What causes "Failed to set writer [%s] " in curl/curl?

Verify the curl handle passed to curl_easy_setopt is non-NULL and was freshly returned by curl_easy_init().

Failed to set writer [%s]

Error message

Failed to set writer [%s]

What it means

This message is printed when curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer) returns a value other than CURLE_OK while registering the write callback that receives downloaded HTTP body data. The bracketed %s is the libcurl error buffer string (CURLOPT_ERRORBUFFER) populated by an earlier setopt call. It indicates libcurl rejected the option configuration, which is rare for a valid handle and usually points to an internal/usage error in option handling. The init() helper returns false and the program aborts setup before any transfer begins.

Source

Thrown at docs/examples/htmltitle.cpp:118

    fprintf(stderr, "Failed to set error buffer [%d]\n", result);
    return false;
  }

  result = curl_easy_setopt(curl, CURLOPT_URL, url);
  if(result != CURLE_OK) {
    fprintf(stderr, "Failed to set URL [%s]\n", errorBuffer);
    return false;
  }

  result = curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
  if(result != CURLE_OK) {
    fprintf(stderr, "Failed to set redirect option [%s]\n", errorBuffer);
    return false;
  }

  result = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);
  if(result != CURLE_OK) {
    fprintf(stderr, "Failed to set writer [%s]\n", errorBuffer);
    return false;
  }

  result = curl_easy_setopt(curl, CURLOPT_WRITEDATA, &buffer);
  if(result != CURLE_OK) {
    fprintf(stderr, "Failed to set write data [%s]\n", errorBuffer);
    return false;
  }

  return true;
}

//
// libxml start element callback function
//
static void StartElement(void *voidContext,
                         const xmlChar *name,
                         const xmlChar **attributes)

View on GitHub (pinned to 527573490e)

Solutions

Verify the curl handle passed to curl_easy_setopt is non-NULL and was freshly returned by curl_easy_init().
Confirm the libcurl shared library version matches the compiled curl/curl.h headers (run curl-config --version vs. ldd on the binary).
Ensure the writer callback matches the exact signature size_t (*)(char *, size_t, size_t, void*) and uses C linkage.
Check CURLOPT_ERRORBUFFER contents for the specific CURLcode to narrow the failing option.

Example fix

// before
result = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);
if(result != CURLE_OK) {
  fprintf(stderr, "Failed to set writer [%s]\n", errorBuffer);
  return false;
}

// after - assert correct callback signature and report the CURLcode
static size_t writer(char *data, size_t size, size_t nmemb, void *userp);
...
result = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);
if(result != CURLE_OK) {
  fprintf(stderr, "Failed to set writer [%d: %s]\n", (int)result,
          curl_easy_strerror(result));
  return false;
}

Defensive patterns

Strategy: validation

Validate before calling

// Before curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer):
if(!curl) return false;          // handle must be initialized first
if(!writer) return false;        // a null callback is never valid
// CURLOPT_WRITEFUNCTION exists in every supported libcurl build,
// so a real failure here almost always means a bad handle.

Type guard

// Confirm 'writer' is a non-null function pointer compatible with
// libcurl's write-callback contract: returns size_t, takes 4 args.
using write_cb = size_t(*)(char*, size_t, size_t, void*);
write_cb cb = reinterpret_cast<write_cb>(&writer);
if(!cb) return false;

Try / catch

CURLcode r = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);
if(r != CURLE_OK) {
  fprintf(stderr, "Failed to set writer [%s]\n", errorBuffer);
  curl_easy_cleanup(curl);
  return false;
}

Prevention

Initialize the CURL* handle with curl_easy_init() and confirm it is non-null before any curl_easy_setopt call.
Define the write callback with the exact prototype size_t(char*, size_t, size_t, void*) to avoid undefined behavior.
Set CURLOPT_ERRORBUFFER first so subsequent setopt failures populate a human-readable message.
Treat any non-CURLE_OK return from curl_easy_setopt as fatal for that option; do not proceed to curl_easy_perform.

When it happens

Trigger: Calling curl_easy_setopt with CURLOPT_WRITEFUNCTION on a CURL handle that is NULL, already cleaned up, or in a corrupted state; or passing a callback pointer of the wrong signature. In this example it occurs inside init() (htmltitle.cpp:116) right after successfully setting CURLOPT_FOLLOWLOCATION, so the handle is valid and failure would stem from a libcurl build/ABI mismatch or a bad function pointer cast.

Common situations: Linking the program against a libcurl shared library whose ABI version differs from the curl/curl.h headers compiled against; passing a non-matching callback prototype (e.g. omitting the size/nmemb parameters); running under a sanitizer that flags the function pointer; using a thread-unsafe global init sequence.

Related errors

AI-assisted analysis of curl/curl@527573490e (2026-08-01). Data as JSON: /data/errors/1a7f4889fb48997c.json. Report an issue: GitHub.