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Post transfer info
Remember how libcurl transfers are associated with an "easy handle"! Each transfer has such a handle and when a transfer is completed, before the handle is cleaned or reused for another transfer, it can be used to extract information from the previous operation.
Your friend for doing this is called curl_easy_getinfo() and you tell it which specific information you are interested in and it will return that to you if it can.
When you use this function, you pass in the easy handle, which information you want and a pointer to a variable to hold the answer. You must pass in a pointer to a variable of the correct type or you risk that things will go side-ways. These information values are designed to be provided after the transfer is completed.
The data you receive can be a long, a 'char *', a 'struct curl_slist *', a double or a socket.
This is how you extract the Content-Type: value from the previous HTTP transfer:
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CURLcode res;
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char *content_type;
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res = curl_easy_getinfo(curl, CURLINFO_CONTENT_TYPE, &content_type);
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If you want to extract the local port number that was used in that connection:
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CURLcode res;
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long port_number;
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res = curl_easy_getinfo(curl, CURLINFO_LOCAL_PORT, &port_number);
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Available information

Getinfo option
Type
Description
CURLINFO_ACTIVESOCKET
curl_socket_t
The session's active socket
CURLINFO_APPCONNECT_TIME
double
Time from start until SSL/SSH handshake completed
CURLINFO_APPCONNECT_TIME_T
curl_off_t
Time from start until SSL/SSH handshake completed (in microseconds)
CURLINFO_CERTINFO
struct curl_slist *
Certificate chain
CURLINFO_CONDITION_UNMET
long
Whether or not a time conditional was met
CURLINFO_CONNECT_TIME
double
Time from start until remote host or proxy completed
CURLINFO_CONNECT_TIME_T
curl_off_t
Time from start until remote host or proxy completed (in microseconds)
CURLINFO_CONTENT_LENGTH_DOWNLOAD
double
Content length from the Content-Length header
CURLINFO_CONTENT_LENGTH_UPLOAD
double
Upload size
CURLINFO_CONTENT_TYPE
char *
Content type from the Content-Type header
CURLINFO_COOKIELIST
struct curl_slist *
List of all known cookies
CURLINFO_EFFECTIVE_METHOD
char *
Last used HTTP request method
CURLINFO_EFFECTIVE_URL
char *
Last used URL
CURLINFO_FILETIME
long
Remote time of the retrieved document
CURLINFO_FTP_ENTRY_PATH
char *
The entry path after logging in to an FTP server
CURLINFO_HEADER_SIZE
long
Number of bytes of all headers received
CURLINFO_HTTP_CONNECTCODE
long
Last proxy CONNECT response code
CURLINFO_HTTP_VERSION
long
The HTTP version used in the connection
CURLINFO_HTTPAUTH_AVAIL
long
Available HTTP authentication methods (bitmask)
CURLINFO_LASTSOCKET
long
Last socket used
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