DosReadAsync
Bindings: C, MASM
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
Remarks
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:
o A byte pipe in byte read mode
o A message pipe in message read mode
o 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.
Created using Inf-PHP v.2 (c) 2003 Yuri Prokushev
Created using Inf-HTML v.0.9b (c) 1995 Peter Childs