curl_global_sslset(3) Library Functions Manual curl_global_sslset(3)

curl_global_sslset - Select SSL backend to use with libcurl

#include <curl/curl.h>
CURLsslset curl_global_sslset(curl_sslbackend id,
                              const char *name,
                              const curl_ssl_backend ***avail);

This function configures at runtime which SSL backend to use with libcurl. This function can only be used to select an SSL backend once, and it must be called before curl_global_init(3).

The backend can be identified by the id (e.g. CURLSSLBACKEND_OPENSSL). The backend can also be specified via the name parameter for a case insensitive match (passing CURLSSLBACKEND_NONE as id). If both id and name are specified, the name is ignored.

If neither id nor name are specified, the function fails with CURLSSLSET_UNKNOWN_BACKEND and set the avail pointer to the NULL-terminated list of available backends. The available backends are those that this particular build of libcurl supports.

Since libcurl 7.60.0, the avail pointer is always set to the list of alternatives if non-NULL.

Upon success, the function returns CURLSSLSET_OK.

If the specified SSL backend is not available, the function returns CURLSSLSET_UNKNOWN_BACKEND and sets the avail pointer to a NULL-terminated list of available SSL backends. In this case, you may call the function again to try to select a different backend.

The SSL backend can be set only once. If it has already been set, a subsequent attempt to change it results in a CURLSSLSET_TOO_LATE getting returned.

This function is thread-safe since libcurl 7.84.0 if curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set (most platforms).

If this is not thread-safe, you must not call this function when any other thread in the program (i.e. a thread sharing the same memory) is running. This does not just mean no other thread that is using libcurl.

The name "OpenSSL" is used for all versions of OpenSSL and its associated forks/flavors in this function. OpenSSL, BoringSSL, libressl, quictls and AmiSSL are all supported by libcurl, but in the eyes of curl_global_sslset(3) they are all just "OpenSSL". They all mostly provide the same API.

curl_version_info(3) can return more specific info about the exact OpenSSL flavor and version number is use.

typedef struct {
  curl_sslbackend id;
  const char *name;
} curl_ssl_backend;
typedef enum {
  CURLSSLBACKEND_OPENSSL = 1, /* or one of its forks */
  CURLSSLBACKEND_GSKIT = 5, /* deprecated */
  CURLSSLBACKEND_POLARSSL = 6, /* deprecated */
  CURLSSLBACKEND_AXTLS = 10, /* deprecated */
  CURLSSLBACKEND_MESALINK = 12, /* deprecated */
} curl_sslbackend;


int main(void)
  int i;
  /* choose a specific backend */
  /* list the available ones */
  const curl_ssl_backend **list;
  curl_global_sslset(CURLSSLBACKEND_NONE, NULL, &list);
  for(i = 0; list[i]; i++)
    printf("SSL backend #%d: '%s' (ID: %d)\n",
           i, list[i]->name, list[i]->id);

This function was added in libcurl 7.56.0. Before this version, there was no support for choosing SSL backends at runtime.

If this function returns CURLSSLSET_OK, the backend was successfully selected.

If the chosen backend is unknown (or support for the chosen backend has not been compiled into libcurl), the function returns CURLSSLSET_UNKNOWN_BACKEND.

If the backend had been configured previously, or if curl_global_init(3) has already been called, the function returns CURLSSLSET_TOO_LATE.

If this libcurl was built completely without SSL support, with no backends at all, this function returns CURLSSLSET_NO_BACKENDS.

curl_global_init(3), libcurl(3)

2024-05-22 libcurl