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• Basic Recursion, Check Balanced Parenthesis

  - by pws5068

  Greetings all, I've written software in the past that uses a stack to check for balanced equations, but now I'm asked to write a similar algorithm recursively to check for properly nested brackets and parenthesis. Good examples: () [] () ([]()[]) Bad examples: ( (] ([)] Suppose my function is called: isBalanced. Should each pass evaluate a smaller substring (until reaching a base case of 2 left)? Or, should I always evaluate the full string and move indices inward?

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• Do you always use the same technologies/frameworks?

  - by James.Elsey

  When given a new task/challenge/application to build, do you always use the same framework, for example spring / struts? Or do you try something new that you haven't used before, such as GWT? What makes you return to the same technology stack? Is it good to be advanced at particular technologies, or to have a broad understanding?

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• play framework NoClassDefFoundError

  - by lhk

  I've downloaded the typesafe stack for windows and created a new project. When I fire up sbt and try to run the new unmodified application there's this error: [error] java.lang.NoClassDefFoundError: org/jboss/netty/channel/ChannelFactory just out of curiosity I also tried to compile the project. The error is different: [error] IO error while decoding .....welcome.template.scala with UTF-8 [error] Please try specifying another one using the -encoding option What can I do to fix this ?

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• Getting the start address of the current process's heap?

  - by beta

  Hey, I am exploring the lower level workings of the system, and was wondering how malloc determines the start address of the heap. Is the heap a constant offset or is there a call of some sort to get the start address? Does the stack effect the start address of the heap? Thanks, Braden McDorman

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• Creating a FIFO queue in C

  - by Tyler

  Is it possible to create a FIFO 'stack' in C without using 2 stacks? Thanks! (Sorry to those who answered the previous one. I was thinking LIFO and meaning FIFO.)

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• For what applications is Forth best suited?

  - by namin

  I am intrigued by stack-based languages like Forth. Are there situations where Forth is the best tool for the job or is it just an intellectual and historical curiosity? What about derivative languages like Factor or Joy? Which of these languages would you recommend learning? And for what purpose (apart from mind expansion)?

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• Neccessity of push and pop operands on CPUs

  - by Hawken

  Why do we have commands like push and pop? From what I understand pop and push are basically the same as doing a (mov then add) and (sub then mov) on esp respectively. For example wouldn't: pushl %eax be equivalent to: subl $4, %esp movl %eax, (%esp-4) please correct me if stack access is not (%esp-4), I'm still learning assembly The only true benefit I can see is if doing both operation simultaneously offers some advantage; however I don't see how it could.

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• Implications of trying to double free memory space in C

  - by SidNoob

  Here' my piece of code: #include <stdio.h> #include<stdlib.h> struct student{ char *name; }; int main() { struct student s; s.name = malloc(sizeof(char *)); // I hope this is the right way... printf("Name: "); scanf("%[^\n]", s.name); printf("You Entered: \n\n"); printf("%s\n", s.name); free(s.name); // This will cause my code to break } All I know is that dynamic allocation on the 'heap' needs to be freed. My question is, when I run the program, sometimes the code runs successfully. i.e. ./struct Name: Thisis Myname You Entered: Thisis Myname I tried reading this I've concluded that I'm trying to double-free a piece of memory i.e. I'm trying to free a piece of memory that is already free? (hope I'm correct here. If Yes, what could be the Security Implications of a double-free?) While it fails sometimes as its supposed to: ./struct Name: CrazyFishMotorhead Rider You Entered: CrazyFishMotorhead Rider *** glibc detected *** ./struct: free(): invalid next size (fast): 0x08adb008 *** ======= Backtrace: ========= /lib/tls/i686/cmov/libc.so.6(+0x6b161)[0xb7612161] /lib/tls/i686/cmov/libc.so.6(+0x6c9b8)[0xb76139b8] /lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0xb7616a9d] ./struct[0x8048533] /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0xb75bdbd6] ./struct[0x8048441] ======= Memory map: ======== 08048000-08049000 r-xp 00000000 08:01 288098 /root/struct 08049000-0804a000 r--p 00000000 08:01 288098 /root/struct 0804a000-0804b000 rw-p 00001000 08:01 288098 /root/struct 08adb000-08afc000 rw-p 00000000 00:00 0 [heap] b7400000-b7421000 rw-p 00000000 00:00 0 b7421000-b7500000 ---p 00000000 00:00 0 b7575000-b7592000 r-xp 00000000 08:01 788956 /lib/libgcc_s.so.1 b7592000-b7593000 r--p 0001c000 08:01 788956 /lib/libgcc_s.so.1 b7593000-b7594000 rw-p 0001d000 08:01 788956 /lib/libgcc_s.so.1 b75a6000-b75a7000 rw-p 00000000 00:00 0 b75a7000-b76fa000 r-xp 00000000 08:01 920678 /lib/tls/i686/cmov/libc-2.11.1.so b76fa000-b76fc000 r--p 00153000 08:01 920678 /lib/tls/i686/cmov/libc-2.11.1.so b76fc000-b76fd000 rw-p 00155000 08:01 920678 /lib/tls/i686/cmov/libc-2.11.1.so b76fd000-b7700000 rw-p 00000000 00:00 0 b7710000-b7714000 rw-p 00000000 00:00 0 b7714000-b7715000 r-xp 00000000 00:00 0 [vdso] b7715000-b7730000 r-xp 00000000 08:01 788898 /lib/ld-2.11.1.so b7730000-b7731000 r--p 0001a000 08:01 788898 /lib/ld-2.11.1.so b7731000-b7732000 rw-p 0001b000 08:01 788898 /lib/ld-2.11.1.so bffd5000-bfff6000 rw-p 00000000 00:00 0 [stack] Aborted So why is it that my code does work sometimes? i.e. the compiler is not able to detect at times that I'm trying to free an already freed memory. Has it got to do something with my stack/heap size?

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• How to change the default behavior of overrifing when GCC -fstack-protector-all is set?

  - by Vicente Botet Escriba

  I would like to use the GCC -fstack-protector-all option on release mode. The main issue is that when there is an override a code is built. I would like to plug something that will just dump the stack and let the program continue. Is there a way to do that?

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• Core Data - Limiting number of saved objects

  - by m_arc

  Hi! Is there way to limit the number of saved objects in coredata like stack, where the last objects are automatically deleted?

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• How to change the default behavior of overwriting when GCC -fstack-protector-all is set?

  - by Vicente Botet Escriba

  I would like to use the GCC -fstack-protector-all option on release mode. The main issue is that when there is an override a code is built. I would like to plug something that will just dump the stack and let the program continue. Is there a way to do that?

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• How is destroying local variables when a block is exited normally called in C++?

  - by sharptooth

  C++ automagically calls destructors of all local variables in the block in reverse order regardless of whether the block is exited normally (control falls through) or an exception is thrown. Looks like the term stack unwinding only applies to the latter. How is the former process (the normal exit of the block) called in terms of destroying local variables?

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• Determining the word width in C

  - by das_weezul

  Hi! I'm learning C right now and so I'm fiddling about with pointers. Is there a way to determine the word width of the CPU in C because I'm writing a small program which prints it's own stack (Because I'm curious how it is structured), so that information would come in handy. Right now I'm using an int pointer, as an integer is 4 Bytes wide and I'm using a 32-bit Intel Atom CPU. Thanks in advance, C gurus ;o)

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• Thread safety with heap-allocated memory

  - by incrediman

  I was reading this: http://en.wikipedia.org/wiki/Thread_safety Is the following function thread-safe? void foo(int y){ int * x = new int[50]; /*...do some stuff with the allocated memory...*/ delete x; } In the article it says that to be thread-safe you can only use variables from the stack. Really? Why? Wouldn't subsequent calls of the above function allocate memory elsewhere?

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• Why do VMs need to be "stack machines" or "register machines" etc.?

  - by Prog

  (This is an extremely newbie-ish question). I've been studying a little about Virtual Machines. Turns out a lot of them are designed very similarly to physical or theoretical computers. I read that the JVM for example, is a 'stack machine'. What that means (and correct me if I'm wrong) is that it stores all of it's 'temporary memory' on a stack, and makes operations on this stack for all of it's opcodes. For example, the source code 2 + 3 will be translated to bytecode similar to: push 2 push 3 add My question is this: JVMs are probably written using C/C++ and such. If so, why doesn't the JVM execute the following C code: 2 + 3..? I mean, why does it need a stack, or in other VMs 'registers' - like in a physical computer? The underlying physical CPU takes care of all of this. Why don't VM writers simply execute the interpreted bytecode with 'usual' instructions in the language the VM is programmed with? Why do VMs need to emulate hardware, when the actual hardware already does this for us? Again, very newbie-ish questions. Thanks for your help

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• by reference in C++

  - by lego69

  I have this snippet of the code Stack& Stack:: operator=(const Stack& stack){ if(this == &stack){ return *this } } here I define operator = but I can't understand, if I receive by reference stack why it should be & in this == &stack and not this == stack and why we return * in return *this and not this thanks in advance for any help

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• Performing a Depth First Search iteratively using async/parallel processing?

  - by Prabhu

  Here is a method that does a DFS search and returns a list of all items given a top level item id. How could I modify this to take advantage of parallel processing? Currently, the call to get the sub items is made one by one for each item in the stack. It would be nice if I could get the sub items for multiple items in the stack at the same time, and populate my return list faster. How could I do this (either using async/await or TPL, or anything else) in a thread safe manner? private async Task<IList<Item>> GetItemsAsync(string topItemId) { var items = new List<Item>(); var topItem = await GetItemAsync(topItemId); Stack<Item> stack = new Stack<Item>(); stack.Push(topItem); while (stack.Count > 0) { var item = stack.Pop(); items.Add(item); var subItems = await GetSubItemsAsync(item.SubId); foreach (var subItem in subItems) { stack.Push(subItem); } } return items; } I was thinking of something along these lines, but it's not coming together: var tasks = stack.Select(async item => { items.Add(item); var subItems = await GetSubItemsAsync(item.SubId); foreach (var subItem in subItems) { stack.Push(subItem); } }).ToList(); if (tasks.Any()) await Task.WhenAll(tasks); The language I'm using is C#.

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• i386 assembly question: why do I need to meddle with the stack pointer?

  - by zneak

  Hello everyone, I decided it would be fun to learn x86 assembly during the summer break. So I started with a very simple hello world program, borrowing on free examples gcc -S could give me. I ended up with this: HELLO: .ascii "Hello, world!\12\0" .text .globl _main _main: pushl %ebp # 1. puts the base stack address on the stack movl %esp, %ebp # 2. puts the base stack address in the stack address register subl $20, %esp # 3. ??? pushl $HELLO # 4. push HELLO's address on the stack call _puts # 5. call puts xorl %eax, %eax # 6. zero %eax, probably not necessary since we didn't do anything with it leave # 7. clean up ret # 8. return # PROFIT! It compiles and even works! And I think I understand most of it. Though, magic happens at step 3. Would I remove this line, my program would die between the call to puts and the xor from a misaligned stack error. And would I change $20 to another value, it'd crash too. So I came to the conclusion that this value is very important. Problem is, I don't know what it does and why it's needed. Can anyone explain me? (I'm on Mac OS, would it ever matter.)

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• How to devise instruction set of a stack based machine?

  - by Anindya Chatterjee

  Stack based virtual machines like CLR and JVM has different set of instructions. Is there any theory behind devising the instruction set while creating a virtual machine? e.g. there are JVM instruction sets to load constants from 0-5 onto the stack iconst_0 iconst_1 iconst_2 iconst_3 iconst_4 iconst_5 whereas in CLR there are instruction set to load number from 0 to 8 onto the stack as follows ldc.i4.0 ldc.i4.1 ldc.i4.2 ldc.i4.3 ldc.i4.4 ldc.i4.5 ldc.i4.6 ldc.i4.7 ldc.i4.8 why there is no ldc.i4.9 and if ldc.i4 is there why we need the above opcodes? And there are others like these. I am eager to know what is the reason behind this difference between opcodes of different VMs? Is there any specific theory to devise these opcodes or it is totally driven by the characteristics of the VM itself or depends on the high-level language constructs?

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• How can I install the Play! framework using typesafe-stack? [migrated]

  - by lhk

  I'd like to create a new project with the Play! framework. My system is Mint 12 64bit. Since the newest version of Play! is already bundled with the typesafe-stack, I thought installation would be easy. I added the typesafe repo, then I apt-get updated and apt-get installed typesafe-stack with the command g8 typesafehub/play-scala. I successfully created a new project in my home folder. Now the problems begin: I don't know how to access Play! with this installation. After creating the project, I tried to convert it into an Eclipse project, it but there's no play command available in the terminal. How can I get a "standard" Play! installation on Linux? What happens to the tools bundled in the typesafe stack - Where do they go?

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• Beginner learning assembly preserving esp after function calls

  - by Daniel

  I'm a beginner learning some assembly, when preserving the ESP register before a function call does it matter if you do it by adding or subtracting? hard to explain, consider the following mov esi, esp sub esp, 12 // on 32bit OS this would mean that there are 3 arguments to the function // push, function call etc cmp esi, esp // should be the same or mov esi, esp // push, function call etc add esp, 12 cmp esi, esp // should be the same Also if for some reason the cmp fails, is it safe to do mov esp, esi to re-align the stack? Thanks EDIT: Also how come i need to do this for a call like sprintf, but MessageBox seems to fix ESP for me? How am i to know what function needs this and what doesn't?

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• can anyone explain this code to me???

  - by Abed

  //shellcode.c char shellcode[] = "\x31\xc0\x31\xdb\xb0\x17\xcd\x80" "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh"; int main() { int *ret; //ret pointer for manipulating saved return. ret = (int *)&ret + 2; //setret to point to the saved return //value on the stack. (*ret) = (int)shellcode; //change the saved return value to the //address of the shellcode, so it executes. } can anyone give me a better explanation

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• Unable to locate the Bug

  - by tzenes

  I was recently on The Daily WTF when I came across this old post. In it the author mentions that one of the programmers changed this code: int main (int argc, char **argv) { int x; char data_string[15]; ... x = 2; strcpy(data_string,"data data data"); ... } To this code: int main (int argc, char **argv) { int x = 2; char data_string[15] = "data data data"; ... } The author goes on to mention: [the coder] changed every single variable to be initiated on the stack For the life of me I cannot see how this change could be harmful, and I am worried that it is a lapse in my C knowledge. What is the WTF?

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• Primary reasons why programming language runtimes use stacks?

  - by manuel aldana

  Many programming language runtime environments use stacks as their primary storage structure (e.g. see JVM bytecode to runtime example). Quickly recalling I see following advantages: Simple structure (pop/push), trivial to implement Most processors are anyway optimized for stack operations, so it is very fast Less problems with memory fragmentation, it is always about moving memory-pointer up and down for allocation and freeing complete blocks of memory by resetting the pointer to the last entry offset. Is the list complete or did I miss something? Are there programming language runtime environments which are not using stacks for storage at all?

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• C++ memory management of reference types

  - by Russel

  Hello, I'm still a fairly novice programmer and I have a question about c++ memory management with refence types. First of all, my understanding of reference types: A pointer is put on the stack and the actual data that the pointer points to is created and placed on the heap. Standard arrays and user defined classes are refence types. Is this correct? Second, my main question is do c and c++'s memory management mechanisms (malloc, free and new, delete) always handle this properly and free the memory that a class or array is pointing to? Does everything still work if those pointers get reassigned somehow to other objects of the same size/type on the heap? What if a class has a pointer member that points to another object? I am assuming that delete/freeing the class object doesn't free what it's member pointer points to, is that correct? Thanks all! -R

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