CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CAINFO, char *path);
If CURLOPT_SSL_VERIFYPEER(3) is zero and you avoid verifying the server's certificate, CURLOPT_CAINFO(3) need not even indicate an accessible file.
This option is by default set to the system path where libcurl's cacert bundle is assumed to be stored, as established at build time.
If curl is built against the NSS SSL library, the NSS PEM PKCS#11 module (libnsspem.so) needs to be available for this option to work properly. Starting with curl-7.55.0, if both CURLOPT_CAINFO(3) and CURLOPT_CAPATH(3) are unset, NSS-linked libcurl tries to load libnssckbi.so, which contains a more comprehensive set of trust information than supported by nss-pem, because libnssckbi.so also includes information about distrusted certificates.
(iOS and macOS) When curl uses Secure Transport this option is supported. If the option is not set, then curl will use the certificates in the system and user Keychain to verify the peer.
(Schannel) This option is supported for Schannel in Windows 7 or later but we recommend not using it until Windows 8 since it works better starting then. If the option is not set, then curl will use the certificates in the Windows' store of root certificates (the default for Schannel).
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/");
curl_easy_setopt(curl, CURLOPT_CAINFO, "/etc/certs/cabundle.pem");
ret = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}