CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ACCEPT_ENCODING, char *enc);
Sets the contents of the Accept-Encoding: header sent in an HTTP request, and enables decoding of a response when a Content-Encoding: header is received.
libcurl potentially supports several different compressed encodings depending on what support that has been built-in.
To aid applications not having to bother about what specific algorithms this particular libcurl build supports, libcurl allows a zero-length string to be set ("") to ask for an Accept-Encoding: header to be used that contains all built-in supported encodings.
Alternatively, you can specify exactly the encoding or list of encodings you want in the response. Four encodings are supported: identity, meaning non-compressed, deflate which requests the server to compress its response using the zlib algorithm, gzip which requests the gzip algorithm, (since curl 7.57.0) br which is brotli and (since curl 7.72.0) zstd which is zstd. Provide them in the string as a comma-separated list of accepted encodings, like:
"br, gzip, deflate".
Set CURLOPT_ACCEPT_ENCODING(3) to NULL to explicitly disable it, which makes libcurl not send an Accept-Encoding: header and not decompress received contents automatically.
You can also opt to just include the Accept-Encoding: header in your request with CURLOPT_HTTPHEADER(3) but then there will be no automatic decompressing when receiving data.
This is a request, not an order; the server may or may not do it. This option must be set (to any non-NULL value) or else any unsolicited encoding done by the server is ignored.
Servers might respond with Content-Encoding even without getting a Accept-Encoding: in the request. Servers might respond with a different Content-Encoding than what was asked for in the request.
The Content-Length: servers send for a compressed response is supposed to indicate the length of the compressed content so when auto decoding is enabled it may not match the sum of bytes reported by the write callbacks (although, sending the length of the non-compressed content is a common server mistake).
The application does not have to keep the string around after setting this option.
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* enable all supported built-in compressions */
curl_easy_setopt(curl, CURLOPT_ACCEPT_ENCODING, "");
/* Perform the request */
curl_easy_perform(curl);
}
The specific libcurl you're using must have been built with zlib to be able to decompress gzip and deflate responses, with the brotli library to decompress brotli responses and with the zstd library to decompress zstd responses.