Openssl SSL_CTX_new is successfull but set errno to ENOSYS (Function not implemented)

Question

The SSL_CTX_new() function works fine but something is strange it sets errno to ENOSYS.

Documentation doesn't say anything about this : https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_new.html

#include <errno.h>
#include <openssl/ssl.h>

int main(int argc, char * argv[]){

    SSL_library_init();
    SSL_load_error_strings();

    perror("A");

    SSL_CTX * sslContext = SSL_CTX_new(TLS_server_method());

    if (sslContext == NULL){
        return EXIT_FAILURE;
    }

    perror("B");

    return 0;
}

this code returns this :

A: Undefined error: 0
B: Function not implemented

My openssl version is : OpenSSL 1.1.1b 26 Feb 2019

Answer 1

Not all libraries use errno to report status. If the documentation does not explicitly say that this function sets errno, then the value is best thought of as undefined: the function might preserve the original value, or set it to 0, or leave some garbage in it.

And indeed the OpenSSL documentation does not say anything about errno; instead the library has its own "error stack" that you can access using ERR_get_error(), and this provides more precise error codes and messages than the very limited set provided by libc.

So the value you find in errno is just a leftover from some or other libc function that SSL_CTX_new() used internally. (For example, it might have tried to seed the RNG by calling libc getrandom(), which then returned ENOSYS due to lack of kernel support.)