List CAs in a trustore always return an empty stack

[freebendy]

I have a folder storing root CAs for TLS verification on an embedded Linux. It works fine with TLS connection (Libcurl and asio). I want to manage the root CAs. One of features is to list all root CAs in the truststore. I tried demo code below but it always says no CA found, is there anything wrong in it? OpenSSL 3.4 is used.

#include <stdio.h>
#include <openssl/x509.h>
#include <openssl/x509_vfy.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <openssl/provider.h>

int main() {
    // Initialize OpenSSL algorithms and error strings
    OpenSSL_add_all_algorithms();
    ERR_load_crypto_strings();

    // Load the default provider
    if (!OSSL_PROVIDER_load(NULL, "fips")) {
        fprintf(stderr, "Error loading default provider\n");
        return 1;
    }

    // Create a new X509_STORE object
    X509_STORE *store = X509_STORE_new();
    if (!store) {
        fprintf(stderr, "Failed to create X509_STORE object\n");
        return 1;
    }

    // Load certificates into the store (example: loading from a directory)
    if (X509_STORE_load_locations(store, NULL, "/mnt/encry/truststore") != 1) {
        fprintf(stderr, "Failed to load certificates into store\n");
        X509_STORE_free(store);
        return 1;
    }

    // Retrieve all certificates in the store
    STACK_OF(X509) *certs = X509_STORE_get1_all_certs(store);
    if (sk_X509_num(certs) == 0) {
        printf("No certificates found in the store\n");
    } else {
        printf("Found %d certificates in the store\n", sk_X509_num(certs));
        for (int i = 0; i < sk_X509_num(certs); i++) {
            X509 *cert = sk_X509_value(certs, i);
            char *subject_str = X509_NAME_oneline(X509_get_subject_name(cert), NULL, 0);
            if (subject_str) {
                printf("Certificate %d subject: %s\n", i + 1, subject_str);
                OPENSSL_free(subject_str);
            }
        }
    }

    // Clean up
    sk_X509_pop_free(certs, X509_free);
    X509_STORE_free(store);
    ERR_free_strings();
    EVP_cleanup();
    return 0;
}