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  • Lots of questions about file I/O (reading/writing message strings)

    - by Nazgulled
    Hi, For this university project I'm doing (for which I've made a couple of posts in the past), which is some sort of social network, it's required the ability for the users to exchange messages. At first, I designed my data structures to hold ALL messages in a linked list, limiting the message size to 256 chars. However, I think my instructors will prefer if I save the messages on disk and read them only when I need them. Of course, they won't say what they prefer, I need to make a choice and justify the best I can why I went that route. One thing to keep in mind is that I only need to save the latest 20 messages from each user, no more. Right now I have an Hash Table that will act as inbox, this will be inside the user profile. This Hash Table will be indexed by name (the user that sent the message). The value for each element will be a data structure holding an array of size_t with 20 elements (20 messages like I said above). The idea is to keep track of the disk file offsets and bytes written. Then, when I need to read a message, I just need to use fseek() and read the necessary bytes. I think this could work nicely... I could use just one single file to hold all messages from all users in the network. I'm saying one single file because a colleague asked an instructor about saving the messages from each user independently which he replied that it might not be the best approach cause the file system has it's limits. That's why I'm thinking of going the single file route. However, this presents a problem... Since I only need to save the latest 20 messages, I need to discard the older ones when I reach this limit. I have no idea how to do this... All I know is about fread() and fwrite() to read/write bytes from/to files. How can I go to a file offset and say "hey, delete the following X bytes"? Even if I could do that, there's another problem... All offsets below that one will be completely different and I would have to process all users mailboxes to fix the problem. Which would be a pain... So, any suggestions to solve my problems? What do you suggest?

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  • Synchronization requirements for FileStream.(Begin/End)(Read/Write)

    - by Doug McClean
    Is the following pattern of multi-threaded calls acceptable to a .Net FileStream? Several threads calling a method like this: ulong offset = whatever; // different for each thread byte[] buffer = new byte[8192]; object state = someState; // unique for each call, hence also for each thread lock(theFile) { theFile.Seek(whatever, SeekOrigin.Begin); IAsyncResult result = theFile.BeginRead(buffer, 0, 8192, AcceptResults, state); } if(result.CompletedSynchronously) { // is it required for us to call AcceptResults ourselves in this case? // or did BeginRead already call it for us, on this thread or another? } Where AcceptResults is: void AcceptResults(IAsyncResult result) { lock(theFile) { int bytesRead = theFile.EndRead(result); // if we guarantee that the offset of the original call was at least 8192 bytes from // the end of the file, and thus all 8192 bytes exist, can the FileStream read still // actually read fewer bytes than that? // either: if(bytesRead != 8192) { Panic("Page read borked"); } // or: // issue a new call to begin read, moving the offsets into the FileStream and // the buffer, and decreasing the requested size of the read to whatever remains of the buffer } } I'm confused because the documentation seems unclear to me. For example, the FileStream class says: Any public static members of this type are thread safe. Any instance members are not guaranteed to be thread safe. But the documentation for BeginRead seems to contemplate having multiple read requests in flight: Multiple simultaneous asynchronous requests render the request completion order uncertain. Are multiple reads permitted to be in flight or not? Writes? Is this the appropriate way to secure the location of the Position of the stream between the call to Seek and the call to BeginRead? Or does that lock need to be held all the way to EndRead, hence only one read or write in flight at a time? I understand that the callback will occur on a different thread, and my handling of state, buffer handle that in a way that would permit multiple in flight reads. Further, does anyone know where in the documentation to find the answers to these questions? Or an article written by someone in the know? I've been searching and can't find anything. Relevant documentation: FileStream class Seek method BeginRead method EndRead IAsyncResult interface

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  • One row is skipped each time the program scans a matrix from file !

    - by ZaZu
    Hello there, I had this code working yesterday, but it seems like I edited it a bit and lost the working version. I cant get this to work anymore. I basically want to scan a matrix from a .txt file. But each time it scans the first row, the second one is skipped, and it reads the third instead :( Here is my code : for(i=0;i<=test->rowmat1;i++){ for(j=0;j<=test->colmat1;j++){ fscanf(fin,"%f\t",&test->mat[i][j]); } fscanf(fin,"%*[^\n]",&test->mat[i][j]); } For example, for a matrix of : 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 If I extract 3 rows and 3 cols, I get : 1.00 2.00 3.00 7.00 8.00 9.00 Then fails, it wants to skip over the second line but there is nothing after 10 11 12 Why did it stop working ? What do I have wrong ? Please help, Thanks in advance.

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  • safely lock a file then move? windows

    - by acidzombie24
    I have a file and need to ensure it exist before inserting a row into the db. After i insert i need to use the PK as part of the filename and move it into another location. How do i check if it exist then lock it so it cant be deleted until i can insert into the db then proceed to move the file without it being deleted upon releasing the lock? also the file may be in use. I am thinking of copying the file into a safe location then moving that file away. In this case i need to copy a file that is being used for reading. How can i do the above steps safely? Using .NET

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  • listFiles() of File not working on symbolic links?

    - by Joset
    I have the following File object pointing to a directory via symbolic link, File directory = new File("/path/symlink/foo/bar"); String[] files = directory.listFiles(); listFiles() returns null, is this because of the symlink? if yes, how will I go about this if I really want to list the files in bar using the path that contains a symlink?

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  • Android: How to copy a SQLite database from one application to another

    - by mahdaeng
    I have a lite version of an application that uses a SQLite database. I want to copy that database over to the full version of the application when the user installs the full version. I have written some code to perform the file copy, but the lite database file always comes up as unreadable. The file is there and I can point to it, but I can't read it to perform the copy. In the Android documentation, we read: You can save files directly on the device's internal storage. By default, files saved to the internal storage are private to your application and other applications cannot access them (nor can the user). Note the words, "by default". Is there a way that I can override that default and make the SQLite file readable by my other application? Thank you.

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  • How can I insert or remove bytes from the middle of a large file in .NET

    - by Eric
    Is it possible to efficiently insert or remove bytes from the middle of a large file, and if so how? Or am I stuck rewriting the entire file after the point where the data was inserted or removed? [A lot of Bytes][Unwanted Bytes][A lot of Bytes] - > [A lot of Bytes][A lot of Bytes] or [A lot of Bytes][A lot of Bytes] - > [A loto f Bytes][New Inserted Bytes][A lot of Bytes]

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  • Android: writing a file to sdcard

    - by Sumit M Asok
    I'm trying to write a file from an Http post reply to a file on the sdcard. Everything works fine until the byte array of data is retrieved. I've tried setting WRITE_EXTERNAL_STORAGE permission in the manifest and tried many different combinations of tutorials I found on the net. All I could find was using the openFileOutput("",MODE_WORLD_READABLE) method, of the activity but how my app writes file is by using a thread. Specifically, a thread is invoked from another thread when a file has to be written, so giving an activity object didn't work even though I tried it. The app has come a long way and I cannot change how the app is currently written. Please, someone help me? CODE: File file = new File(bgdmanip.savLocation); FileOutputStream filecon = null; filecon = new FileOutputStream(file); // bgdmanip.savLocation holds the whole files path byte[] myByte; myByte = Base64Coder.decode(seReply); Log.d("myBytes", String.valueOf(myByte)); bos.write(myByte); filecon.write(myByte); myvals = x * 11024; seReply is a string reply from HttpPost response. the second set of code is looped with reference to x. the file is created but remains 0 bytes

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  • How can I walk through two files simultaneously in Perl?

    - by Alex Reynolds
    I have two text files that contain columnar data of the variety position-value, sorted by position. Here is an example of the first file (file A): 100 1 101 1 102 0 103 2 104 1 ... Here is an example of the second file (B): 20 0 21 0 ... 100 2 101 1 192 3 193 1 ... Instead of reading one of the two files into a hash table, which is prohibitive due to memory constraints, what I would like to do is walk through two files simultaneously, in a stepwise fashion. What this means is that I would like to stream through lines of either A or B and compare position values. If the two positions are equal, then I perform a calculation on the values associated with that position. Otherwise, if the positions are not equal, I move through lines of file A or file B until the positions are equal (when I again perform my calculation) or I reach EOF of both files. Is there a way to do this in Perl?

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  • Java: Send BufferedImage through Socket with a low bitdepth

    - by Martijn Courteaux
    Hi, The title says enough I think. I have a full quality BufferedImage and I want to send it through an OutputStream with a low bitdepth. I don't want an algorithm to change pixel by pixel the quality, so it is still a full-quality. So, the goal is to write the image (with the full resolution) through the OuputStream with a very small size. Thanks, Martijn

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  • Is it possible to stream a file to and from servers without holding the data in ram?

    - by Chris Denman
    Hi everyone! PHP question (new to PHP after 10 years of Perl, arrggghhh!). I have a 100mb file that I want to send to another server. I have managed to read the file in and "post" it without curl (cannot use curl for this app). Everything is working fine on smaller files. However, with the larger files, PHP complains about not being able to allocate memory. Is there a way to open a file, line by line, and send it as a post ALSO line by line? This way nothing is held in ram thus preventing my errors and getting around strict limitations. Chris

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  • .NET: IOException for permissions when creating new file?

    - by Rosarch
    I am trying to create a new file and write XML to it: FileStream output = File.Create(Path.Combine(PATH_TO_DATA_DIR, fileName)); The argument evaluates to: C:\path\to\Data\test.xml The exception is: The process cannot access the file 'C:\path\to\Data\test.xml' because it is being used by another process. What am I doing wrong here? UPDATE: This code throws the same exception: StreamWriter writer = new StreamWriter(Path.Combine(PATH_TO_DATA_DIR, fileName)); UPDATE 2: The file I am trying to create does not exist in the file system. So how can be it in use?

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  • C# File Exception: cannot access the file because it is being used by another process

    - by Lirik
    I'm trying to download a file from the web and save it locally, but I get an exception: C# The process cannot access the file 'blah' because it is being used by another process. This is my code: File.Create("data.csv"); // create the file request = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url)); request.Timeout = 30000; response = (HttpWebResponse)request.GetResponse(); using (Stream file = File.OpenWrite("data.csv"), // <-- Exception here input = response.GetResponseStream()) { // Save the file using Jon Skeet's CopyStream method CopyStream(input, file); } I've seen numerous other questions with the same exception, but none of them seem to apply here. Any help?

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  • lseek/write suddenly returns -1 with errno = 9 (Bad file descriptor)

    - by Ger Teunis
    My application uses lseek to seek the desired position to write data. The file is opened using open() command successfully and my application was able to use lseek and wite lots of times. At a given time, for some users and not easily reproducible the lseek returns -1 with an errno of 9. File is not closed before this and the filehandle (int) isn't reset. After this an other file is created open is okay again and lseek and write works again. To make it even worse, this user tried the complete sequence again and all was well. So my question is, can the OS close the file handle for me for some reason? What could cause this? A file indexer or file scanner of some sort? What is the best way to solve this; is this pseudo code the best solution? (never mind the code layout, will create functions for it) int fd=open(...); if (fd>-1) { long result = lseek(fd,....); if (result == -1 && errno==9) { close(fd..); //make sure we try to close nicely fd=open(...); result = lseek(fd,....); } } Anybody experience with something similar? Summary: file seek and write works okay for a given fd and suddenly gives back errno=9 without a reason.

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  • Possible to open a text file in a MYSQL stored procedure?

    - by futureelite7
    Is it possible to open and read from a text file in a MYSQL stored procedure? I have a text file with a list of about 50k telephone numbers, and want to write a stored procedure that will open the file, read the 50k lines and store it as rows in a table. I cannot load the file directly using LOAD IN FILE as the table has additional columns that I have to set. Thanks!

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  • C# .Net file in use issue

    - by Dan
    I'm having an issue opening files that have recently been closed by the .Net framework. Basically, what happens is the following: -Read in an XML file using DataSet.ReadXml() -Make some changes to the data -Write out the XML file using DataSet.WriteXml() -Copy the XML file to a new location using File.Copy -FTP the file using a custom control This sequence can intermittently fail either after the WriteXML or the File.Copy with a file in use exception. I'm guessing it could be the Windows write cache not flushing right away. Can anyone confirm that this could be causing my issue? Any solutions to suggest? Thanks, Dan

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  • According to MSDN ReadFile() Win32 function may incorrectly report read operation completion. When?

    - by Martin Dobšík
    The MSDN states in its description of ReadFile() function (http://msdn.microsoft.com/en-us/library/aa365467%28VS.85%29.aspx): “If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.” I have some applications that are violating the above recommendation and I would like to know the severity of the problem. I mean the program uses named pipe that has been created with FILE_FLAG_OVERLAPPED, but it reads from it using the following call: ReadFile(handle, &buf, n, &n_read, NULL); That means it passes NULL as the lpOverlapped parameter. That call should not work correctly in some circumstances according to documentation. I have spent a lot of time trying to reproduce the problem, but I was unable to! I always got all data in right place at right time. I was testing only Named Pipes though. Would anybody know when can I expect that ReadFile() will incorrectly return and report successful completion even the data are not yet in the buffer? What would have to happen in order to reproduce the problem? Does it happen with files, pipes, sockets, consoles, or other devices? Do I have to use particular version of OS? Or particular version of reading (like register the handle to I/O completion port)? Or particular synchronization of reading and writing processes/threads? Or when would that fail? It works for me :/ Please help! With regards Martin

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  • C file read leaves garbage characters

    - by KJ
    Hi. I'm trying to read the contents of a file into my program but I keep occasionally getting garbage characters at the end of the buffers. I haven't been using C a lot (rather I've been using C++) but I assume it has something to do with streams. I don't really know what to do though. I'm using MinGW. Here is the code (this gives me garbage at the end of the second read): include include char* filetobuf(char *file) { FILE *fptr; long length; char *buf; fptr = fopen(file, "r"); /* Open file for reading */ if (!fptr) /* Return NULL on failure */ return NULL; fseek(fptr, 0, SEEK_END); /* Seek to the end of the file */ length = ftell(fptr); /* Find out how many bytes into the file we are */ buf = (char*)malloc(length+1); /* Allocate a buffer for the entire length of the file and a null terminator */ fseek(fptr, 0, SEEK_SET); /* Go back to the beginning of the file */ fread(buf, length, 1, fptr); /* Read the contents of the file in to the buffer */ fclose(fptr); /* Close the file */ buf[length] = 0; /* Null terminator */ return buf; /* Return the buffer */ } int main() { char* vs; char* fs; vs = filetobuf("testshader.vs"); fs = filetobuf("testshader.fs"); printf("%s\n\n\n%s", vs, fs); free(vs); free(fs); return 0; } The filetobuf function is from this example http://www.opengl.org/wiki/Tutorial2:_VAOs,_VBOs,_Vertex_and_Fragment_Shaders_%28C_/_SDL%29. It seems right to me though. So anyway, what's up with that?

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  • C# double-quoted path name being escaped when read from file

    - by KrisTrip
    I am trying to read in a text input file that contains a list of filenames (one per line). However, I am running into an issue if the user double-quotes the path (because it has a space in it). For example, a normal input file might have: C:\test\test.tiff C:\test\anothertest.tiff C:\test\lasttest.tiff These get read in fine by my code ("C:\\test\\test.tiff" etc) However, if I have the following input file: "C:\test with spaces\test.tiff" "C:\test with spaces\anothertest.tiff" "C:\test with spaces\lasttest.tiff" These get read in double-quotes and all ("\"C:\\test with spaces\\test.tiff\"" etc). This becomes a problem when I try to open the files (I understandably get invalid character exceptions). My question is, how do I fix this? I want to allow users to input quoted strings and handle them correctly. My first impression was to just write a little method that strips off beginning or ending quotes, but I thought there might be a better way.

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  • Reading Binary file in C

    - by darkie15
    Hi All, I am having following issue with reading binary file in C. I have read the first 8 bytes of a binary file. Now I need to start reading from the 9th byte. Following is the code: fseek(inputFile, 2*sizeof(int), SEEK_SET); However, when I print the contents of the array where I store the retrieved values, it still shows me the first 8 bytes which is not what I need. Can anyone please help me out with this? Regards, darkie

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  • Blackberry read local properties file in project

    - by Dachmt
    Hi, I have a config.properties file at the root of my blackberry project (same place as Blackberry_App_Descriptor.xml file), and I try to access the file to read and write into it. See below my class: public class Configuration { private String file; private String fileName; public Configuration(String pathToFile) { this.fileName = pathToFile; try { // Try to load the file and read it System.out.println("---------- Start to read the file"); file = readFile(fileName); System.out.println("---------- Property file:"); System.out.println(file); } catch (Exception e) { System.out.println("---------- Error reading file"); System.out.println(e.getMessage()); } } /** * Read a file and return it in a String * @param fName * @return */ private String readFile(String fName) { String properties = null; try { System.out.println("---------- Opening the file"); //to actually retrieve the resource prefix the name of the file with a "/" InputStream is = this.getClass().getResourceAsStream(fName); //we now have an input stream. Create a reader and read out //each character in the stream. System.out.println("---------- Input stream"); InputStreamReader isr = new InputStreamReader(is); char c; System.out.println("---------- Append string now"); while ((c = (char)isr.read()) != -1) { properties += c; } } catch (Exception e) { } return properties; } } I call my class constructor like this: Configuration config = new Configuration("/config.properties"); So in my class, "file" should have all the content of the config.properties file, and the fileName should have this value "/config.properties". But the "name" is null because the file cannot be found... I know this is the path of the file which should be different, but I don't know what i can change... The class is in the package com.mycompany.blackberry.utils Thank you!

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  • Parse values from a text file in C

    - by Mohit Deshpande
    Say I have written to a text file in this format: key1/value1 key2/value2 akey/withavalue anotherkey/withanothervalue I have a linked list like: struct Node { char *key; char *value; struct Node *next; }; to hold the values. How would I read key1 and value1? I was thinking of putting line by line in a buffer and using strtok(buffer, '/'). Would that work? What other ways could work, maybe a bit faster or less prone to error? Please include a code sample if you can!

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