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• casting char[][] to char** causes segfault?

  - by Earlz

  Ok my C is a bit rusty but I figured I'd make my next(small) project in C so I could polish back up on it and less than 20 lines in I already have a seg fault. This is my complete code: #define ROWS 4 #define COLS 4 char main_map[ROWS][COLS+1]={ "a.bb", "a.c.", "adc.", ".dc."}; void print_map(char** map){ int i; for(i=0;i<ROWS;i++){ puts(map[i]); //segfault here } } int main(){ print_map(main_map); //if I comment out this line it will work. puts(main_map[3]); return 0; } I am completely confused as to how this is causing a segfault. What is happening when casting from [][] to **!? That is the only warning I get. rushhour.c:23:3: warning: passing argument 1 of ‘print_map’ from incompatible pointer type rushhour.c:13:7: note: expected ‘char **’ but argument is of type ‘char (*)[5]’ Are [][] and ** really not compatible pointer types? They seem like they are just syntax to me.

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• Accessing any structs members at run-time.

  - by jmgunn

  Is it possible to get access to an individual member of a struct or class without knowing the names of its member variables? I would like to do an "offsetof(struct, tyname)" without having the struct name or member variable name hard coded amoungst other things. thanks.

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• Detect pointer arithmetics because of LARGEADDRESSAWARE

  - by Suma

  I would like to switch my application to LARGEADDRESSAWARE. One of issues to watch for is pointer arithmetic, as pointer difference can no longer be represented as signed 32b. Is there some way how to find automatically all instances of pointer subtraction in a large C++ project? If not, is there some "least effort" manual or semi-automatic method how to achieve this?

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• Determining if Memory Pointer is Valid - C++

  - by Jim Fell

  It has been my observation that if free( ptr ) is called where ptr is not a valid pointer to system-allocated memory, an access violation occurs. Let's say that I call free like this: LPVOID ptr = (LPVOID)0x12345678; free( ptr ); This will most definitely cause an access violation. Is there a way to test that the memory location pointed to by ptr is valid system-allocated memory? It seems to me that the the memory management part of the Windows OS kernel must know what memory has been allocated and what memory remains for allocation. Otherwise, how could it know if enough memory remains to satisfy a given request? (rhetorical) That said, it seems reasonable to conclude that there must be a function (or set of functions) that would allow a user to determine if a pointer is valid system-allocated memory. Perhaps Microsoft has not made these functions public. If Microsoft has not provided such an API, I can only presume that it was for an intentional and specific reason. Would providing such a hook into the system prose a significant threat to system security? Situation Report Although knowing whether a memory pointer is valid could be useful in many scenarios, this is my particular situation: I am writing a driver for a new piece of hardware that is to replace an existing piece of hardware that connects to the PC via USB. My mandate is to write the new driver such that calls to the existing API for the current driver will continue to work in the PC applications in which it is used. Thus the only required changes to existing applications is to load the appropriate driver DLL(s) at startup. The problem here is that the existing driver uses a callback to send received serial messages to the application; a pointer to allocated memory containing the message is passed from the driver to the application via the callback. It is then the responsibility of the application to call another driver API to free the memory by passing back the same pointer from the application to the driver. In this scenario the second API has no way to determine if the application has actually passed back a pointer to valid memory.

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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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• C++ deleting a pointer

  - by eSKay

  On this page, its written that One reason is that the operand of delete need not be an lvalue. Consider: delete p+1; delete f(x); Here, the implementation of delete does not have a pointer to which it can assign zero. Adding a number to a pointer shifts it forward in memory by those many number of sizeof(*p) units. So, what is the difference between delete p and delete p+1, and why would making the pointer 0 only be a problem with delete p+1?

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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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• glibc detected ./.a.out: free(): invalid pointer

  - by ExtremeBlue

  typedef struct _PERSON { size_t age; unsigned char* name; }PERSON; int init(PERSON** person) { (* person) = (PERSON *) malloc(sizeof(struct _PERSON)); (* person)->age = 1; (* person)->name = (unsigned char *) malloc(sizeof(4)); (* person)->name = "NAME"; return 0; } void close(PERSON** person) { (* person)->age = 0; if((* person)->name != NULL) { free((* person)->name); } if((* person) != NULL) { free((* person)); } } int main(int argc, char* argv[]) { PERSON* p; init(&p); printf("%d\t%s\n", (int) p->age, p->name); close(&p); return 0; } 1 NAME *** glibc detected *** ./a.out: free(): invalid pointer: 0x000000000040079c *** ======= Backtrace: ========= /lib/libc.so.6(+0x774b6)[0x7fa9027054b6] /lib/libc.so.6(cfree+0x73)[0x7fa90270bc83] ./a.out(close+0x3d)[0x400651] ./a.out[0x40069f] /lib/libc.so.6(__libc_start_main+0xfe)[0x7fa9026acd8e] ./a.out[0x4004f9] ... 7fa8fc000000-7fa8fc021000 rw-p 00000000 00:00 0 7fa8fc021000-7fa900000000 ---p 00000000 00:00 0 7fa902478000-7fa90248d000 r-xp 00000000 08:12 23068732 /lib/libgcc_s.so.1 7fa90248d000-7fa90268c000 ---p 00015000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268c000-7fa90268d000 r--p 00014000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268d000-7fa90268e000 rw-p 00015000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268e000-7fa902808000 r-xp 00000000 08:12 23068970 /lib/libc-2.12.1.so 7fa902808000-7fa902a07000 ---p 0017a000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a07000-7fa902a0b000 r--p 00179000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a0b000-7fa902a0c000 rw-p 0017d000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a0c000-7fa902a11000 rw-p 00000000 00:00 0 7fa902a11000-7fa902a31000 r-xp 00000000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c25000-7fa902c28000 rw-p 00000000 00:00 0 7fa902c2e000-7fa902c31000 rw-p 00000000 00:00 0 7fa902c31000-7fa902c32000 r--p 00020000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c32000-7fa902c33000 rw-p 00021000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c33000-7fa902c34000 rw-p 00000000 00:00 0 7fff442d5000-7fff442f6000 rw-p 00000000 00:00 0 [stack] 7fff44308000-7fff44309000 r-xp 00000000 00:00 0 [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] Aborted

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• C String input confusion

  - by ahref

  C really isn't my strong point and after reading 3 chapters of a book on the subject and spending ages trying to get stuff working it just doesn't: #include <stdio.h> char *a,*b; int main( ) { char input[10]; fgets(input,sizeof input, stdin); a = input; fgets(input,sizeof input, stdin); b = input; printf("%s : %s",a,b); } I've isolated the problem from my main project. This code is meant to read in two strings and then print them however it seems to be setting a and b to point to input. Sample output from this code when A and B are entered is(don't worry about the \n's i can remove them): A B B : B How do i store the value of input in another variable eg. a or b so that in the above case A B A : B Is output? Thanks

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• NULL pointer comparison fails

  - by Ilya

  Hello, I'm initializing in a class a pointer to be NULL. Afterwards I check if it is NULL in the same class. But it's not always 0x0. Sometimes it's 0x8 or 0xfeffffff or 0x3f800000 or 0x80 or other strange stuff. In most case the pointer is 0x0 but sometimes it gets altered somehow. I'm sure that I'm not changing it anywhere in my code. Is there a way it gets changed by "itself"? Here's my code: MeshObject::MeshObject() { mesh.vertexColors = NULL; } MeshObject::MeshObject(const MeshObject &_copyFromMe) { SimpleLog("vertexColors pointer: %p", _copyFromMe.mesh.vertexColors); if (_copyFromMe.mesh.vertexColors != NULL) { SimpleLog("vertexColors"); this->mesh.vertexColors = new tColor4i[_copyFromMe.mesh.vertexCount]; memcpy(this->mesh.vertexColors, _copyFromMe.mesh.vertexColors, _copyFromMe.mesh.vertexCount * sizeof(tColor4i) ); } } My application crashes, because vertexColors wasn't initialized and is being copied. However it is NULL and shouldn't be copied. Thanks.

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• Decayed multidimensional array return from function

  - by paul simmons

  related to http://stackoverflow.com/questions/2520535/gcc-multi-dim-array-or-double-pointer-for-warning-free-compile , is there a way to return so-called "decayed array pointer" from a function? in summary (suppose 2 dim array) returning int (*a)[5] format rather than int** format? as far as I see, when returned int** pointer is sent to another function waiting (int*)[] parameter, it is not working correctly.

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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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• 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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• Passing an array of an array of char to a function

  - by L.A. Rabida

  In my program, I may need to load a large file, but not always. So I have defined: char** largefilecontents; string fileName="large.txt"; When I need to load the file, the program calles this function: bool isitok=LoadLargeFile(fileName,largefilecontents); And the function is: bool LoadLargeFile(string &filename, char ** &lines) { if (lines) delete [] lines; ifstream largeFile; #ifdef LINUX largeFile.open(filename.c_str()); #endif #ifdef WINDOWS largeFile.open(filename.c_str(),ios::binary); #endif if (!largeFile.is_open()) return false; lines=new char *[10000]; if (!lines) return false; largeFile.clear(); largeFile.seekg(ios::beg); for (int i=0; i>-1; i++) { string line=""; getline(largeFile,line); if (largeFile.tellg()==-1) break; //when end of file is reached, tellg returns -1 lines[i]=new char[line.length()]; lines[i]=const_cast<char*>(line.c_str()); cout << lines[i] << endl; //debug output } return true; } When I view the debug output of this function, "cout << lines[i] << endl;", it is fine. But when I then check this in the main program like this, it is all messed up: for (i=0; i<10000; i++) cout << largefilecontents[i] << endl; So within the function LoadLargeFile(), the results are fine, but without LoadLargeFile(), the results are all messed up. My guess is that the char ** &lines part of the function isn't right, but I do not know what this should be. Could someone help me? Thank you in advance!

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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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• what does deleting a pointer mean

  - by ashish yadav

  Is deleting a pointer same as freeing a pointer(that deallocates the memory)?

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• Pointer to current function

  - by pythonic metaphor

  Is there any way to get a pointer to the current function, maybe through gcc extensions or some other trickery?

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• Could I ever want to access the address zero?

  - by Joel

  The constant 0 is used as the null pointer in C and C++. But as in http://stackoverflow.com/questions/2389251/pointer-to-a-specific-fixed-address there seems to be some possible use of assigning fixed addresses. Is there ever any conceivable need, in any system, for whatever low level task, for accessing the address 0? If there is, how is that solved with 0 being the null pointer and all? If not, what makes it certain that there is not such a need?

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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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• Class lookup structure array in C++

  - by wyatt

  I'm trying to create a structure array which links input strings to classes as follows: struct {string command; CommandPath cPath;} cPathLookup[] = { {"set an alarm", AlarmCommandPath}, {"send an email", EmailCommandPath}, {"", NULL} }; which will be used as follows: CommandPath *cPath = NULL; string input; getline(cin, input); for(int i = 0; cPathLookup[i] != ""; i++) { if(cPathLookup[i].command == input) cPath = new cPathLookup[i].cPath; } Obviously, this code is meaningless, but I think my intention is apparent - depending on input, I'd like cPath to be initialized as either a new AlarmCommandPath or a new EmailCommandPath. I could handle it with a function returning an instance depending on input, but a whole sequence of ifs just seems inelegant. I should also note that, in case it's not apparent and important, that AlarmCommandPath and EmailCommandPath are derived from CommandPath, and CommandPath is an abstract class. Thanks for any help you can offer. EDIT: I just noticed that, in spite of CommandPath being abstract, I have a declaration: CommandPath *cPath = NULL; in working code. Why does that compile?

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• Fortran pointer as an argument to interface procedure

  - by icarusthecow

  Im trying to use interfaces to call different subroutines with different types, however, it doesnt seem to work when i use the pointer attribute. for example, take this sample code MODULE ptr_types TYPE, abstract :: parent INTEGER :: q END TYPE TYPE, extends(parent) :: child INTEGER :: m END TYPE INTERFACE ptr_interface MODULE PROCEDURE do_something END INTERFACE CONTAINS SUBROUTINE do_something(atype) CLASS(parent), POINTER :: atype ! code determines that this allocation is correct from input ALLOCATE(child::atype) WRITE (*,*) atype%q END SUBROUTINE END MODULE PROGRAM testpass USE ptr_types CLASS(child), POINTER :: ctype CALL ptr_interface(ctype) END PROGRAM This gives error Error: There is no specific subroutine for the generic 'ptr_interface' at (1) however if i remove the pointer attribute in the subroutine it compiles fine. Now, normally this wouldnt be a problem, but for my use case i need to be able to treat that argument as a pointer, mainly so i can allocate it if necessary. Any suggestions? Mind you I'm new to fortran so I may have missed something edit: forgot to put the allocation in the parents subroutine, the initial input is unallocated EDIT 2 this is my second attempt, with caller side casting MODULE ptr_types TYPE, abstract :: parent INTEGER :: q END TYPE TYPE, extends(parent) :: child INTEGER :: m END TYPE TYPE, extends(parent) :: second INTEGER :: meow END TYPE CONTAINS SUBROUTINE do_something(this, type_num) CLASS(parent), POINTER :: this INTEGER type_num IF (type_num == 0) THEN ALLOCATE (child::this) ELSE IF (type_num == 1) THEN ALLOCATE (second::this) ENDIF END SUBROUTINE END MODULE PROGRAM testpass USE ptr_types CLASS(child), POINTER :: ctype SELECT TYPE(ctype) CLASS is (parent) CALL do_something(ctype, 0) END SELECT WRITE (*,*) ctype%q END PROGRAM however this still fails. in the select statement it complains that parent must extend child. Im sure this is due to restrictions when dealing with the pointer attribute, for type safety, however, im looking for a way to convert a pointer into its parent type for generic allocation. Rather than have to write separate allocation functions for every type and hope they dont collide in an interface or something. hopefully this example will illustrate a little more clearly what im trying to achieve, if you know a better way let me know

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• C++ dynamic array causes segmentation fault at assigment

  - by opc0de

  I am doing a application witch uses sockets so I am holding in an array the sockets handles.I have the following code: while(0 == 0){ int * tx = (int*)(malloc((nr_con + 2) * sizeof(int))); if (conexiuni != NULL) { syslog(LOG_NOTICE,"Ajung la eliberare %d",nr_con); memcpy(&tx[0],&conexiuni[0],(sizeof(int) * (nr_con))); syslog(LOG_NOTICE,"Ajung la eliberare %d",nr_con); free(conexiuni); } conexiuni = tx; syslog(LOG_NOTICE,"Ajung la mama %d",nr_con); //The line bellow causes a segfault at second connection if ((conexiuni[nr_con] = accept(hsock,(sockaddr*)(&sadr),&addr_size)) != -1) { nr_con++; syslog(LOG_NOTICE,"Primesc de la %s",inet_ntoa(sadr.sin_addr)); syslog(LOG_NOTICE,"kkt %d",conexiuni[nr_con - 1]); int * sz = (int*)malloc(sizeof(int)); *sz = conexiuni[nr_con - 1]; syslog(LOG_NOTICE,"after %d",*sz); pthread_create(&tidi,0,&ConexiuniHandler, sz); } } When I connect the second time when I assign the array the program crashes. What am I doing wrong? I tried the same code on Windows and it works well but on Linux it crashes.

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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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• How to return a copy of the data in C++

  - by Josh Curren

  I am trying to return a new copy of the data in a C++ Template class. The following code is getting this error: invalid conversion from ‘int*’ to ‘int’. If I remove the new T then I am not returning a copy of the data but a pointer to it. template<class T> T OrderedList<T>::get( int k ) { Node<T>* n = list; for( int i = 0; i < k; i++ ) { n=n->get_link(); } return new T( n->get_data() ); // This line is getting the error ********** }

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• Problem with pointer copy in C

  - by Stefano Salati

  I radically re-edited the question to explain better my application, as the xample I made up wasn't correct in many ways as you pointed out: I have one pointer to char and I want to copy it to another pointer and then add a NULL character at the end (in my real application, the first string is a const, so I cannot jsut modify it, that's why I need to copy it). I have this function, "MLSLSerialWriteBurst" which I have to fill with some code adapt to my microcontroller. tMLError MLSLSerialWriteBurst( unsigned char slaveAddr, unsigned char registerAddr, unsigned short length, const unsigned char *data ) { unsigned char *tmp_data; tmp_data = data; *(tmp_data+length) = NULL; // this function takes a tmp_data which is a char* terminated with a NULL character ('\0') if(EEPageWrite2(slaveAddr,registerAddr,tmp_data)==0) return ML_SUCCESS; else return ML_ERROR; } I see there's a problem here: tha fact that I do not initialize tmp_data, but I cannot know it's length.

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