I/O States
I/O States |
|
Most Recursion Software I/O objects maintain state information that can be used to determine if the stream or device is ready for a read or write. All I/O objects respond to the following messages.
eof()
-Returns true if the device is at end-of-input. Most sequential I/O classes
have end-of-input. End-of-input occurs when the end of an input data set is
reached or when the write end of a pipe or socket is closed.good()
-Returns true if the device is ready for I/O; that is, the device is not in
a failed state and is not at end-of-input.clear()
-Clears the device from a failed state and returns the state of the device
to good() . Clearing the device may or may not
permit a successful read or write, depending on the reason the device
failed.One commonly used I/O object state is end-of-input. This state can be used even in no-error situations. The following example illustrates a simple use of the end-of-input state.
#include <iostream>
#include <ospace/file.h>
void
write()
{
os_file file
(
"state1.txt",
os_open_control::create_always,
os_io_control::write_only_access
);
file.write( "123", 3 );
}
void
read()
{
os_file file( "state1.txt" ); // Open existing file.
while ( file.good() )
{
char result = '.';
file.read( &result, 1 );
cout << "eof() = " << file.eof();
cout << " good() = " << file.good();
cout << " result = " << result << endl;
}
}
void
main()
{
os_file_toolkit initialize;
write();
read();
}
eof() = 0 good() = 1 result = 1
eof() = 0 good() = 1 result = 2
eof() = 0 good() = 1 result = 3
eof() = 1 good() = 0 result = .
You can program most I/O objects to generate an exception on end-of-input. Refer to the "Exception Handling" section of Introduction to Communications for more information on errors.
To enable exception generation
when end-of-input occurs, use the event_on_eof()
message. The following example illustrates error generation when os_file
encounters end-of-input.
#include <iostream>
#include <ospace/file.h>
void
main()
{
os_file_toolkit initialize;
os_file file( "state2.cpp" ); // Open this file for read-only.
file.event_on_eof ( true ); // Generate event on end-of-file.
int bytes = 0;
char line[ 256 ]; // Line buffer.
try
{
while ( file.good() )
bytes += file.read( line, sizeof( line ) );
}
catch ( os_file_toolkit_error& error )
{
cout << "Caught os_file_toolkit_error: " << endl;
cout << 't' << error << endl;
cout << "Read " << bytes << " bytes" << endl;
}
}
Caught os_file_toolkit_error:
eof_encountered: [file.cpp:204]
Read 890 bytes
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