Unsuccessful read() after successful select() and FD_ISSET() - c++

Question

In a Linux environment, I have an application that gets a file descriptor from an API call to a driver. The following function is what I use to read the data read in by a card on my system. About 1 out of 10 reads fail. I am puzzled as to why, after a successful select, and checking to see if the read_fd is set, no data is returned.
int MyClass::Read(int file_descriptor)
{
unsigned short read_buffer[READ_BUFFER_SIZE];
fd_set read_set;
time_val timeout;
int return_value = 0;
int count = 0;
int status = -1;
// Initialize read file descriptor
FD_ZERO(&read_set)
// Add driver file descriptor
FD_SET(file_descriptor, &read_set)
// Set timeout
timeout.tv_sec = 0;
timeout.tv_usec = 10000;
while(!count)
{
// Wait for data to be available to read
return_value = select(file_descriptor + 1, &read_set, NULL, NULL, &timeout);
// Make sure an error or a timeout didn't occur
if (-1 == return_value)
{
cout << "an error occurred" << endl;
}
else if (0 == return_value)
{
cout << "a timeout occurred" << endl;
}
else
{
// If the read file descriptor is set, read in the data
if (FD_ISSET(file_descriptor, &read_set))
{
count = read(file_descriptor, read_buffer, sizeof(read_buffer));
// Double check that data was read in
if (!count)
{
cout << "read failed" << endl;
}
else
{
// Set status to success
status = 0;
}
}
}
}
return status;
}

Answer 1

A return value of 0 from read (your if (!count) check) does not mean that the read failed -- it means that the read succeeded and got an EOF.
In any case, select returning with the file descriptor set does not mean that a read of that fd will not fail -- it means that a read of that fd will not block, and will return something immediately, either failure or success.

Answer 2

You are not using select() correctly. It modifies the fd_set, and possibly the time_val, so you have to reset them on each loop iteration.
Also, you are not handling errors correctly. read() returns -1 on error, 0 on disconnect, and > 0 on bytes read. You are not handling error and disconnect conditions correctly.
Try something more like this instead:
int MyClass::Read(int file_descriptor)
{
unsigned short read_buffer[READ_BUFFER_SIZE];
fd_set read_set;
time_val timeout;
int return_value, count;
do
{
// Initialize read file descriptor
FD_ZERO(&read_set);
// Add driver file descriptor
FD_SET(file_descriptor, &read_set);
// Set timeout
timeout.tv_sec = 0;
timeout.tv_usec = 10000;
// Wait for data to be available to read
return_value = select(file_descriptor + 1, &read_set, NULL, NULL, &timeout);
// Make sure an error or a timeout didn't occur
if (-1 == return_value)
{
cout << "select failed" << endl;
return -1;
}
if (0 == return_value)
{
cout << "select timed out" << endl;
continue; // or return, your choice...
}
// select() returned > 0, so the fd_set MUST be set,
// so no need to check it with FD_ISSET()...
// read in the data
count = read(file_descriptor, read_buffer, sizeof(read_buffer));
// Double check that data was actually read in
if (-1 == count)
{
cout << "read failed" << endl;
return -1;
}
if (0 == count)
{
cout << "peer disconnected" << endl;
return 0; // or -1, or whatever you want...
}
// success
break;
}
while (true);
return 0;
}