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Using Fibers

The CreateFiber function creates a new fiber for a thread. The creating thread must specify the starting address of the code that the new fiber is to execute. Typically, the starting address is the name of a user-supplied function. Multiple fibers can execute the same function.

The following example demonstrates how to create, schedule, and delete fibers. The fibers execute the locally defined functions ReadFiberFunc and WriteFiberFunc. This example implements a fiber-based file copy operation. When running the example, you must specify the source and destination files. Note that there are many other ways to copy file programmatically; this example exists primarily to illustrate the use of the fiber functions.


#include <windows.h>
#include <tchar.h>
#include <stdio.h>

VOID
__stdcall
ReadFiberFunc(LPVOID lpParameter);

VOID
__stdcall
WriteFiberFunc(LPVOID lpParameter);

void DisplayFiberInfo(void);

typedef struct
{
   DWORD dwParameter;          // DWORD parameter to fiber (unused)
   DWORD dwFiberResultCode;    // GetLastError() result code
   HANDLE hFile;               // handle to operate on
   DWORD dwBytesProcessed;     // number of bytes processed
} FIBERDATASTRUCT, *PFIBERDATASTRUCT, *LPFIBERDATASTRUCT;

#define RTN_OK 0
#define RTN_USAGE 1
#define RTN_ERROR 13

#define BUFFER_SIZE 32768   // read/write buffer size
#define FIBER_COUNT 3       // max fibers (including primary)

#define PRIMARY_FIBER 0 // array index to primary fiber
#define READ_FIBER 1    // array index to read fiber
#define WRITE_FIBER 2   // array index to write fiber

LPVOID g_lpFiber[FIBER_COUNT];
LPBYTE g_lpBuffer;
DWORD g_dwBytesRead;

int __cdecl _tmain(int argc, TCHAR *argv[])
{
   LPFIBERDATASTRUCT fs;

   if (argc != 3)
   {
      printf("Usage: %s <SourceFile> <DestinationFile>\n", argv[0]);
      return RTN_USAGE;
   }

   //
   // Allocate storage for our fiber data structures
   //
   fs = (LPFIBERDATASTRUCT) HeapAlloc(
                              GetProcessHeap(), 0,
                              sizeof(FIBERDATASTRUCT) * FIBER_COUNT);

   if (fs == NULL)
   {
      printf("HeapAlloc error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   //
   // Allocate storage for the read/write buffer
   //
   g_lpBuffer = (LPBYTE)HeapAlloc(GetProcessHeap(), 0, BUFFER_SIZE);
   if (g_lpBuffer == NULL)
   {
      printf("HeapAlloc error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   //
   // Open the source file
   //
   fs[READ_FIBER].hFile = CreateFile(
                                    argv[1],
                                    GENERIC_READ,
                                    FILE_SHARE_READ,
                                    NULL,
                                    OPEN_EXISTING,
                                    FILE_FLAG_SEQUENTIAL_SCAN,
                                    NULL
                                    );

   if (fs[READ_FIBER].hFile == INVALID_HANDLE_VALUE)
   {
      printf("CreateFile error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   //
   // Open the destination file
   //
   fs[WRITE_FIBER].hFile = CreateFile(
                                     argv[2],
                                     GENERIC_WRITE,
                                     0,
                                     NULL,
                                     CREATE_NEW,
                                     FILE_FLAG_SEQUENTIAL_SCAN,
                                     NULL
                                     );

   if (fs[WRITE_FIBER].hFile == INVALID_HANDLE_VALUE)
   {
      printf("CreateFile error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   //
   // Convert thread to a fiber, to allow scheduling other fibers
   //
   g_lpFiber[PRIMARY_FIBER]=ConvertThreadToFiber(&fs[PRIMARY_FIBER]);

   if (g_lpFiber[PRIMARY_FIBER] == NULL)
   {
      printf("ConvertThreadToFiber error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   //
   // Initialize the primary fiber data structure.  We don't use
   // the primary fiber data structure for anything in this sample.
   //
   fs[PRIMARY_FIBER].dwParameter = 0;
   fs[PRIMARY_FIBER].dwFiberResultCode = 0;
   fs[PRIMARY_FIBER].hFile = INVALID_HANDLE_VALUE;

   //
   // Create the Read fiber
   //
   g_lpFiber[READ_FIBER]=CreateFiber(0,ReadFiberFunc,&fs[READ_FIBER]);

   if (g_lpFiber[READ_FIBER] == NULL)
   {
      printf("CreateFiber error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   fs[READ_FIBER].dwParameter = 0x12345678;

   //
   // Create the Write fiber
   //
   g_lpFiber[WRITE_FIBER]=CreateFiber(0,WriteFiberFunc,&fs[WRITE_FIBER]);

   if (g_lpFiber[WRITE_FIBER] == NULL)
   {
      printf("CreateFiber error (%d)\n", GetLastError());
      return RTN_ERROR;
   }

   fs[WRITE_FIBER].dwParameter = 0x54545454;

   //
   // Switch to the read fiber
   //
   SwitchToFiber(g_lpFiber[READ_FIBER]);

   //
   // We have been scheduled again. Display results from the 
   // read/write fibers
   //
   printf("ReadFiber: result code is %lu, %lu bytes processed\n",
   fs[READ_FIBER].dwFiberResultCode, fs[READ_FIBER].dwBytesProcessed);

   printf("WriteFiber: result code is %lu, %lu bytes processed\n",
   fs[WRITE_FIBER].dwFiberResultCode, fs[WRITE_FIBER].dwBytesProcessed);

   //
   // Delete the fibers
   //
   DeleteFiber(g_lpFiber[READ_FIBER]);
   DeleteFiber(g_lpFiber[WRITE_FIBER]);

   //
   // Close handles
   //
   CloseHandle(fs[READ_FIBER].hFile);
   CloseHandle(fs[WRITE_FIBER].hFile);

   //
   // Free allocated memory
   //
   HeapFree(GetProcessHeap(), 0, g_lpBuffer);
   HeapFree(GetProcessHeap(), 0, fs);

   return RTN_OK;
}

VOID
__stdcall
ReadFiberFunc(
             LPVOID lpParameter
             )
{
   LPFIBERDATASTRUCT fds = (LPFIBERDATASTRUCT)lpParameter;

   //
   // If this fiber was passed NULL for fiber data, just return,
   // causing the current thread to exit
   //
   if (fds == NULL)
   {
      printf("Passed NULL fiber data; exiting current thread.\n");
      return;
   }

   //
   // Display some information pertaining to the current fiber
   //
   DisplayFiberInfo();

   fds->dwBytesProcessed = 0;

   while (1)
   {
      //
      // Read data from file specified in the READ_FIBER structure
      //
      if (!ReadFile(fds->hFile, g_lpBuffer, BUFFER_SIZE, 
         &g_dwBytesRead, NULL))
      {
         break;
      }

      //
      // if we reached EOF, break
      //
      if (g_dwBytesRead == 0) break;

      //
      // Update number of bytes processed in the fiber data structure
      //
      fds->dwBytesProcessed += g_dwBytesRead;

      //
      // Switch to the write fiber
      //
      SwitchToFiber(g_lpFiber[WRITE_FIBER]);
   } // while

   //
   // Update the fiber result code
   //
   fds->dwFiberResultCode = GetLastError();

   //
   // Switch back to the primary fiber
   //
   SwitchToFiber(g_lpFiber[PRIMARY_FIBER]);
}

VOID
__stdcall
WriteFiberFunc(
              LPVOID lpParameter
              )
{
   LPFIBERDATASTRUCT fds = (LPFIBERDATASTRUCT)lpParameter;
   DWORD dwBytesWritten;

   //
   // If this fiber was passed NULL for fiber data, just return,
   // causing the current thread to exit
   //
   if (fds == NULL)
   {
      printf("Passed NULL fiber data; exiting current thread.\n");
      return;
   }

   //
   // Display some information pertaining to the current fiber
   //
   DisplayFiberInfo();

   //
   // Assume all writes succeeded.  If a write fails, the fiber
   // result code will be updated to reflect the reason for failure
   //
   fds->dwBytesProcessed = 0;
   fds->dwFiberResultCode = ERROR_SUCCESS;

   while (1)
   {
      //
      // Write data to the file specified in the WRITE_FIBER structure
      //
      if (!WriteFile(fds->hFile, g_lpBuffer, g_dwBytesRead, 
         &dwBytesWritten, NULL))
      {
         //
         // If an error occurred writing, break
         //
         break;
      }

      //
      // Update number of bytes processed in the fiber data structure
      //
      fds->dwBytesProcessed += dwBytesWritten;

      //
      // Switch back to the read fiber
      //
      SwitchToFiber(g_lpFiber[READ_FIBER]);
   }  // while

   //
   // If an error occurred, update the fiber result code...
   //
   fds->dwFiberResultCode = GetLastError();

   //
   // ...and switch to the primary fiber
   //
   SwitchToFiber(g_lpFiber[PRIMARY_FIBER]);
}

void
DisplayFiberInfo(
                void
                )
{
   LPFIBERDATASTRUCT fds = (LPFIBERDATASTRUCT)GetFiberData();
   LPVOID lpCurrentFiber = GetCurrentFiber();

   //
   // Determine which fiber is executing, based on the fiber address
   //
   if (lpCurrentFiber == g_lpFiber[READ_FIBER])
      printf("Read fiber entered");
   else
   {
      if (lpCurrentFiber == g_lpFiber[WRITE_FIBER])
         printf("Write fiber entered");
      else
      {
         if (lpCurrentFiber == g_lpFiber[PRIMARY_FIBER])
            printf("Primary fiber entered");
         else
            printf("Unknown fiber entered");
      }
   }

   //
   // Display dwParameter from the current fiber data structure
   //
   printf(" (dwParameter is 0x%lx)\n", fds->dwParameter);
}


This example makes use of a fiber data structure which is used to determine the behavior and state of the fiber. One data structure exists for each fiber; the pointer to the data structure is passed to the fiber at fiber creation time using the parameter of the FiberProc function.

The calling thread calls the ConvertThreadToFiber function, which enables fibers to be scheduled by the caller. This also allows the fiber to be scheduled by another fiber. Next, the thread creates two additional fibers, one that performs read operations against a specified file, and another that performs the write operations against a specified file.

The primary fiber calls the SwitchToFiber function to schedule the read fiber. After a successful read, the read fiber schedules the write fiber. After a successful write in the write fiber, the write fiber schedules the read fiber. When the read/write cycle has completed, the primary fiber is scheduled, which results in the display of the read/write status. If an error occurs during the read or write operations, the primary fiber is scheduled and example displays the status of the operation.

Prior to process termination, the process frees the fibers using the DeleteFiber function, closes the file handles, and frees the allocated memory.

Related topics

Fibers

 

 

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