{"id":"1a7f4889fb48997c","repo":"curl/curl","slug":"failed-to-set-writer-s","errorCode":null,"errorMessage":"Failed to set writer [%s]\n","messagePattern":"Failed to set writer \\[(.+?)\\]\n","errorType":"console","errorClass":null,"httpStatus":null,"severity":"error","filePath":"docs/examples/htmltitle.cpp","lineNumber":118,"sourceCode":"    fprintf(stderr, \"Failed to set error buffer [%d]\\n\", result);\n    return false;\n  }\n\n  result = curl_easy_setopt(curl, CURLOPT_URL, url);\n  if(result != CURLE_OK) {\n    fprintf(stderr, \"Failed to set URL [%s]\\n\", errorBuffer);\n    return false;\n  }\n\n  result = curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);\n  if(result != CURLE_OK) {\n    fprintf(stderr, \"Failed to set redirect option [%s]\\n\", errorBuffer);\n    return false;\n  }\n\n  result = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);\n  if(result != CURLE_OK) {\n    fprintf(stderr, \"Failed to set writer [%s]\\n\", errorBuffer);\n    return false;\n  }\n\n  result = curl_easy_setopt(curl, CURLOPT_WRITEDATA, &buffer);\n  if(result != CURLE_OK) {\n    fprintf(stderr, \"Failed to set write data [%s]\\n\", errorBuffer);\n    return false;\n  }\n\n  return true;\n}\n\n//\n// libxml start element callback function\n//\nstatic void StartElement(void *voidContext,\n                         const xmlChar *name,\n                         const xmlChar **attributes)","sourceCodeStart":100,"sourceCodeEnd":136,"githubUrl":"https://github.com/curl/curl/blob/527573490eb2564b3d7c9dd51d8bff963b5d6303/docs/examples/htmltitle.cpp#L100-L136","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","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."],"exampleFix":"// before\nresult = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);\nif(result != CURLE_OK) {\n  fprintf(stderr, \"Failed to set writer [%s]\\n\", errorBuffer);\n  return false;\n}\n\n// after - assert correct callback signature and report the CURLcode\nstatic size_t writer(char *data, size_t size, size_t nmemb, void *userp);\n...\nresult = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);\nif(result != CURLE_OK) {\n  fprintf(stderr, \"Failed to set writer [%d: %s]\\n\", (int)result,\n          curl_easy_strerror(result));\n  return false;\n}","handlingStrategy":"validation","validationCode":"// Before curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer):\nif(!curl) return false;          // handle must be initialized first\nif(!writer) return false;        // a null callback is never valid\n// CURLOPT_WRITEFUNCTION exists in every supported libcurl build,\n// so a real failure here almost always means a bad handle.","typeGuard":"// Confirm 'writer' is a non-null function pointer compatible with\n// libcurl's write-callback contract: returns size_t, takes 4 args.\nusing write_cb = size_t(*)(char*, size_t, size_t, void*);\nwrite_cb cb = reinterpret_cast<write_cb>(&writer);\nif(!cb) return false;","tryCatchPattern":"CURLcode r = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);\nif(r != CURLE_OK) {\n  fprintf(stderr, \"Failed to set writer [%s]\\n\", errorBuffer);\n  curl_easy_cleanup(curl);\n  return false;\n}","preventionTips":["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."],"tags":["libcurl","curl-easy-setopt","configuration","callback","c-plus-plus"],"analyzedSha":"527573490eb2564b3d7c9dd51d8bff963b5d6303","analyzedAt":"2026-08-01T21:00:26.351Z","schemaVersion":2}