经常性遇到libcurl的问题，而且都特别奇怪，记录一下CURLcode的定义：
http://curl.haxx.se/libcurl/c/libcurl-errors.html
#include <curl/curl.h>
enum CURLcode
{
CURLE_OK = 0,
CURLE_UNSUPPORTED_PROTOCOL, /* 1 */
CURLE_FAILED_INIT, /* 2 */
CURLE_URL_MALFORMAT, /* 3 */
CURLE_OBSOLETE4, /* 4 - NOT USED */
CURLE_COULDNT_RESOLVE_PROXY, /* 5 */
CURLE_COULDNT_RESOLVE_HOST, /* 6 */
CURLE_COULDNT_CONNECT, /* 7 */
CURLE_FTP_WEIRD_SERVER_REPLY, /* 8 */
CURLE_REMOTE_ACCESS_DENIED, /* 9 a service was denied by the server
due to lack of access - when login fails
this is not returned. */
CURLE_OBSOLETE10, /* 10 - NOT USED */
CURLE_FTP_WEIRD_PASS_REPLY, /* 11 */
CURLE_OBSOLETE12, /* 12 - NOT USED */
CURLE_FTP_WEIRD_PASV_REPLY, /* 13 */
CURLE_FTP_WEIRD_227_FORMAT, /* 14 */
CURLE_FTP_CANT_GET_HOST, /* 15 */
CURLE_OBSOLETE16, /* 16 - NOT USED */
CURLE_FTP_COULDNT_SET_TYPE, /* 17 */
CURLE_PARTIAL_FILE, /* 18 */
CURLE_FTP_COULDNT_RETR_FILE, /* 19 */
CURLE_OBSOLETE20, /* 20 - NOT USED */
CURLE_QUOTE_ERROR, /* 21 - quote command failure */
CURLE_HTTP_RETURNED_ERROR, /* 22 */
CURLE_WRITE_ERROR, /* 23 */
CURLE_OBSOLETE24, /* 24 - NOT USED */
CURLE_UPLOAD_FAILED, /* 25 - failed upload "command" */
CURLE_READ_ERROR, /* 26 - couldn't open/read from file */
CURLE_OUT_OF_MEMORY, /* 27 */
/* Note: CURLE_OUT_OF_MEMORY may sometimes indicate a conversion error
instead of a memory allocation error if CURL_DOES_CONVERSIONS
is defined
*/
CURLE_OPERATION_TIMEDOUT, /* 28 - the timeout time was reached */
CURLE_OBSOLETE29, /* 29 - NOT USED */
CURLE_FTP_PORT_FAILED, /* 30 - FTP PORT operation failed */
CURLE_FTP_COULDNT_USE_REST, /* 31 - the REST command failed */
CURLE_OBSOLETE32, /* 32 - NOT USED */
CURLE_RANGE_ERROR, /* 33 - RANGE "command" didn't work */
CURLE_HTTP_POST_ERROR, /* 34 */
CURLE_SSL_CONNECT_ERROR, /* 35 - wrong when connecting with SSL */
CURLE_BAD_DOWNLOAD_RESUME, /* 36 - couldn't resume download */
CURLE_FILE_COULDNT_READ_FILE, /* 37 */
CURLE_LDAP_CANNOT_BIND, /* 38 */
CURLE_LDAP_SEARCH_FAILED, /* 39 */
CURLE_OBSOLETE40, /* 40 - NOT USED */
CURLE_FUNCTION_NOT_FOUND, /* 41 */
CURLE_ABORTED_BY_CALLBACK, /* 42 */
CURLE_BAD_FUNCTION_ARGUMENT, /* 43 */
CURLE_OBSOLETE44, /* 44 - NOT USED */
CURLE_INTERFACE_FAILED, /* 45 - CURLOPT_INTERFACE failed */
CURLE_OBSOLETE46, /* 46 - NOT USED */
CURLE_TOO_MANY_REDIRECTS, /* 47 - catch endless re-direct loops */
CURLE_UNKNOWN_TELNET_OPTION, /* 48 - User specified an unknown option */
CURLE_TELNET_OPTION_SYNTAX, /* 49 - Malformed telnet option */
CURLE_OBSOLETE50, /* 50 - NOT USED */
CURLE_PEER_FAILED_VERIFICATION, /* 51 - peer's certificate or fingerprint
wasn't verified fine */
CURLE_GOT_NOTHING, /* 52 - when this is a specific error */
CURLE_SSL_ENGINE_NOTFOUND, /* 53 - SSL crypto engine not found */
CURLE_SSL_ENGINE_SETFAILED, /* 54 - can not set SSL crypto engine as
default */
CURLE_SEND_ERROR, /* 55 - failed sending network data */
CURLE_RECV_ERROR, /* 56 - failure in receiving network data */
CURLE_OBSOLETE57, /* 57 - NOT IN USE */
CURLE_SSL_CERTPROBLEM, /* 58 - problem with the local certificate */
CURLE_SSL_CIPHER, /* 59 - couldn't use specified cipher */
CURLE_SSL_CACERT, /* 60 - problem with the CA cert (path?) */
CURLE_BAD_CONTENT_ENCODING, /* 61 - Unrecognized transfer encoding */
CURLE_LDAP_INVALID_URL, /* 62 - Invalid LDAP URL */
CURLE_FILESIZE_EXCEEDED, /* 63 - Maximum file size exceeded */
CURLE_USE_SSL_FAILED, /* 64 - Requested FTP SSL level failed */
CURLE_SEND_FAIL_REWIND, /* 65 - Sending the data requires a rewind
that failed */
CURLE_SSL_ENGINE_INITFAILED, /* 66 - failed to initialise ENGINE */
CURLE_LOGIN_DENIED, /* 67 - user, password or similar was not
accepted and we failed to login */
CURLE_TFTP_NOTFOUND, /* 68 - file not found on server */
CURLE_TFTP_PERM, /* 69 - permission problem on server */
CURLE_REMOTE_DISK_FULL, /* 70 - out of disk space on server */
CURLE_TFTP_ILLEGAL, /* 71 - Illegal TFTP operation */
CURLE_TFTP_UNKNOWNID, /* 72 - Unknown transfer ID */
CURLE_REMOTE_FILE_EXISTS, /* 73 - File already exists */
CURLE_TFTP_NOSUCHUSER, /* 74 - No such user */
CURLE_CONV_FAILED, /* 75 - conversion failed */
CURLE_CONV_REQD, /* 76 - caller must register conversion
callbacks using curl_easy_setopt options
CURLOPT_CONV_FROM_NETWORK_FUNCTION,
CURLOPT_CONV_TO_NETWORK_FUNCTION, and
CURLOPT_CONV_FROM_UTF8_FUNCTION */
CURLE_SSL_CACERT_BADFILE, /* 77 - could not load CACERT file, missing
or wrong format */
CURLE_REMOTE_FILE_NOT_FOUND, /* 78 - remote file not found */
CURLE_SSH, /* 79 - error from the SSH layer, somewhat
generic so the error message will be of
interest when this has happened */
CURLE_SSL_SHUTDOWN_FAILED, /* 80 - Failed to shut down the SSL
connection */
CURLE_AGAIN, /* 81 - socket is not ready for send/recv,
wait till it's ready and try again (Added
in 7.18.2) */
CURLE_SSL_CRL_BADFILE, /* 82 - could not load CRL file, missing or
wrong format (Added in 7.19.0) */
CURLE_SSL_ISSUER_ERROR, /* 83 - Issuer check failed. (Added in
7.19.0) */
CURL_LAST /* never use! */
};