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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