This call asynchronously transfers the specified number of bytes from a file, pipe, or device to a buffer.
DosReadAsync (FileHandle, RamSemaphore, ReturnCode, BufferArea, BufferLength, BytesRead)
- FileHandle (HFILE) - input
- File handle obtained from DosOpen.
- RamSemaphore (PULONG) - input
- Address used by the system to signal the caller that the read operation is complete.
- ReturnCode (PUSHORT) - output
- Address of the I/O error return code.
- BufferArea (PVOID) - output
- Address of the input buffer.
- BufferLength (USHORT) - input
- Length, in bytes, to be read.
- BytesRead (PUSHORT) - output
- Address of the number of bytes read.
- rc (USHORT) - return
- Return code descriptions are:
- 0 NO_ERROR
- 5 ERROR_ACCESS_DENIED
- 6 ERROR_INVALID_HANDLE
- 26 ERROR_NOT_DOS_DISK
- 33 ERROR_LOCK_VIOLATION
- 89 ERROR_NO_PROC_SLOTS
- 109 ERROR_BROKEN_PIPE
The requested number of bytes may not be read. If the value in BytesRead is zero, the process tried to read from the end of the file.
A BufferLength value of zero is not considered an error. In the case where BufferLength is zero, the system treats the request as a null operation.
The file pointer is moved to the desired position by reading, writing, or issuing DosChgFilePtr.
A call to DosReadAsync may return before the read is complete. To wait for an asynchronous read to complete, RamSemaphore must be set by the application before the DosReadAsync call is made. The application issues DosSemSet to set the semaphore, calls DosReadAsync, and then issues DosSemWait, to wait to be signaled by the system that the asynchronous read is complete. When RamSemaphore is cleared and the read operation is complete, ReturnCode can be checked.
Note: If it is necessary to make a DosReadAsync request from within a segment that has I/O privilege, DosCallback may be used to invoke a privilege level 3 segment that actually issues the DosReadAsync request.
Named Pipe Considerations
A named pipe is read as one of the following:
- A byte pipe in byte read mode
- A message pipe in message read mode
- A message pipe in byte read mode.
A byte pipe must be in byte read mode to be read; an error is returned if it is in message read mode. All currently available data, up to the size requested, is returned.
When a message pipe is read in message read mode, a read that is larger than the next available message returns only that message and BytesRead is set to indicate the size of the message returned.
A read that is smaller than the next available message returns with the number of bytes requested and an ERROR_MORE_DATA return code. When resuming the reading of a message after ERROR_MORE_DATA is returned, a read always blocks until the next piece (or rest) of the message can be transferred. DosPeekNmPipe may be used to determine how many bytes are left in the message.
A message pipe in byte read mode is read as if it were a byte stream, skipping over message headers. This is like reading a byte pipe in byte read mode.
When blocking mode is set for a named pipe, a read blocks until data is available. In this case, the read never returns with BytesRead = 0 except at EOF. In message read mode, messages are always read in their entirety, except in the case where the message is bigger than the size of the read.
Non-blocking mode allows a return with BytesRead = 0 only when trying to read at the start of a message and no message is available.
#define INCL_DOSFILEMGR USHORT rc = DosReadAsync(FileHandle, RamSemaphore, ReturnCode, BufferArea, BufferLength, BytesRead); HFILE FileHandle; /* File handle */ PULONG RamSemaphore; /* Ram semaphore */ PUSHORT ReturnCode; /* I/O operation return code (returned) */ PVOID BufferArea; /* User buffer (returned) */ USHORT BufferLength; /* Buffer length */ PUSHORT BytesRead; /* Bytes read (returned) */ USHORT rc; /* return code */
EXTRN DosReadAsync:FAR INCL_DOSFILEMGR EQU 1 PUSH WORD FileHandle ;File handle PUSH@ DWORD RamSemaphore ;Ram semaphore PUSH@ WORD ReturnCode ;I/O operation return code (returned) PUSH@ OTHER BufferArea ;User buffer (returned) PUSH WORD BufferLength ;Buffer length PUSH@ WORD BytesRead ;Bytes read (returned) CALL DosReadAsync Returns WORD