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• C read part of file into cache

  - by Pete Jodo

  I have to do a program (for Linux) where there's an extremely large index file and I have to search and interpret the data from the file. Now the catch is, I'm only allowed to have x-bytes of the file cached at any time (determined by argument) so I have to remove certain data from the cache if it's not what I'm looking for. If my understanding is correct, fopen (r) doesn't put anything in the cache, only when I call getc or fread(specifying size) does it get cached. So my question is, lets say I use fread and read 100 bytes but after checking it, only 20 of the 100 bytes contains the data I need; how would I remove the useless 80 bytes from cache (or overwrite it) in order to read more from the file.

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• Python to extract data from a file

  - by user297003

  Hi, I am new to python. I am trying to extract the text between that has specific text file: ---- data1 data1 data1 extractme ---- data2 data2 data2 ---- data3 data3 extractme ---- and then dump it to text file so that ---- data1 data1 data1 extractme --- data3 data3 extractme --- Thanks for the help.

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• writing structs and classes to disk

  - by Phenom

  The following function writes a struct to a file. int btwrite(short rrn, BTPAGE *page_ptr) { long addr; addr = (long) rrn * (long) PAGESIZE + HEADERSIZE; lseek(btfd, addr, 0); return (write(btfd, page_ptr, PAGESIZE)); } The following is the struct. typedef struct { short keycount; /* number of keys in page */ int key[MAXKEYS]; /* the actual keys */ int value[MAXKEYS]; /* the actual values */ short child[MAXKEYS+1]; /* ptrs to rrns of descendants */ } BTPAGE; What would happen if I changed the struct to a class, would it still work the same? If I added class functions, would the size it takes up on disk increase?

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• (C++) Loading a file into a vector

  - by Alden

  This is probably a simple question, however I am new to C++ and I cannot figure this out. I am trying to load a binary file and load each byte to a vector. This works fine with a small file, but when I try to read larger than 410 bytes the program crashes and says: This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information. I am using code::blocks on windows. This is the code: #include <iostream> #include <fstream> #include <vector> using namespace std; int main() { std::vector<char> vec; std::ifstream file; file.exceptions( std::ifstream::badbit | std::ifstream::failbit | std::ifstream::eofbit); file.open("file.bin"); file.seekg(0, std::ios::end); std::streampos length(file.tellg()); if (length) { file.seekg(0, std::ios::beg); vec.resize(static_cast<std::size_t>(length)); file.read(&vec.front(), static_cast<std::size_t>(length)); } int firstChar = static_cast<unsigned char>(vec[0]); cout << firstChar <<endl; return 0; } Thank you for your help!

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• which one consume less resources? opening text file or make an sql query,both a thousand times ?

  - by imin

  hi I've a php website which displays recipes www.trymasak.my, to be exact. The recipes being displayed at the index page is updated about once a day. To get the latest recipes, I just use a mysql query which is something like "select recipe_name, page_views, image from table order by last_updated". So if I got 10000 visitors a day, obviously the query would be made 10000 times a day. A friend told me a better way (in terms of reducing server load) is when I update the recipes, I just put in the latest recipe details (names,images etc) into a text file, and make my page instead of querying a same query for 10,000 times, just get the data from the text file. Is his suggestion really better? If yes, which is the best php command should I use to open, read and close the text file? thanks

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• How to replace same text in a text file

  - by user1688220

  i created a c# windows login form and i am saving username or password to a text file but every time i use same username or password that i have saved before it takes new place in that text file. But What i want is to replace the same username or password that is already saved in that text file. this is my code: private void button1_Click(object sender, EventArgs e) { try { FileStream fs = new FileStream("data.txt", FileMode.Append, FileAccess.Write); StreamWriter sw = new StreamWriter(fs); sw.Write("Email ID: "); sw.WriteLine(textBox1.Text); sw.Write("Password: "); sw.Write(textBox2.Text); sw.WriteLine(); sw.WriteLine(); sw.Flush(); sw.Close(); fs.Close(); } catch (Exception) { MessageBox.Show("Error", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); this.Close(); } MessageBox.Show("DONE", "Done", MessageBoxButtons.OK, MessageBoxIcon.Information); textBox1.Clear(); textBox2.Clear(); }

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• Detecting metadata-only read requests in windows filesystem

  - by HyLian

  Hello, I'm developing a kind of filesystem driver. All of read requests that windows makes to my filesystem goes by the driver implementation. I would like to distinguish between "normal" read requests and those who want to get only the metadata from the file. ( Windows reads first 4K of the file and then stop reading ). Does Windows mark this metadata reads in some way? It would be very useful in order to treat that two kind of operations in a different way. In a typical CreateFile call, we have AccessMode, ShareMode, CreationDisposition and FlagsAndAttributes parameters ( being DWORD ), i'm not sure if it's possible to extract some clue of the operation requested. Thanks for reading :)

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• Why are some programs writing on stderr instead of stdout their output?

  - by Zagorax

  I've recently added to my .bashrc file an ssh-add command. I found that ssh-add $HOME/.ssh/id_rsa_github > /dev/null results on a message "identity added and something else" every time I open a shell. While ssh-add $HOME/.ssh/id_rsa_github > /dev/null 2>&1 did the trick and my shell is now 'clean'. Reading on internet, I found that other command do it, (for example time). Could you please explain why it's done?

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• Which is faster: glob() or opendir()

  - by silent

  Which is faster between glob() and opendir(), for reading around 1-2K file(s)?

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• explain this macro

  - by deostroll

  #define __T(x) L ## x Found in code from one of the MFC source header file. It is mostly used for converting strings to ........ (I don't know what). If I am correct it converts strings to LPCTSTR...don't know what that type is either... I can't seem to convert char* into LPCTSTR. While MFC file handling, the following code will always return error while trying to open the file... char* filepath = "C:\\Program Files\\Microsoft Office\\Office12\\BITMAPS\\STYLES\\GLOBE.WMF"; if( !file.Open((LPCTSTR)filepath , CFile::modeRead, &fexp) ) { fexp.ReportError(); return 1; } But instead if I wrote it this way, it doesn't give error: if( !file.Open( _T("C:\\Program Files\\Microsoft Office\\Office12\\BITMAPS\\STYLES\\GLOBE.WMF") , CFile::modeRead, &fexp) ) { fexp.ReportError(); return 1; } I am looking at passing a variable as the first argument to the CFile::Open() method.

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• How to write or append to text file using sql?

  - by aron

  Hello, in short i need to convert following C# code into SQL stored procedure foreach (var e in Entity) { if (File.Exists(e.Filename)) File.AppendAllText(e.Filename,e.Content); else File.WriteAllText(e.Filename, e.Content); }

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• Naming a file downloaded from url in iPhone

  - by hgpc

  I would like to save a file downloaded from the internet in iPhone. Can I use the url as the file name? If not, what transformation should I apply to the url to obtain a valid file name? I need to find the local copy of the file later using its url.

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• Random Loss of precision in Python ReadLine()

  - by jackyouldon

  Hi all, We have a process which takes a very large csv (1.6GB) and breaks it down into pieces (in this case 3). This runs nightly and normally doesn't give us any problems. When it ran last night, however, the first of the output files had lost precision on the numeric fields in the data. The active ingredient in the script are the lines: while lineCounter <= chunk: oOutFile.write(oInFile.readline()) lineCounter = lineCounter + 1 and the normal output might be something like StringField1; StringField2; StringField3; StringField4; 1000000; StringField5; 0.000054454 etc. On this one occasion and in this one output file the numeric fields were all output with 6 zeros at the end i.e. StringField1; StringField2; StringField3; StringField4; 1000000.000000; StringField5; 0.000000 We are using Python v2.6 (and don't want to upgrade unless we really have to) but we can't afford to lose this data. Does anyone have any idea why this might have happened? If the readline is doing some kind of implicit conversion is there a way to do a binary read, because we really just want this data to pass through untouched? It is very wierd to us that this only affected one of the output files generated by the same script, and when it was rerun the output was as expected. thanks Jack

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• Can you call Directory.GetFiles() with multiple filters?

  - by Jason Z

  I am trying to use the Directory.GetFiles() method to retrieve a list of files of multiple types, such as mp3's and jpg's. I have tried both of the following with no luck: Directory.GetFiles("C:\\path", "*.mp3|*.jpg", SearchOption.AllDirectories); Directory.GetFiles("C:\\path", "*.mp3;*.jpg", SearchOption.AllDirectories); Is there a way to do this in one call?

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• How can I modify the application file of an application that is currently running (on Linux)?

  - by Hach-Que

  I have an application running called AppFS. This application has an ext2 filesystem just attached to the end of the file (it's positioned so that the application binary exists in a 1MB spacing area, followed by the ext2 data). Now I've got FUSE embedded in the program and I've managed to extract the filesystem out of the application data into a temporary file so that FUSE can mount / use it. The problem I have now is writing the temporary file back into the application file. I get "Text file busy" presumably because the application has locked itself and won't let writes occur. Is there a way I can force the file to become unlocked so I can write data to it? (It's important to note that I'm not changing the application binary area - just rewriting the ext2 component.) It needs to be unlocked without requiring root permissions (unlocked by the same user who started the application).

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• How do I read and write to a file using threads in java?

  - by WarmWaffles

  I'm writing an application where I need to read blocks in from a single file, each block is roughly 512 bytes. I am also needing to write blocks simultaneously. One of the ideas I had was BlockReader implements Runnable and BlockWriter implements Runnable and BlockManager manages both the reader and writer. The problem that I am seeing with most examples that I have found was locking problems and potential deadlock situations. Any ideas how to implement this?

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• Read a file to multiple array byte[]

  - by hankol

  I have an encryption algorithm (AES) that accepts file converted to array byte and encrypt it. Since I am going to process a very big size files, the JVM may go out of memory. I am planing to read the files in multiple array byte. each containing some part of the file. Then I teratively feed the algorithm. Finally merge them to produce encrypted file. So my question is: there any way to read a file part by part to multiple array byte? I thought I can use the following to read the file to array byte: IOUtils.toByteArray(InputStream input). And then split the array into multiple bytes using: Arrays.copyOfRange(). But I am afraid that the first code that reads file to byte will make the JVM to go out of memory. any suggestion please ? thanks

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• Enter custom file name to be read ?

  - by ZaZu

  Hello, I want to allow users to type the name of any .txt file to be read/written. This is my code : printf("Enter .txt file name\n"); scanf("%s",&fname); FILE *inputf; inputf=fopen(&fname,"w"); Problem is this method does not work (having &fname) as a parameter. I can imagine its because C needs "filename.txt" for it work ... even if I enter for example : "custom.txt", the program returns an error of "Storage block not big enough for this operation" What is the correct method to accomplish this ? Im using C and im pretty much using basic commands .. (not too advanced) Thanks alot !!!

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• USB device Set Attribute in C#

  - by p19lord

  I have this bit of code: DriveInfo[] myDrives = DriveInfo.GetDrives(); foreach (DriveInfo myDrive in myDrives) { if (myDrive.DriveType == DriveType.Removable) { string path = Convert.ToString(myDrive.RootDirectory); DirectoryInfo mydir = new DirectoryInfo(path); String[] dirs = new string[] {Convert.ToString(mydir.GetDirectories())}; String[] files = new string[] {Convert.ToString(mydir.GetFiles())}; foreach (var file in files) { File.SetAttributes(file, ~FileAttributes.Hidden); File.SetAttributes(file, ~FileAttributes.ReadOnly); } foreach (var dir in dirs) { File.SetAttributes(dir, ~FileAttributes.Hidden); File.SetAttributes(dir, ~FileAttributes.ReadOnly); } } } I have a problem. It is trying the code for Floppy Disk drive first which and because no Floppy disk in it, it threw the error The device is not ready. How can I prevent that?

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• Perl, redirect stdout to file

  - by Mike

  I'm looking for an example of redirecting stdout to a file using Perl. I'm doing a fairly straightforward fork/exec tool, and I want to redirect the child's output to a file instead of the parents stdout. Is there an equivilant of dup2() I should use? I can't seem to find it

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• Scan file contents into an array of a structure.

  - by ZaZu

  Hello, I have a structure in my program that contains a particular array. I want to scan a random file with numbers and put the contents into that array. This is my code : ( NOTE : This is a sample from a bigger program, so I need the structure and arrays as declared ) The contents of the file are basically : 5 4 3 2 5 3 4 2 #include<stdio.h> #define first 500 #define sec 500 struct trial{ int f; int r; float what[first][sec]; }; int trialtest(trial *test); main(){ trial test; trialtest(&test); } int trialtest(trial *test){ int z,x,i; FILE *fin; fin=fopen("randomfile.txt","r"); for(i=0;i<5;i++){ fscanf(fin,"%5.2f\t",(*test).what[z][x]); } fclose(fin); return 0; } But the problem is, whenever this I run this code, I get this error : (25) : warning 508 - Data of type 'double' supplied where a pointer is required I tried adding do{ for(i=0;i<5;i++){ q=fscanf(fin,"%5.2f\t",(*test).what[z][x]); } }while(q!=EOF); But that didnt work either, it gives the same error. Does anyone have a solution to this problem ?

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• C++ how to store integer into a binary file??

  - by blaxc

  i gt a struct with 2 integer, i want to store them in a binary file and read it again... here is my code... struct pw { int a; int b; }; void main(){ pw* p = new pw(); pw* q = new pw(); std::ofstream fout(ADMIN_FILE, ios_base::out | ios_base::binary | ios_base::trunc); std::ifstream fin(ADMIN_FILE, ios_base::in | ios_base::binary); p->a=123; p->b=321; fout.write((const char*)p, sizeof(pw)); fin.write((char*)q, sizeof(pw)); fin.close(); cout<< q->a << endl;} my output is 0. anyone can tell me what is the problem?

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• Generic file container for quick read of data

  - by DreamCodeR

  Since there are some major privacy issues with alot of social networking sites I am trying to think about alternatives. One is to let the user keep all the information stored in some kind of file container. Now, I haven't found a single type of container that can hold "generic" information. Only for audio/video. What I want is a container that can be read by PHP with some kind of index file that lists up the users pictures in a image/ directory in the container, FOAF files (or some alternative XML-file describing users information and friends, etc.). My thoughts was to let the user keep all their information and data stored in a container that can be imported/exported and deleted from my server (the prototype social networking site I am trying to create), and then uploaded to another site that might use the same format (not that I think that will ever happen, but the user still keeps all their pictures, data, comments, messages, etc). The only thing I have come up with yet is to create a tar archive with the Archive_tar library which extracts and creates Tar-archives with an index-file describing which files are holding the messages (there might be several so each file won't be so large), what pictures are in the image/ folder and what are their names and what comments they have gotten etc. Maybe also the permissions for viewing each type of content. Does there exist any generic file format of a container that I can use to keep all this information in one file with a tree-like index file? Or must i try and create something like this myself?

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• Writing at the end of file

  - by user342534

  Hi, I'm working on a system that requires high file I/O performance (with C#). Basically, I'm filling up large files (~100MB) from the start of the file until the end of the file. Every ~5 seconds I'm adding ~5MB to the file (sequentially from the start of the file), on every bulk I'm flushing the stream. Every few minutes I need to update a structure which I write at the end of the file (some kind of metadata). When flushing each one of the bulks I have no performance issue. However, when updating the metadata at the end of the file I get really low performance. My guess is that when creating the file (which also should be done extra fast), the file doesn't really allocates the entire 100MB on the disk and when I flush the metadata it must allocates all space until the end of file. Guys/Girls, any Idea how I can overcome this problem? Thanks a lot!

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• Split a binary file into chunks c++

  - by L4nce0

  I've been bashing my head against trying to first divide up a file into chunks, for the purpose of sending over sockets. I can read / write a file easily without splitting it into chunks. The code below runs, works, kinda. It will write a textfile and has a garbage character. Which if this was just for txt, no problem. Jpegs aren't working with said garbage. Been at it for a few days, so I've done my research, and it's time to get some help. I do want to stick strictly to binary readers, as this need to handle any file. I've seen a lot of slick examples out there. (none of them worked for me with jpgs) Mostly something along the lines of while(file)... I subscribe to the, if you know the size, use a for-loop, not a while-loop camp. Thank you for the help!! vector<char*> readFile(const char* fn){ vector<char*> v; ifstream::pos_type size; char * memblock; ifstream file; file.open(fn,ios::in|ios::binary|ios::ate); if (file.is_open()) { size = fileS(fn); file.seekg (0, ios::beg); int bs = size/3; // arbitrary. Actual program will use the socket send size int ws = 0; int i = 0; for(i = 0; i < size; i+=bs){ if(i+bs > size) ws = size%bs; else ws = bs; memblock = new char [ws]; file.read (memblock, ws); v.push_back(memblock); } } else{ exit(-4); } return v; } int main(int argc, char **argv) { vector<char*> v = readFile("foo.txt"); ofstream myFile ("bar.txt", ios::out | ios::binary); for(vector<char*>::iterator it = v.begin(); it!=v.end(); ++it ){ myFile.write(*it,strlen(*it)); } }

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