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• What is "null pointer assignment error"?

  - by sharptooth

  One of job interview questions on C pointer here is the following: what is null pointer assignment error? I've googled for a while and don't see any reasonable explanation. What is that? Trying to write through a null pointer? Something architecture- or environment-specific? What exactly is that error?

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• Can a pointer ever point to itself?

  - by eSKay

  This question was mentioned here. My doubt is: If a pointer variable has the same address as its value, is it really pointing to itself? For example - in the following piece of code, is a a pointer to itself? #include<stdio.h> int main(){ int* a; int b = (int)&a; a = b; printf("address of a = %d\n", &a); printf(" value of a = %d\n", a); } If a is not a pointer to itself, then the same question poses again: Can a pointer point to itself? Also, how is a self pointing pointer useful?

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• how to write binary copy of structure array to file

  - by cerr

  I would like to write a binary image of a structure array to a binary file. I have tried this so far: #include <stdio.h> #include <string.h> #define NUM 256 const char *fname="binary.bin"; typedef struct foo_s { int intA; int intB; char string[20]; }foo_t; void main (void) { foo_t bar[NUM]; bar[0].intA = 10; bar[0].intB = 999; strcpy(bar[0].string,"Hello World!"); Save(bar); printf("%s written succesfully!\n",fname); } int Save(foo_t* pData) { FILE *pFile; int ptr = 0; int itr = 0; pFile = fopen(fname, "w"); if (pFile == NULL) { printf("couldn't open %s\n", fname); return; } for (itr = 0; itr<NUM; itr++) { for (ptr=0; ptr<sizeof(foo_t); ptr++) { fputc((unsigned char)*((&pData[itr])+ptr), pFile); } fclose(pFile); } } but the compiler is saying aggregate value used where an integer was expected fputc((unsigned char)*((&pData[itr])+ptr), pFile); and I don't quite understand why, what am I doing wrong? Thanks!

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• pointer to a pointer in a linked list

  - by user1596497

  I'm trying to set a linked list head through pointer to a pointer. I can see inside the function that the address of the head pointer is changing but as i return to the main progran it becomes NULL again. can someone tell me what I'm doing wrong ?? #include <stdio.h> #include <stdlib.h> typedef void(*fun_t)(int); typedef struct timer_t { int time; fun_t func; struct timer_t *next; }TIMER_T; void add_timer(int sec, fun_t func, TIMER_T *head); void run_timers(TIMER_T **head); void timer_func(int); int main(void) { TIMER_T *head = NULL; int time = 1; fun_t func = timer_func; while (time < 1000) { printf("\nCalling add_timer(time=%d, func=0x%x, head=0x%x)\n", time, func, &head); add_timer(time, func, head); time *= 2; } run_timers(&head); return 0; } void add_timer(int sec, fun_t func, TIMER_T *head) { TIMER_T ** ppScan=&head; TIMER_T *new_timer = NULL; new_timer = (TIMER_T*)malloc(sizeof(TIMER_T)); new_timer->time = sec; new_timer->func = func; new_timer->next = NULL; while((*ppScan != NULL) && (((**ppScan).time)<sec)) ppScan = &(*ppScan)->next; new_timer->next = *ppScan; *ppScan = new_timer; }

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• C Structure Pointer Problem

  - by Halo

  I have this struct; #define BUFSIZE 10 struct shared_data { pthread_mutex_t th_mutex_queue; int count; int data_buffer_allocation[BUFSIZE]; int data_buffers[BUFSIZE][100]; }; and I want to allocate one of the data_buffers for a process, for that purpose I execute the following function; int allocate_data_buffer(int pid) { int i; for (i = 0; i < BUFSIZE; i++) { if (sdata_ptr->data_buffer_allocation[i] == NULL) { sdata_ptr->data_buffer_allocation[i] = pid; return i; } } return -1; } but the compiler warns me that I'm comparing pointer to a value. When I put a & in front of sdata_ptr it calms down but I'm not sure if it will work. Isn't what I wrote above supposed to be true?

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• C++ compilation error when passing a function into remove_if

  - by garsh0p

  So here's a snippet of my code. void RoutingProtocolImpl::removeAllInfinity() { dv.erase(std::remove_if(dv.begin(), dv.end(), hasInfCost), dv.end()); } bool RoutingProtocolImpl::hasInfCost(RoutingProtocolImpl::dv_entry *entry) { if (entry-link_cost == INFINITY_COST) { free(entry); return true; } else { return false; } } I'm getting the following error when compiling: RoutingProtocolImpl.cc:368: error: argument of type bool (RoutingProtocolImpl::)(RoutingProtocolImpl::dv_entry*)' does not matchbool (RoutingProtocolImpl::*)(RoutingProtocolImpl::dv_entry*)' Sorry, I'm kind of a C++ newb.

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• How does compiler understand the pointer type?

  - by Narek

  How c++ compiler understands the pointer type? As I know pointer has a size equal to WORD of the OS (32 or 64). So does it store dome info in that 32(or 64) bits about type? Just because you can not have a pointer on one type and assign to that pointer another pointer with a different type.

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• The unary increment operator in pointer arithmetic

  - by RhymesWithDuck

  Hello, this is my first post. I have this function for reversing a string in C that I found. void reverse(char* c) { if (*c != 0) { reverse(c + 1); } printf("%c",*c); } It works fine but if I replace: reverse(c + 1); with: reverse(++c); the first character of the original string is truncated. My question is why would are the statements not equivalent in this instance? Thanks

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• A way of allocating multidimensional arrays dynamically

  - by C77431

  salute.. I am learning dynamic allocations for multidimensional arrays in a book and I found some ways for that, And now haven't problem in it. But the author of the book shows us a way, but it doesn't work correctly. It is this: pbeans = new double [3][4]; // Allocate memory for a 3x4 array And this is the error: error C2440: '=' : cannot convert from 'int (*)[4]' to 'int *' how should i define pbeans ( if this type of coding is legal)? and what is the problem exactly? Regards.

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• add space to every word's end in a string in C

  - by hlx98007

  Here I have a string: *line = "123 567 890 "; with 2 spaces at the end. I wish to add those 2 spaces to 3's end and 7's end to make it like this: "123 567 890" I was trying to achieve the following steps: parse the string into words by words list (array of strings). From upstream function I will get values of variables word_count, *line and remain. concatenate them with a space at the end. add space distributively, with left to right priority, so when a fair division cannot be done, the second to last word's end will have (no. of spaces) spaces, the previous ones will get (spaces + 1) spaces. concatenate everything together to make it a new *line. Here is a part of my faulty code: int add_space(char *line, int remain, int word_count) { if (remain == 0.0) return 0; // Don't need to operate. int ret; char arr[word_count][line_width]; memset(arr, 0, word_count * line_width * sizeof(char)); char *blank = calloc(line_width, sizeof(char)); if (blank == NULL) { fprintf(stderr, "calloc for arr error!\n"); return -1; } for (int i = 0; i < word_count; i++) { ret = sscanf(line, "%s", arr[i]); // gdb shows somehow it won't read in. if (ret != 1) { fprintf(stderr, "Error occured!\n"); return -1; } arr[i] = strcat(arr[i], " "); // won't compile. } size_t spaces = remain / (word_count * 1.0); memset(blank, ' ', spaces + 1); for (int i = 0; i < word_count - 1; i++) { arr[0] = strcat(arr[i], blank); // won't compile. } memset(blank, ' ', spaces); arr[word_count-1] = strcat(arr[word_count-1], blank); for (int i = 1; i < word_count; i++) { arr[0] = strcat(arr[0], arr[i]); } free(blank); return 0; } It is not working, could you help me find the parts that do not work and fix them please? Thank you guys.

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• pointer uchar* question in opencv vc++ 2008

  - by kaushalyjain

  img_hsv is a Mat element of an hsv image! when i give cout<<*img_hsv.data+10; it gives true value of the pixel ie 79 as output. but when i assign uchar * a=img_hsv.data+10; it gives me some other value... can you please explain me why is this the case? thanks!

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• Send copy of class to view class so it can render him? ( iPhone )

  - by Johannes Jensen

  I'm making a game for the iPhone, and I have a class called Robot. Then I have a class called View, which renders everything. I want to send a copy of my Robot, which I defined in my ViewController, and I send it to gameView (which is View *gameView), like this: robot = [Robot new]; [gameView setRobot: [robot copy]]; I tried to make a copy but that didn't work, I could also do it with a pointer to Robot (&robot) but sometimes it just crashes ? I tried this in my View.h @interface definition: @property (copy) Robot* robot; but I get the error /RobotsAdventure/Classes/View.h:24: error: setter '-robot' argument type does not match property type :/ Help? I'm pretty new at this, heh.

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• Property Scope (Iphone)

  - by Hank

  Hello All. I am having trouble accessing a declared property and I think I am missing something fundamental about the nature of properties and perhaps view controllers. Here's what I'm doing so far: declaring a property "myPhone" in a root view controller called RootViewController. grabbing a phone number from a modally presented people picker setting "myPhone" to the value from the people picker (from within shouldContinueAfterSelectingPerson of ABPeoplePickerNavigationController) trying to access "myPhone" from another modally presented view controller "myPhone" continues to NSLog to null despite trying every permutation of self.myPhone, super, RootViewController, etc. to try and access the value I set. What am I missing?

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• Loop with pointer arithmetic refuse to stay within boundary in C. Gives me segfault.

  - by Fred

  Hi have made this function which is made to replicate an error that I can't get past. It looks like this: void enumerate(double *c, int size){ while(c < &c[size]){ printf("%lf\n", *c); c++; } } I have added some printf's in there and it gives me: Adressof c: 0x100100080, Adressof c + size: 0x1001000a8 I then also print the address of c for each iteration of the loop, it reaches 0x1001000a8 but continues past this point even though the condition should be false as far as I can tell until I get a segfault. If anyone can spot the problem, please tell me, I have been staring at this for a while now. Thanks.

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• Freeing a character pointer returns error

  - by Kraffs

  I'm trying to free a character pointer after having used it but it returns a strange error. The error says: "_CrtDbgREport: String too long or IO Error" The debugger itself returns no errors while compiling. The code currently looks like this: void RespondToUser(SOCKET client, SOCKET server) { char buffer[80]; char *temp = malloc(_scprintf("HTTP/1.1 200 OK\r\n%s\r\nServer: %s\r\nConnection: close\r\n\r\nHi!", buffer, SERVER_NAME)); sprintf(temp, "HTTP/1.1 200 OK\r\n%s\r\nServer: %s\r\nConnection: close\r\n\r\nHi!", buffer, SERVER_NAME); send(client, temp, strlen(temp), 0); closesocket(client); free(temp); ListenToUsers(server); } The problem only occurs when I try to free the temp pointer from the memory and not otherwise. What might be causing this?

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• C++ multidimensional dynamic array

  - by dmessf

  Let's say I have this to create a multidimensional array dynamically: int* *grid = new int*[gridSizeX]; for (int i=0; i<gridSizeX; i++) { grid[i] = new int[gridSizeY]; } Shouldn't be possible now to access elements like grid[x][y] = 20?

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• Alternates to C++ Reference/Pointer Syntax

  - by Jon Purdy

  What languages other than C and C++ have explicit reference and pointer type qualifiers? People seem to be easily confused by the right-to-left reading order of types, where char*& is "a reference to a pointer to a character", or a "character-pointer reference"; do any languages with explicit references make use of a left-to-right reading order, such as &*char/ref ptr char? I'm working on a little language project, and legibility is one of my key concerns. It seems to me that this is one of those questions to which it's easy for a person but hard for a search engine to provide an answer. Thanks in advance!

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• Pointer to another classs as a property

  - by arjacsoh

  Why I receive an error when I try to create a property to another class through a pointer like that: #ifndef SQUARE_H #define SQUARE_H #include <string> //using namespace std; #include "Player.h" class Square { public: Square(int); void process(); protected: int ID; Player* PlayerOn; <--- }; #endif and the Player class is : #ifndef PLAYER_H #define PLAYER_H #include <string> //using namespace std; #include "Square.h" class Player { public: Player(int,int); // ~Player(void); int playDice(); private: int ID; int money; }; #endif I receive: syntax error missing ; before * (on the declaration of Player* PlayerOn;) and missing type specifier (on the same line...)

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• Event Dispatching, void pointer and alternatives

  - by PeeS

  i have my event dispatching / handling functionality working fine, but there is one issue that i need to resolve. Long story short, here are the details. // The event structure struct tEventMessage { // Type of the event int Type; // (void*) Allows those to be casted into per-Type objects void *pArgument1; void *pArgument2; }; I am sending events from different modules in my engine by using the above structure, which requires a pointer to an argument. All messages are queued, and then dispatched on the next ::Tick(). It works fine, untill i try to send something that doesn't exist in next ::Tick, for example: When a mouse click is being handled, it calculates the click coordinates in world space. This is being sent with a pointer to a vector representing that position, but after my program quits that method, this pointer gets invalid obviously, cause local CVector3 is destructed: CVector2 vScreenSpacePosition = vAt; CVector3 v3DPositionA = CVector3(0,0,0); CVector3 v3DPositionB = CVector3(0,0,0); // Screen space to World space calculation for depth zNear v3DPositionA = CMath::UnProject(vScreenSpacePosition, m_vScreenSize, m_Level.GetCurrentCamera()->getViewMatrix(), m_Level.GetCurrentCamera()->getProjectionMatrix(), -1.0 ); // Screen space to World space calculation for depth zFar v3DPositionB = CMath::UnProject(vScreenSpacePosition, m_vScreenSize, m_Level.GetCurrentCamera()->getViewMatrix(), m_Level.GetCurrentCamera()->getProjectionMatrix(), 1.0); // Send zFar position and ScreenSpace position to the handlers // Obviously both vectors won't be valid after this method quits.. CEventDispatcher::Get()->SendEvent(CIEventHandler::EVENT_SYSTEM_FINGER_DOWN, static_cast<void*>(&v3DPositionB), static_cast<void*>(&vScreenSpacePosition)); What i want to ask is, if there is any chance i could make my tEventMessage more 'template', so i can handle sending objects like in the above situation + use what is already implemented? Can't figure it out at the moment.. What else can be done here to allow me to pass some locally available data ? Please can somebody shed a bit of light on this please?

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• pointer, malloc and char in C

  - by user2534078

  im trying to copy a const char array to some place in the memory and point to it . lets say im defining this var under the main prog : char *p = NULL; and sending it to a function with a string : myFunc(&p, "Hello"); now i want that at the end of this function the pointer will point to the letter H but if i puts() it, it will print Hello . here is what i tried to do : void myFunc(char** ptr , const char strng[] ) { *ptr=(char *) malloc(sizeof(strng)); char * tmp=*ptr; int i=0; while (1) { *ptr[i]=strng[i]; if (strng[i]=='\0') break; i++; } *ptr=tmp; } i know its a rubbish now, but i would like to understand how to do it right, my idea was to allocate the needed memory, copy a char and move forward with the pointer, etc.. also i tried to make the ptr argument byreferenec (like &ptr) but with no success due to a problem with the lvalue and rvalue . the only thing is changeable for me is the function, and i would like not to use strings, but chars as this is and exercise . thanks for any help in advance.

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• Meaning of wild pointer in C ?

  - by sagar

  Hi,can please explain me,the meaning of wild pointer in C, and how it obtain ?

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• Writing a printList method for a Scheme interpreter in C

  - by Rehan Rasool

  I am new to C and working on making an interpreter for Scheme. I am trying to get a suitable printList method to traverse through the structure. The program takes in an input like: (a (b c)) and internally represent it as: [""][ ][ ]--> [""][ ][/] | | ["A"][/][/] [""][ ][ ]--> [""][ ][/] | | ["B"][/][/] ["C"][/][/] Right now, I just want the program to take in the input, make the appropriate cell structure internally and print out the cell structure, thereby getting (a (b c)) at the end. Here is my struct: typedef struct conscell *List; struct conscell { char symbol; struct conscell *first; struct conscell *rest; }; void printList(char token[20]){ List current = S_Expression(token, 0); printf("("); printf("First Value? %c \n", current->first->symbol); printf("Second value? %c \n", current->rest->first->first->symbol); printf("Third value? %c \n", current->rest->first->rest->first->symbol); printf(")"); } In the main method, I get the first token and call: printList(token); I tested the values again for the sublists and I think it is working. However, I will need a method to traverse through the whole structure. Please look at my printList code again. The print calls are what I have to type, to manually get the (a (b c)) list values. So I get this output: First value? a First value? b First value? c It is what I want, but I want a method to do it using a loop, no matter how complex the structure is, also adding brackets where appropriate, so in the end, I should get: (a (b c)) which is the same as the input. Can anyone please help me with this?

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• (gcc) Multi-Dim Array or Double Pointer for Warning-free Compile

  - by paul simmons

  Hi, I have a function, which is called sometimes with regular, sometimes dynamic arrays. If I define the function as function_name(int[10][10] a) and send int** as a parameter, I get a warning. Opposite, if I declare function_name(int** a) and send int[][] as a parameter (after casting) I cannot access to array elements inside function. What is the correctest way?

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• Alternatives to C++ Reference/Pointer Syntax

  - by Jon Purdy

  What languages other than C and C++ have explicit reference and pointer type qualifiers? People seem to be easily confused by the right-to-left reading order of types, where char*& is "a reference to a pointer to a character", or a "character-pointer reference"; do any languages with explicit references make use of a left-to-right reading order, such as &*char/ref ptr char? I'm working on a little language project, and legibility is one of my key concerns. It seems to me that this is one of those questions to which it's easy for a person but hard for a search engine to provide an answer. Thanks in advance!

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• system crash after declaring global object of the class

  - by coming out of void

  hi, i am very new to c++. i am getting system crash (not compilation error) in doing following: i am declaring global pointer of class. BGiftConfigFile *bgiftConfig; class BGiftConfigFile : public EftBarclaysGiftConfig { } in this class i am reading tags from xml file. it is crashing system when this pointer is used to retrieve value. i am doing coding for verifone terminal. int referenceSetting = bgiftConfig->getreferencesetting(); //system error getreferencesetting() is member fuction of class EftBarclaysGiftConfig i am confused about behavior of pointer in this case. i know i am doing something wrong but couldn't rectify it. When i declare one object of class locally it retrieves the value properly. BGiftConfigFile bgiftConfig1; int referenceSetting = bgiftConfig1->getreferencesetting(); //working But if i declare this object global it also crashes the system. i need to fetch values at different location in my code so i forced to use someting global. please suggest me how to rectify this problem.

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