curl_global_sslset(3) Library Functions Manual curl_global_sslset(3)
NAME
curl_global_sslset - select SSL backend to use
SYNOPSIS
#include <curl/curl.h>
CURLsslset curl_global_sslset(curl_sslbackend id,
const char *name,
const curl_ssl_backend ***avail);
DESCRIPTION
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 in-
sensitive 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-termi-
nated 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 al-
ternatives 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-termi-
nated 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 sub-
sequent attempt to change it results in a CURLSSLSET_TOO_LATE getting
returned.
This function is thread-safe since libcurl 7.84.0 if curl_ver-
sion_info(3) has the CURL_VERSION_THREADSAFE feature bit set (most plat-
forms).
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.
Names
SSL backend names (case-insensitive): BearSSL, GnuTLS, mbedTLS, OpenSSL,
Rustls, Schannel, Secure-Transport, wolfSSL
The name "OpenSSL" is used for all versions of OpenSSL and its associ-
ated 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 pro-
vide the same API. curl_version_info(3) can return more specific info
about the exact OpenSSL flavor and version number in use.
struct
typedef struct {
curl_sslbackend id;
const char *name;
} curl_ssl_backend;
typedef enum {
CURLSSLBACKEND_NONE = 0,
CURLSSLBACKEND_OPENSSL = 1, /* or one of its forks */
CURLSSLBACKEND_GNUTLS = 2,
CURLSSLBACKEND_NSS = 3,
CURLSSLBACKEND_GSKIT = 5, /* deprecated */
CURLSSLBACKEND_POLARSSL = 6, /* deprecated */
CURLSSLBACKEND_WOLFSSL = 7,
CURLSSLBACKEND_SCHANNEL = 8,
CURLSSLBACKEND_SECURETRANSPORT = 9,
CURLSSLBACKEND_AXTLS = 10, /* deprecated */
CURLSSLBACKEND_MBEDTLS = 11,
CURLSSLBACKEND_MESALINK = 12, /* deprecated */
CURLSSLBACKEND_BEARSSL = 13,
CURLSSLBACKEND_RUSTLS = 14
} curl_sslbackend;
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
int i;
/* choose a specific backend */
curl_global_sslset(CURLSSLBACKEND_WOLFSSL, NULL, NULL);
/* 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);
}
AVAILABILITY
Added in curl 7.56.0
RETURN VALUE
If this function returns CURLSSLSET_OK, the backend was successfully se-
lected.
If the chosen backend is unknown (or support for the chosen backend has
not been compiled into libcurl), the function returns CURLSSLSET_UN-
KNOWN_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 back-
ends at all, this function returns CURLSSLSET_NO_BACKENDS.
SEE ALSO
curl_global_init(3), libcurl(3)
libcurl 2026-07-12 curl_global_sslset(3)
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