Line-Oriented Messages, DLL Entry Points, and TLS in Visual Studio .NET

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Line-Oriented Messages, DLL Entry Points, and TLS
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serverSK and clientSK communicate using messages, where each message is composed of a 4-byte length header followed by the message content A common alternative to this approach is to have the messages delimited by end-of-line (or new-line) characters The difficulty with delimited messages is that there is no way to know the message length in advance, and each incoming character must be examined Receiving a single character at a time would be inefficient, however, so incoming characters are stored in a buffer, and the buffer contents might include one or more end-of-line characters and parts of one or more messages Buffer contents and state must be retained between calls to the message receive function In a single-threaded environment, static storage can be used, but multiple threads cannot share the same static storage In more general terms, we have a multithreaded persistent state problem This problem occurs any time a thread-safe function must maintain information from one call to the next The Standard C library strtok function, which scans a string for successive instances of a token, is a common alternative example of this problem
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Solving the Multithreaded Persistent State Problem
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The solution requires a combination of the following components
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A DLL for the message send and receive functions An entry point function in the DLL Thread Local Storage (TLS, 7) The DLL index is created when the process attaches, and it is destroyed when the process detaches The index number is stored in static storage to be accessed by all the threads A structure containing a buffer and its current state A structure is allocated every time a thread attaches, and the address is stored in the TLS entry for that thread A thread's structure is deallocated when the thread detaches
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The TLS, then, plays the role of static storage, and each thread has its own unique copy of the static storage
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Example: A Thread-Safe DLL for Socket Messages
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Program 12-4 is the DLL containing two character string ("CS" in names in this example) or socket streaming functions: SendCSMessage and ReceiveCSMessage, along with a DllMain entry point (see 5) These two functions are similar to and essentially replace ReceiveMessage, listed earlier in this chapter, and the functions used in Program 12-1 and 12-2 The DllMain function is a representative solution of a multithreaded persistent state problem, and it combines TLS and DLLs The resource deallocation in the DLL_THREAD_DETACH case is especially important in a server environment; without it, the server would eventually exhaust resources, typically resulting in either failure or performance degradation or both Note: This example illustrates concepts that are not directly related to sockets, but it is included here, rather than in earlier chapters, because this is a convenient place to illustrate thread-safe DLL techniques in a realistic example The book's Web site contains client and server code, slightly modified from Program 12-1 and 12-2, that uses this DLL
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Program 12-4 SendReceiveSKST: THRead-Safe DLL
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/* /* /* /* /* /* SendReceiveSKSTc -- Multithreaded streaming socket DLL */ Messages are delimited by end-of-line characters ('\0') */ so the message length is not known ahead of time Incoming */ data is buffered and preserved from one function call to */ the next Therefore, use Thread Local Storage (TLS) */ so that each thread has its own private "static storage" */
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#define _NOEXCLUSIONS #include "EvryThngh" #include "ClntSrvrh" /* Defines request and response records */ typedef struct STATIC_BUF_T { /* "static_buf" contains "static_buf_len" bytes of residual data */ /* There may or may not be end-of-string (null) characters */ char static_buf [MAX_RQRS_LEN]; LONG32 static_buf_len; } STATIC_BUF; static DWORD TlsIx = 0; /* TLS index -- EACH PROCESS HAS ITS OWN */ /* A single-threaded library would use the following: static char static_buf [MAX_RQRS_LEN]; static LONG32 static_buf_len; */ /* DLL main function */ BOOL WINAPI DllMain (HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved) { STATIC_BUF * pBuf; switch (fdwReason) { case DLL_PROCESS_ATTACH: TlsIx = TlsAlloc (); /* There is no thread attach for the primary thread, so it is necessary to carry out the thread attach operations as well during process attach */
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case DLL_THREAD_ATTACH: /* Indicate that memory has not been allocated */ TlsSetValue (TlsIx, NULL); return TRUE; /* This value is actually ignored */ case DLL_PROCESS_DETACH: /* Detach the primary thread as well */ pBuf = TlsGetValue (TlsIx); if (pBuf != NULL) { free (pBuf); pBuf = NULL; } return TRUE; case DLL_THREAD_DETACH: pBuf = TlsGetValue (TlsIx); if (pBuf != NULL) { free (pBuf); pBuf = NULL; } return TRUE; } } _declspec (dllexport) BOOL ReceiveCSMessage (REQUEST *pRequest, SOCKET sd) { /* TRUE return indicates an error or disconnect */ BOOL Disconnect = FALSE; LONG32 nRemainRecv = 0, nXfer, k; /* Must be signed integers */ LPSTR pBuffer, message; CHAR TempBuf [MAX_RQRS_LEN + 1]; STATIC_BUF *p; p = (STATIC_BUF *) TlsGetValue (TlsIx); if (p == NULL) { /* First-time initialization */ /* Only threads that need this storage will allocate it */ /* Other thread types can use the TLS for other purposes */ p = malloc (sizeof (STATIC_BUF)); TlsSetValue (TlsIx, p); if (p == NULL) return TRUE; /* Error */ p->static_buf_len = 0; /* Initialize state */ } message = pRequest->Record; /* Read up to the new-line character, leaving residual data in the static buffer */ for (k = 0; k < p->static_buf_len && p->static_buf [k] != '\0'; k++) { message [k] = p->static_buf [k]; } /* k is the number of characters transferred */ if (k < p->static_buf_len) { /* A null was found in static buf */ message [k] = '\0'; p->static_buf_len -= (k + 1); /* Adjust static buffer state */ memcpy (p->static_buf, &(p->static_buf [k + 1]), p->static_buf_len); return FALSE; /* No socket input required */ }
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/* The entire static buffer was transferred No eol found */ nRemainRecv = sizeof (TempBuf) - 1 - p->static_buf_len; pBuffer = message + p->static_buf_len; p->static_buf_len = 0; while (nRemainRecv > 0 && !Disconnect) { nXfer = recv (sd, TempBuf, nRemainRecv, 0); if (nXfer <= 0) { Disconnect = TRUE; continue; } nRemainRecv -= nXfer; /* Transfer to target message up to null, if any */ for (k = 0; k < nXfer && TempBuf [k] != '\0'; k++) { *pBuffer = TempBuf [k]; pBuffer++; } if (k >= nXfer) { /* End of line not found, read more */ nRemainRecv -= nXfer; } else { /* End of line has been found */ *pBuffer = '\0'; nRemainRecv = 0; memcpy (p->static_buf, &TempBuf [k + 1], nXfer - k - 1); p->static_buf_len = nXfer - k - 1; } } return Disconnect; } _declspec (dllexport) BOOL SendCSMessage (RESPONSE *pResponse, SOCKET sd) { /* Send the request to the server on socket sd */ BOOL Disconnect = FALSE; LONG32 nRemainSend, nXfer; LPSTR pBuffer; pBuffer = pResponse->Record; nRemainSend = strlen (pBuffer) + 1; while (nRemainSend > 0 && !Disconnect) { /* Send does not guarantee that the entire message is sent */ nXfer = send (sd, pBuffer, nRemainSend, 0); if (nXfer <= 0) { fprintf (stderr, "\nServer disconnect before complete request sent"); Disconnect = TRUE; } nRemainSend -=nXfer; pBuffer += nXfer; } return Disconnect; }
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