asynchronous io is working but never ends in linux - c++

Question

In a previous question, I asked how to implement asynchronous I/O. This code now works, except that at the end it never stops. It seems that aio_read reads starting at offset, for length, and if it is past the end of the file, the operation succeeds? This code builds and runs on Ubuntu 20.04LTS and successfully reads blocks 1-5, each 512 bytes, then when it runs out of file it keeps oscillating between block 4 and 5. It never terminates.
Here is the code:
#include <aio.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>
#include <condition_variable>
#include <cstring>
#include <iostream>
#include <thread>
using namespace std;
using namespace std::chrono_literals;
constexpr uint32_t blockSize = 512;
mutex readMutex;
bool readReady = false;
condition_variable cv;
bool operation_completed = false;
int fh;
int bytesRead;
void process(char* buf, uint32_t bytesRead) {
cout << "processing..." << endl;
usleep(100000);
}
void aio_completion_handler(sigval_t sigval) {
struct aiocb* req = (struct aiocb*)sigval.sival_ptr;
// check whether asynch operation is complete
int status;
if ((status = aio_error(req)) != 0) {
cout << "Error: " << status << '\n';
return;
}
int ret = aio_return(req);
bytesRead = req->aio_nbytes;
cout << "ret == " << ret << endl;
cout << (char*)req->aio_buf << endl;
unique_lock<mutex> readLock(readMutex);
operation_completed = true;
cv.notify_one();
}
void thready() {
char* buf1 = new char[blockSize];
char* buf2 = new char[blockSize];
aiocb cb;
char* processbuf = buf1;
char* readbuf = buf2;
fh = open("smallfile.dat", O_RDONLY);
if (fh < 0) {
throw std::runtime_error("cannot open file!");
}
memset(&cb, 0, sizeof(aiocb));
cb.aio_fildes = fh;
cb.aio_nbytes = blockSize;
cb.aio_offset = 0;
// Fill in callback information
/*
Using SIGEV_THREAD to request a thread callback function as a notification
method
*/
cb.aio_sigevent.sigev_notify_attributes = nullptr;
cb.aio_sigevent.sigev_notify = SIGEV_THREAD;
cb.aio_sigevent.sigev_notify_function = aio_completion_handler;
/*
The context to be transmitted is loaded into the handler (in this case, a
reference to the aiocb request itself). In this handler, we simply refer to
the arrived sigval pointer and use the AIO function to verify that the request
has been completed.
*/
cb.aio_sigevent.sigev_value.sival_ptr = &cb;
int cursor = 0;
int currentBytesRead = read(fh, buf1, blockSize); // read the 1st block
while (true) {
cb.aio_buf = readbuf;
operation_completed = false; // set predicate to true and wait until asynch changes it
cb.aio_offset = cursor;
aio_read(&cb); // each next block is read asynchronously
process(processbuf, currentBytesRead); // process while waiting
{
unique_lock<mutex> readLock(readMutex);
cv.wait( readLock, []{ return operation_completed; } );
}
if (!operation_completed)
break;
currentBytesRead = bytesRead; // make local copy of global modified by the asynch code
cursor += bytesRead;
if (currentBytesRead < blockSize) {
break; // last time, get out
}
cout << "back from wait" << endl;
swap(processbuf, readbuf); // switch to other buffer for next time
currentBytesRead = bytesRead; // create local copy
}
delete[] buf1;
delete[] buf2;
}
int main() {
try {
thready();
} catch (std::exception& e) {
cerr << e.what() << '\n';
}
return 0;
}
First, is the above code an appropriate way to do this to get the length of the file and figure out exactly how many reads to do?
Second, if this is so, fine, but how can aio_read just return success if I try to read past the end of file? Error status is always zero. I am confused about what it is supposed to do.
with 512 bytes of each of 1,2,3,4,5