syncro multiprocess with FileLock() read/write on same file c++ win32 - c++

Question

i'm traing to do 2 process that do this:
Process A (the client) :
- It opens a temporary file for writing (use GetTempFileName to
obtain a unique file name).
- It starts Process B passing as argument the name of the file.
- It reads from standard input strings of characters, and it writes
those strings on the intermediate file.
Use string of constant length (e.g., #define MAX_LEN 100).
- It stops reading strings from standard input when it receives an
end-of-file (ctrl-Z).
- Before terminating, it writes the string
".end"
as last line of the file.
Process B (the server) works as follows:
- It opens the intermediate file for reading.
- It reads lines as soon as they are available, and it prints them
out on standard output.
Process A keeps the file open, and it always locks the next part of
the file before unlocking the previous part.
Process B runs in parallel, and it reads a shared file, waiting to
have access to single lines to read them.
but seams that the ReadFile on process B do not wait until the fileLock is removed so it terminate befor the first line is inserted
process A:
#include <windows.h>
#include <process.h>
#include <tchar.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>
#include <io.h>
#define MAX_LEN 100
int _tmain(int argc, LPTSTR argv[]){
TCHAR tmpFileName[MAX_PATH];
TCHAR tmppath[MAX_PATH];
TCHAR moment[MAX_PATH];
TCHAR command[MAX_PATH + 256];
UINT uRetVal = 0;
HANDLE bufferW,next;
DWORD n;
PROCESS_INFORMATION piS;
STARTUPINFO siS;
TCHAR buffer[MAX_LEN];
int i=0;
OVERLAPPED ov = { 0, 0, 0, 0, NULL };
LARGE_INTEGER FilePos;
ZeroMemory(&siS, sizeof(siS));
siS.cb = sizeof(siS);
ZeroMemory(&piS, sizeof(piS));
uRetVal = GetTempFileName(_T("."), // directory for tmp files
_T(""), // temp file name prefix
0, // create unique name
tmpFileName); // buffer for name
if (uRetVal == 0)
{
_tprintf("\n errore apertura temp file");
return (3);
}
GetCurrentDirectory(MAX_PATH, moment);
//_stprintf(tmppath, "%s\\ %s",moment,tmpFileName);
_tprintf("\n CLIENT PATH %s \n",moment);
//_stprintf(command,"%s %s","C:\\Users\\zio gianni\\Documents\\Visual Studio 2013\\Projects\\lab11server\\Debug\\lab11server.exe",tmpFileName);
_stprintf(command, "%s %s", "server.exe", tmpFileName);
bufferW = CreateFile(tmpFileName, GENERIC_WRITE, FILE_SHARE_WRITE | FILE_SHARE_READ, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
next = CreateFile(tmpFileName, GENERIC_WRITE, FILE_SHARE_WRITE | FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (next == INVALID_HANDLE_VALUE) {
_tprintf(_T("Cannot open client file. Error: %x\n"),
GetLastError());
_tprintf(" \n Nome File: %s\n", "./prova.bin");
return 2;
}
if (bufferW == INVALID_HANDLE_VALUE) {
_tprintf(_T("Cannot open client file. Error: %x\n"),
GetLastError());
_tprintf(" \n Nome File: %s\n", "./prova.bin");
return 2;
}
FilePos.QuadPart = i*MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
LockFileEx(bufferW, LOCKFILE_EXCLUSIVE_LOCK, 0, FilePos.LowPart, FilePos.HighPart, &ov);
printf("file bloccato avvio server con comand: %s\n", command);
if (!CreateProcess(NULL, // No module name (use command line)
command, // Command line
NULL, // Process handle not inheritable
NULL, // Thread handle not inheritable
TRUE, // Set handle inheritance to FALSE
0, // No creation flags
NULL, // Use parent's environment block
NULL, // Use parent's starting directory
&siS, // Pointer to STARTUPINFO structure
&piS) // Pointer to PROCESS_INFORMATION structure
)
{
printf("CreateProcess failed (%d).\n", GetLastError());
return -5;
}
while (TRUE){
scanf("%s",buffer);
if (strcmp(buffer, "EOF")==0){
break;
}
WriteFile(bufferW, buffer, MAX_LEN*sizeof(TCHAR), &n, NULL);
FilePos.QuadPart =(i+1)* MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
if (i % 2 == 0){
LockFile(next, LOCKFILE_EXCLUSIVE_LOCK, 0, FilePos.LowPart, FilePos.HighPart, &ov);
FilePos.QuadPart = (i )* MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
UnlockFile(bufferW, 0, 0, 0, &ov);
}
else{
LockFile(bufferW, LOCKFILE_EXCLUSIVE_LOCK, 0, FilePos.LowPart, FilePos.HighPart, &ov);
FilePos.QuadPart = (i )* MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
UnlockFile(next, 0, 0, 0, &ov);
}
i++;
}
sprintf(buffer, ".end");
WriteFile(bufferW, buffer, MAX_LEN*sizeof(TCHAR), &n ,NULL);
if (i % 2 == 0){
FilePos.QuadPart = (i)* MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
UnlockFile(bufferW, 0, FilePos.LowPart, FilePos.HighPart, &ov);
}
else{
FilePos.QuadPart = (i)* MAX_LEN * sizeof (TCHAR);
ov.Offset = FilePos.LowPart;
ov.OffsetHigh = FilePos.HighPart;
ov.hEvent = 0;
UnlockFile(next, 0, FilePos.LowPart, FilePos.HighPart, &ov);
}
// Wait until child process exits.
WaitForSingleObject(piS.hProcess, INFINITE);
// Close process and thread handles.
CloseHandle(piS.hProcess);
CloseHandle(piS.hThread);
return 0;
}
process B:
#include <windows.h>
#include <process.h>
#include <tchar.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>
#include <io.h>
#define MAX_LEN 100
int _tmain(int argc, LPTSTR argv[]){
HANDLE read;
DWORD n;
int i = 0;
TCHAR buffer[MAX_LEN], path[MAX_LEN];
OVERLAPPED ov = { 0, 0, 0, 0, NULL };
LARGE_INTEGER FilePos;
read = CreateFile(argv[1], GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (read == INVALID_HANDLE_VALUE) {
_tprintf(_T("Cannot open input file. Error: %x\n"),
GetLastError());
_tprintf(" \n Nome File: %s\n", argv[1]);
return 2;
}
_tprintf(" \n Nome File APERTO: %s\n", argv[1]);
GetCurrentDirectory(MAX_LEN,path);
_tprintf(" \n path server: %s\n%s\n", path,argv[1]);
while (ReadFile(read, buffer, MAX_LEN*sizeof(TCHAR), &n, &ov) && n>0 ){
if (n == 0){
fprintf(stderr,"error readfle\n");
return -5;
}
if (strncmp(buffer, ".end", 4) == 0){
_tprintf(" \n STAMPA SERVER: esco\n");
break;
}
_tprintf(" \n STAMPA SERVER: %s\n", buffer);
}
if (n == 0){
fprintf(stderr, "error readfle\n");
return -5;
}
_tprintf("\n END SERVER");
return 0;
}

Answer 1

Do not use actual files for inter process communication. Use named pipes!
Search for CreateNamedPipe, there are plenty of examples.