libcurl and expected string encodings

An attempt to assess and document the expected string encoding for libcurl APIs where strings are used as inputs or return values.

Notes:

• Encodings describe how the string should work, based on content and how/where it is being used. This may differ from reality depending on build configuration.
• Please feel free to add any missing API to the list.
• Entries marked with ? are yet to be finalized.
• An interesting category is for example the referrer string which is normally a URL, where UTF-8 support is expected, but the value is effectively a raw, zero-terminate string.

String types:

• binary = raw bytes with an explicit length
• ASCII = zero terminated, 7-bit ASCII string
  usually strings internal to libcurl. Internally, these behave the same as raw, but documented/accepted/valid/typical values are 7-bit ASCII.
• UTF-8 = zero terminated, UTF-8 encoded string
  URLs, filenames, human-readable text, etc.
• raw = zero terminated, 8-bit string
  none of the above, codepage to be determined by context on either the local or remote side

APIs:

verified	encoding	API
✓	binary	CURLFORM_COPYCONTENTS (ptr + length)
✓	binary	CURLFORM_COPYNAME (ptr + length)
?	UTF-8	CURLFORM_FILE
?		CURLFORM_FILECONTENT
?	ASCII	CURLINFO_CONTENT_TYPE
✓	ASCII	CURLINFO_EFFECTIVE_METHOD
✓	UTF-8	CURLINFO_EFFECTIVE_URL (URL)
?	UTF-8	CURLINFO_FTP_ENTRY_PATH
✓	ASCII	CURLINFO_LOCAL_IP
✓	ASCII	CURLINFO_PRIMARY_IP
✓	UTF-8	CURLINFO_REDIRECT_URL (URL)
?	UTF-8	CURLINFO_REFERER
✓	raw	CURLINFO_RTSP_SESSION_ID
✓	ASCII	CURLINFO_SCHEME
✓	UTF-8	CURLOPT_ABSTRACT_UNIX_SOCKET (local filename)
✓	ASCII	CURLOPT_ACCEPT_ENCODING / CURLOPT_ENCODING (sent to remote, normally ASCII)
✓	UTF-8	CURLOPT_ALTSVC (local filename)
✓	ASCII	CURLOPT_AWS_SIGV4 (sent to remote, normally ASCII)
✓	UTF-8	CURLOPT_CAINFO (local filename)
✓	binary	CURLOPT_CAINFO_BLOB (ptr + length)
✓	UTF-8	CURLOPT_CAPATH (local directory)
✓	raw	CURLOPT_COOKIE (cookie string sent to remote)
✓	UTF-8	CURLOPT_COOKIEFILE (local filename)
✓	UTF-8	CURLOPT_COOKIEJAR (local filename)
✓	raw	CURLOPT_COOKIELIST (ASCII internal string ALL/SESS/FLUSH/RELOAD or raw string sent to remote)
✓	binary	CURLOPT_COPYPOSTFIELDS (ptr + length)
✓	UTF-8	CURLOPT_CRLFILE (local filename)
✓	ASCII	CURLOPT_CUSTOMREQUEST (sent to remote, normally ASCII)
✓	ASCII	CURLOPT_DEFAULT_PROTOCOL (normally ASCII)
✓	UTF-8	CURLOPT_DNS_SERVERS (URL host:port)
✓	ASCII	CURLOPT_DNS_INTERFACE (local ASCII)
✓	UTF-8	CURLOPT_DNS_LOCAL_IPV4 (local ASCII)
✓	UTF-8	CURLOPT_DNS_LOCAL_IPV6 (local ASCII)
✓	UTF-8	CURLOPT_DOH_URL (URL)
✓	UTF-8	CURLOPT_EGDSOCKET (local filename)
✓	ASCII	CURLOPT_FTPPORT (URL IP:port)
✓	raw	CURLOPT_FTP_ACCOUNT (remote credentials)
✓	raw	CURLOPT_FTP_ALTERNATIVE_TO_USER (remote credentials)
✓	UTF-8	CURLOPT_HSTS (local filename)
✓	UTF-8	CURLOPT_INTERFACE (URL address or host)
✓	UTF-8	CURLOPT_ISSUERCERT (local filename)
✓	binary	CURLOPT_ISSUERCERT_BLOB (ptr + length)
✓	raw	CURLOPT_KEYPASSWD (local credentials)
✓	ASCII	CURLOPT_KRBLEVEL / CURLOPT_KRB4LEVEL (ASCII)
✓	raw	CURLOPT_LOGIN_OPTIONS (sent to remote)
✓	raw	CURLOPT_MAIL_AUTH (sent to remote)
✓	raw	CURLOPT_MAIL_FROM (sent to remote)
✓	UTF-8	CURLOPT_NETRC_FILE (local filename)
✓	UTF-8	CURLOPT_NOPROXY (URL domain/IP list)
✓	raw	CURLOPT_PASSWORD (remote credentials)
✓	UTF-8	CURLOPT_PINNEDPUBLICKEY (local filename)
✓	binary	CURLOPT_POSTFIELDS (ptr + length)
✓	UTF-8	CURLOPT_PRE_PROXY (URL server:port)
✓	UTF-8	CURLOPT_PROXY (URL server:port)
✓	raw	CURLOPT_PROXYPASSWORD (remote credentials)
✓	raw	CURLOPT_PROXYUSERNAME (remote credentials)
✓	raw	CURLOPT_PROXYUSERPWD (remote credentials)
✓	UTF-8	CURLOPT_PROXY_CAINFO (local filename)
✓	binary	CURLOPT_PROXY_CAINFO_BLOB (ptr + length)
✓	UTF-8	CURLOPT_PROXY_CAPATH (local directory)
✓	UTF-8	CURLOPT_PROXY_CRLFILE (local filename)
✓	UTF-8	CURLOPT_PROXY_ISSUERCERT (local filename)
✓	binary	CURLOPT_PROXY_ISSUERCERT_BLOB (ptr + length)
✓	raw	CURLOPT_PROXY_KEYPASSWD (local credentials)
✓	UTF-8	CURLOPT_PROXY_PINNEDPUBLICKEY (local filename)
✓	ASCII	CURLOPT_PROXY_SERVICE_NAME (normally ASCII)
✓	UTF-8	CURLOPT_PROXY_SSLCERT (local filename)
✓	ASCII	CURLOPT_PROXY_SSLCERTTYPE (local ASCII)
✓	binary	CURLOPT_PROXY_SSLCERT_BLOB (ptr + length)
✓	UTF-8	CURLOPT_PROXY_SSLKEY (local filename)
✓	ASCII	CURLOPT_PROXY_SSLKEYTYPE (local ASCII)
✓	binary	CURLOPT_PROXY_SSLKEY_BLOB (ptr + length)
✓	ASCII	CURLOPT_PROXY_SSL_CIPHER_LIST
✓	ASCII	CURLOPT_PROXY_TLS13_CIPHERS
✓	raw	CURLOPT_PROXY_TLSAUTH_PASSWORD (remote credentials)
✓	ASCII	CURLOPT_PROXY_TLSAUTH_TYPE (internal ASCII)
✓	raw	CURLOPT_PROXY_TLSAUTH_USERNAME (remote credentials)
✓	UTF-8	CURLOPT_RANDOM_FILE (local filename)
✓	ASCII	CURLOPT_RANGE (ASCII, sent to remote)
?	raw	CURLOPT_REFERER (URL or any string)
?	UTF-8	CURLOPT_REQUEST_TARGET (URL)
✓	raw	CURLOPT_RTSP_SESSION_ID (sent to remote)
✓	UTF-8	CURLOPT_RTSP_STREAM_URI (URL)
✓	ASCII	CURLOPT_RTSP_TRANSPORT (sent to remote, normally ASCII)
✓	raw	CURLOPT_SASL_AUTHZID (remote credentials)
✓	ASCII	CURLOPT_SERVICE_NAME (normally ASCII)
✓	ASCII	CURLOPT_SOCKS5_GSSAPI_SERVICE (normally ASCII)
✓	ASCII	CURLOPT_SSH_HOST_PUBLIC_KEY_MD5 (local ASCII)
✓	UTF-8	CURLOPT_SSH_KNOWNHOSTS (local filename)
✓	UTF-8	CURLOPT_SSH_PRIVATE_KEYFILE (local filename)
✓	UTF-8	CURLOPT_SSH_PUBLIC_KEYFILE (local filename)
✓	UTF-8	CURLOPT_SSLCERT (local filename)
✓	ASCII	CURLOPT_SSLCERTTYPE (local ASCII)
✓	binary	CURLOPT_SSLCERT_BLOB (ptr + length)
✓	ASCII	CURLOPT_SSLENGINE (internal)
✓	UTF-8	CURLOPT_SSLKEY (local filename)
✓	ASCII	CURLOPT_SSLKEYTYPE (local ASCII)
✓	binary	CURLOPT_SSLKEY_BLOB (ptr + length)
✓	ASCII	CURLOPT_SSL_CIPHER_LIST
✓	ASCII	CURLOPT_SSL_EC_CURVES (local ASCII)
✓	ASCII	CURLOPT_TLS13_CIPHERS
✓	raw	CURLOPT_TLSAUTH_PASSWORD (remote credentials)
✓	ASCII	CURLOPT_TLSAUTH_TYPE (internal ASCII)
✓	raw	CURLOPT_TLSAUTH_USERNAME (remote credentials)
✓	UTF-8	CURLOPT_UNIX_SOCKET_PATH (local filename)
✓	UTF-8	CURLOPT_URL (URL)
✓	raw	CURLOPT_USERAGENT (sent to remote)
✓	raw	CURLOPT_USERNAME (remote credentials)
✓	raw	CURLOPT_USERPWD (remote credentials)
✓	raw	CURLOPT_XOAUTH2_BEARER (sent to remote)
✓	binary	curl_mime_data() (ptr + length)
✓	ASCII	curl_mime_encoder() (internal ASCII)
✓	raw	curl_mime_filedata() (sent to remote)
✓	raw	curl_mime_filename() (sent to remote)
✓	ASCII	curl_mime_name() (sent to remote, normally ASCII)
✓	ASCII	curl_mime_type() (sent to remote, normally ASCII)