apache/hadoop · error · IOException

Unexpected return of {} from transferTo()

Error message

Unexpected return of {} from transferTo()

What it means

Error "Unexpected return of {} from transferTo()" thrown in apache/hadoop.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/SocketOutputStream.java:234

       * Once we move to JAVA SE 7, wait should be moved to correct place.
       */
      long start = System.nanoTime();
      waitForWritable();
      long wait = System.nanoTime();

      int nTransfered = (int) fileCh.transferTo(position, count, getChannel());
      
      if (nTransfered == 0) {
        //check if end of file is reached.
        if (position >= fileCh.size()) {
          throw new EOFException("EOF Reached. file size is " + fileCh.size() + 
                                 " and " + count + " more bytes left to be " +
                                 "transfered.");
        }
        //otherwise assume the socket is full.
        //waitForWritable(); // see comment above.
      } else if (nTransfered < 0) {
        throw new IOException("Unexpected return of " + nTransfered + 
                              " from transferTo()");
      } else {
        position += nTransfered;
        count -= nTransfered;
      }
      long transfer = System.nanoTime();
      waitTime += wait - start;
      transferTime += transfer - wait;
    }
    
    if (waitForWritableTime != null) {
      waitForWritableTime.set(waitTime);
    }
    if (transferToTime != null) {
      transferToTime.set(transferTime);
    }
  }

View on GitHub (pinned to 2add963021)

Solutions

1. Check the underlying FileChannel and filesystem for errors; transferTo returning an unexpected count usually indicates an I/O problem or concurrent modification of the file.
2. Verify the file size did not change during the transfer and retry the operation.
3. Review OS-level limits (sendfile limits) and fall back to stream-based copy if zero-copy transfer is unreliable on this platform.

When it happens

Trigger: Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/SocketOutputStream.java:234 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/b36c0ffc3c5b83f2. Report an issue: GitHub.