brain teaser: file sharing by an application and its child(s)

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  12 years ago

  by russbude

  Russell Brown

  Cornwall, United Kingdom

  Joined 13 years ago

  Hi guys,

  I have got something to test your understanding of  multitasking processes.  Have a fun!!

  An application opens a file and starts a child process passing the file handle to the child.  The application(parent)  and its child share the file.  When these two processes write to or read the file, a big problem can occur if they do not snychronise their wirting and reading of the file!!  Why?

  Try the following example programs (testp.cpp & testc.cpp).  Run testp.exe and observe the problem!!

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  // testp.cpp : the parent application
  //
  // When running, this program prints the following to the screen:
  //  End of the child
  //  child*
  //  parent
  //  Parent: exiting
  //
  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <process.h>
  #include <windows.h>  // for Sleep( )
  #include <winbase.h>  // for Sleep( )
  //#include <winnt.h>  // For SECURITY_ATTRIBUTES

  #define len_of_record  6  // for length of a record in the file
  #define filename  "test.txt"  // our database file

  int main( )
  {

   char buf[100]; unsigned long nbytes;

   HANDLE filehandle;
   char filehandle_str[20];
   int err;

   SECURITY_ATTRIBUTES sa;  // For CreateProcess( ) & CreateFile( )

   // For the file handle to be inherited by a child
   sa.nLength = sizeof(SECURITY_ATTRIBUTES);
   sa.lpSecurityDescriptor = NULL;
   sa.bInheritHandle = TRUE; // We want the file to be inherited by a child

   // *** Open our database file ***
   // Note the file attributes flag FILE_FLAG_WRITE_THROUGH|FILE_FLAG_RANDOM_ACCESS
   // to avoid lazy flush when writing to the disk.
   if ((filehandle=CreateFile(filename,GENERIC_READ|GENERIC_WRITE,FILE_SHARE_READ|
     FILE_SHARE_WRITE,&sa,OPEN_ALWAYS,FILE_FLAG_WRITE_THROUGH|
     FILE_FLAG_RANDOM_ACCESS,NULL)) ==
     INVALID_HANDLE_VALUE ) {
    // an error to open the file. we have to terminate this session
    fprintf(stderr,"Fatal(parent):failed to open our database file %s",filename);
    Sleep(5000);
    exit(3);
   } // if

   // We could use either CreateProcess( ) or spwanlp( ) to 
   // start/create a child.                                

   itoa((int)filehandle, filehandle_str, 10);

   // Create the child using spawnlp( )
   if (spawnlp(P_NOWAIT,"testc.exe","testc.exe",filehandle_str,NULL)==-1){
    // Error: failed to create a child process
    fprintf(stderr,"Fatal: failed with CreateProcess( )\n");
    fprintf(stderr,"Parent exits\n");
    Sleep(4000); // To keep the output window open for 4 secs
    exit(1);
   } // if

   // Set the file pointer to the beginning of the file
   if (SetFilePointer(filehandle,(LONG)0,(PLONG)NULL,FILE_BEGIN)
    == 0xFFFFFFFF && GetLastError() != NO_ERROR) {
    CloseHandle(filehandle);
    printf("Fatal(parent): SetFilePointer() failed\n");
    Sleep(10000);
    exit(2);
   } // if

   Sleep(2000); // Give the child enough time to exit!

   // Write "parent" into the beginning of the file!
   strcpy(buf,"parent");  //note strlen(buf) = len_of_record!
   if (WriteFile(filehandle,buf,len_of_record,&nbytes,NULL) == 0 ||
    nbytes != len_of_record) {
    printf("Parent: error for WriteFile()=%d\n",err=GetLastError());
    CloseHandle(filehandle);
    Sleep(5000);
    exit(4);
   } // if 

  
   // *** We now read the records from the file ***

   // Read the records from the file & print them to the screen
   // Note that the position # of the 1st byte in a file is 0(zero), not 1.

   if (SetFilePointer(filehandle,(LONG)0,(PLONG)NULL,FILE_BEGIN)
    == 0xFFFFFFFF && GetLastError() != NO_ERROR) {
    CloseHandle(filehandle);
    printf("Fatal(parent): SetFilePointer() failed\n");
    Sleep(10000);
    exit(2);
   } // if

   while (1) {
    if (ReadFile(filehandle,buf,len_of_record,&nbytes,NULL) == 0) {
     printf("Parent: error for ReadFile()=%d\n",err=GetLastError());
     CloseHandle(filehandle);
     Sleep(5000);
     exit(4);
    } else if (nbytes == 0) {
     // end of file reached
     break;
    }// if & else if
    buf[len_of_record] = '\0';
    printf("%s\n",buf);
   } // while

   _flushall();

   CloseHandle(filehandle); // This does not affect the file open for the child
   printf("Parent: exiting\n");
   Sleep(20000); // Sleep for 20 seconds
  }  // main

  
  // testc.cpp :the child application

  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <fcntl.h>
  #include <io.h>
  #include <windows.h>  // For Sleep( )
  #include <winbase.h>  // For Sleep( )

  #define  len_of_record 6  // a record is 6 chars long
  #define  filename        "test.txt" // our database file

  
  int main(int argc, char **argv)
  {

   char buf[len_of_record+1]; // buf to write to the file
   unsigned long nbytes;    // # of bytes written to or read from the file
   HANDLE filehandle;      // for our database file (opened for the parent too)
    
   filehandle = (HANDLE) atoi(argv[1]);  // file handle inherited from the parent

  
   // write "child*" to the beginning of the file
   strcpy(buf,"child*"); // note strlen(buf) = 6!
   if (SetFilePointer(filehandle,0,(PLONG)0,FILE_BEGIN)==0xFFFFFFFF ){
    CloseHandle(filehandle);
    printf("child: SetFilePointer() failed\n");
    Sleep(5000);
    exit(2);
   } // if

   if (WriteFile(filehandle,buf,len_of_record,&nbytes,NULL)==0){
    printf("child: WriteFile() failed\n");
    printf("            error = %d\n",GetLastError()); //c.f. ERROR_LOCK_VIOLATION=33
    CloseHandle(filehandle);
    Sleep(20000);
    exit(4);
   } // if
   

   printf("End of the child\n");
   CloseHandle(filehandle);
  } // main
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