Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/x509_cert_check_demo.c

@@ -0,0 +1,88 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/x509_cer.h>
+
+char *pem =
+"-----BEGIN CERTIFICATE-----\n"
+"MIIBszCCAVegAwIBAgIIaeL+wBcKxnswDAYIKoEcz1UBg3UFADAuMQswCQYDVQQG\n"
+"EwJDTjEOMAwGA1UECgwFTlJDQUMxDzANBgNVBAMMBlJPT1RDQTAeFw0xMjA3MTQw\n"
+"MzExNTlaFw00MjA3MDcwMzExNTlaMC4xCzAJBgNVBAYTAkNOMQ4wDAYDVQQKDAVO\n"
+"UkNBQzEPMA0GA1UEAwwGUk9PVENBMFkwEwYHKoZIzj0CAQYIKoEcz1UBgi0DQgAE\n"
+"MPCca6pmgcchsTf2UnBeL9rtp4nw+itk1Kzrmbnqo05lUwkwlWK+4OIrtFdAqnRT\n"
+"V7Q9v1htkv42TsIutzd126NdMFswHwYDVR0jBBgwFoAUTDKxl9kzG8SmBcHG5Yti\n"
+"W/CXdlgwDAYDVR0TBAUwAwEB/zALBgNVHQ8EBAMCAQYwHQYDVR0OBBYEFEwysZfZ\n"
+"MxvEpgXBxuWLYlvwl3ZYMAwGCCqBHM9VAYN1BQADSAAwRQIgG1bSLeOXp3oB8H7b\n"
+"53W+CKOPl2PknmWEq/lMhtn25HkCIQDaHDgWxWFtnCrBjH16/W3Ezn7/U/Vjo5xI\n"
+"pDoiVhsLwg==\n"
+"-----END CERTIFICATE-----\n";
+
+static int prepare_pem_file(void)
+{
+	FILE *fp;
+
+	if (!(fp = fopen("cert.pem", "wb"))) {
+		fprintf(stderr, "fopen() error\n");
+		return -1;
+	}
+
+	if (fwrite(pem, 1, strlen(pem), fp) != strlen(pem)) {
+		fprintf(stderr, "fwrite() error\n");
+		return -1;
+	}
+
+	fclose(fp);
+	return 1;
+}
+
+int main(int argc, char **argv)
+{
+	int ret = -1;
+	FILE *fp = NULL;
+	uint8_t cert[2048];
+	size_t certlen;
+	int cert_type = X509_cert_root_ca;
+	/*
+	cert_type options:
+		X509_cert_server_auth,
+		X509_cert_client_auth,
+		X509_cert_server_key_encipher,
+		X509_cert_client_key_encipher,
+		X509_cert_ca,
+		X509_cert_root_ca,
+		X509_cert_crl_sign,
+	*/
+	int path_len_constraint;
+
+	(void)prepare_pem_file();
+
+	if (!(fp = fopen("cert.pem", "rb"))) {
+		fprintf(stderr, "fopen() cert.pem error\n");
+		goto err;
+	}
+
+	if (x509_cert_from_pem(cert, &certlen, sizeof(cert), fp) != 1) {
+		fprintf(stderr, "x509_cert_from_pem() error\n");
+		goto err;
+	}
+
+	if (x509_cert_check(cert, certlen, cert_type, &path_len_constraint) != 1) {
+		fprintf(stderr, "invalid cert\n");
+		goto err;
+	}
+
+	ret = 0;
+err:
+	if (fp) fclose(fp);
+	return ret;
+}