c++,pointers,casting

I think using intptr_t is the correct intermediate here, since it's guaranteed to be large enough to contain the integral value of the void*, and once I have an integer value I can then truncate it without causing the compiler to complain. Yes, for the reason you mentioned that's...

Conversion between a void pointer and a pointer to function pointer

c,pointers,function-pointers

There is a bug in your code: You first allocate space for a function pointer and store the address in pp. void (**pp)(void) = malloc(sizeof(void (*)(void))); Then you immediately overwrite that with the address of a local function function p: pp = &p; You call f(), which in turn calls...

double* android ndk crash

c++,pointers,android-ndk

I see several possible causes. First of all, casting char* to double* and then accessing it through that pointer is undefined behavior. Most of time it works, but you are warned. Pointer misalignment. double is most likely supposed to be aligned to 8 bytes, and you read it through pointer...

C++, Use [] notation on pointer?

c++,arrays,pointers,operator-keyword

In C/C++, given a pointer p and integral value k, p[k] is evaluated as *(p+k). Either form is fine to use as long as p+k points to valid memory. If you have access to the C99 Standard, see section 6.5.2.1 Array subscripting, Paragraph 2. It says: A postfix expression followed...

Call template function for the value of a pointer out of a template function, in C++

c++,templates,pointers,c++11

The whole body of your template function needs to compile for the types it's instantiated with, regardless of whether or not a branch will ever be taken. To get around this issue, you can define separate functions for when T is a pointer and when it's not. Using SFINAE: template...

Malloc confusion

c,pointers,malloc

In the original C language - C89/90 - calling an undeclared function is not an error. For this reason, a pre-C99 compiler will not produce any "error" due to a missing function declaration. The compiler will simply assume that function returns an int. It will also automatically and quietly "guess"...

Array of constant pointers to functions

c,function,pointers,constants,ansi

The const is misplaced. As it stands, the functions are supposed to return const int (which makes little sense). What you want is: int (*const x[])(int) This way it reads: array of "const pointers to function"....

modern c++ alternative to function pointers

c++,pointers,c++11,function-pointers

As an alternative to traditional function pointers, C++11 introduced template alias which combined with variadic templates could simplify the function pointer sintax. below, an example of how to create a "template" function pointer: template <typename R, typename ...ARGS> using function = R(*)(ARGS...); It can be used this way: void foo()...

How to call a non-class member function with pointers as parameters with QtConcurrent::run?

c++,qt,pointers,qtconcurrent

The GUI can be updated only from the main thread. You are trying to update your dialog from another thread. Try this QMetaObject::invokeMethod(poProgressDialog, "setValue", Qt::AutoConnection, Q_ARG(qint32, *itemDeletedCounter)); instead of this : poProgressDialog->setValue(*itemDeletedCounter); ...

Why is a pointer to NULL-charater not converted to false

You're a layer of indirection out! A "null pointer" and "a pointer to null/zero" are two different things. At its most basic: A null pointer itself has the value zero. It doesn't point to some other object with the value zero. p == 0; // yes *p == 0; //...

How to store result of type char in pointer?

c,pointers,types,pointer-arithmetic

Also another way (no dynamic memory): #include<stdio.h> void try(char *); int main() { char result[100] = {0}; try(result); printf("%s",result); return 0; } void try(char *result) { strcpy(result,"try this"); } Note: When you say you got null, that doesn't mean anything - actually you had undefined behaviour there - because result...

Program to reverse a string in C without declaring a char[]

c,string,pointers,char

Important: scanf(" %s", name); has no bounds checking on the input. If someone enters more than 255 characters into your program, it may give undefined behaviour. Now, you have the char array you have the count (number of char in the array), why do you need to bother doing stuffs...

How much bytes will an int*** be moved forward when incremented? [duplicate]

Incrementing a pointer adds to the memory address the size of the thing the pointer points to. If d is of type int***, it is a pointer to int**. ++d will cause the pointer to move over sizeof(int**) bytes and sizeof(int**) is 4 because it is a pointer. So if...

In C language, What is the difference between extern buffer[] and extern *buffer?

c,arrays,function,pointers,unsigned-char

See these questions in the C FAQ List: http://c-faq.com/aryptr/aryptr1.html http://c-faq.com/aryptr/aryptr2.html http://c-faq.com/aryptr/aryptrparam.html ...

C Program: Update unsigned char pointer array with function

With this *arr = ptr; you are storing a pointer to a variable with automatic storage duration. The behaviour undefined. You can dynamically allocate and return a pointer that way: void changeArray(unsigned char **arr) { unsigned char ptr[3] = {100, 101, 102}; unsigned char *p = malloc(sizeof ptr); memcpy(p, ptr,...

Initializing a pointer to a struct with malloc [duplicate]

c,pointers,linked-list

When you passed head to the function init_List, a local copy of head is made and then memory is allocated to this local pointer. In main, head still points to NULL. You need to use pointer to pointer in function parameter. void init_List(Node **head, int data) { *head = malloc(sizeof(Node));...

Looking into dynamic variables created using pointers?

c,pointers,memory,memory-address

Any decent IDE that comes with debugger should allow you to view dereferenced value of an pointer. I am not familiar with CodeBlocks, but for instance Eclipse CDT does such thing pretty easily: By default it prints just *ptr, but you may set it to view as array of particular...

What does (unsigned char*)&niz[10] mean?