Understanding and Fixing the curl 23 Failed Writing Body Error
This error shows up when libcurl tries to write downloaded data and your write callback returns an error. It's not one of the most common curl errors, but when it hits, it can be frustrating because the error message itself doesn't tell you much. The download might look like it's in progress, then suddenly stops with that message and you're left wondering what went wrong. At its core, curl 23 fails writing body happens when your custom write callback function signals failure. The callback is the piece of code you give curl that tells it where to put the data it receives. If that callback returns anything other than zero (or the number of bytes successfully written, depending on your callback type), curl throws error 23. Common culprits include a closed file handle, a full disk, a buffer overflow in your own code, or a network issue inside the callback logic itself. The tricky part is that sometimes the error isn't even in your callback. I ran into this with a Python script using pycurl where the error was coming from a logging library that was trying to write to a rotated log file that didn't exist yet. The callback itself was fine, but an exception inside it caused curl to interpret it as a write failure. Took me about three hours to find because the traceback was pointing at pycurl, not my logging call.
Another frequent source is writing to a file descriptor that got closed by something else while curl still has it open. This happens more often than you'd think with threaded applications where another thread closes the file to do a rotation or cleanup.
How to Fix the Curl 23 Error
The first thing to check is always your write callback. Make sure it returns the correct value. In C-style callbacks, returning zero or the number of bytes written means success. Returning any other value triggers curl 23. In Python with pycurl, your callback should return None (which signals success) or the number of bytes consumed. If your callback raises an exception, curl won't give you useful debug information about the exception itself. If you're not using a custom callback and just writing to a file, verify the file handle is still open and the disk isn't full. Check with os.path.exists() for the file and run a df command on Unix systems or check free space in Windows. This sounds obvious but I've seen it in production twice in the last year alone. For threaded applications, make sure no other thread is touching the same file descriptor. Use a mutex or restructure your code so each thread writes to its own file. This was the exact fix for the case I mentioned earlier with the log rotation issue.
Get the Full Details
Debugging Strategies That Actually Work
Enable curl's debug output with CURLOPT_VERBOSE set to true. This gives you detailed information about what curl is doing at each step. Look for any anomalies right before the error occurs, like unexpected resets or partial writes. The verbose output will show you the exact byte count curl thinks it wrote, which helps you identify if the write callback is losing track of bytes somewhere. When using pycurl specifically, wrap your write callback in a try-except block and log any exceptions. The default behavior silently swallows Python exceptions inside the callback, so you'll never know what actually went wrong without explicit error handling. Check for resource leaks. If you're opening and closing file handles repeatedly during a long download, make sure you're not hitting the file descriptor limit. This becomes relevant on systems with small ulimits, especially on older Linux distributions.
Common Scenarios and Solutions
Web scraping with curl often triggers this error when you're parsing responses and trying to write them to a database inside the write callback. The database connection might fail or the parser might throw an exception. Move that logic out of the callback entirely. Collect the data in the callback and process it afterward. Streaming downloads are another common case. If you're downloading large files and your output buffer fills up unexpectedly, you'll get error 23. Make sure your buffer size is reasonable and that you're flushing it properly. A buffer that's too small causes excessive overhead. Too large and you risk memory issues on constrained systems. When using libcurl in a non-blocking mode, make sure you're calling curl_easy_write_cb in the correct context. Mixing blocking and non-blocking operations on the same handle can cause subtle failures that manifest as curl 23.
Edge Case: The Curl 23 Failed Writing Body Issue with glibc Memory Allocators
I hit a weird one recently where the error only appeared on systems using a specific glibc version with certain memory allocators enabled. The issue was that the malloc implementation was fragmenting memory during large transfers, causing intermittent write failures. Switching to a different allocator with mallopt fixed it. This won't affect most users, but if you're seeing intermittent curl 23 failures on high-memory systems with large transfers, it's worth checking. The practical takeaway is to always validate your write callback, check system resources, and add proper error handling around any non-trivial operations inside the callback. Most cases of this error resolve within ten minutes once you know where to look.
