why undefined reference to system function（such as SHA512_Init） on linux gcc?

Question

我想问一下用以下代码我以及表明了头文件而且也存在这些头文件并且这些函数确
实存在于这些头文件中，为什还显示undefined reference to？
第一次用这个，很希望大家能够帮我看看谢谢

#include <string.h>
#include <stdio.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <openssl/sha.h> 
#include <openssl/crypto.h> 



#define PUBLIC_KEY_PATH ("/home/yang_mingkai/rsa_private_key.pem")
#define PRIVATE_KEY_PATH ("/home/yang_mingkai/rsa_public_key.pem")


void printHash(unsigned char *md, int len)
{

    int i = 0;
    for (i = 0; i < len; i++)
    {
        printf("%02x", md[i]);
    }

    printf("\n");
}

RSA* ReadPrivateKey(char* p_KeyPath)

{   
    FILE *fp = NULL;

    char szKeyPath[1024];

    RSA  *priRsa = NULL, *pubRsa = NULL, *pOut = NULL;



    printf("PrivateKeyPath[%s] \n", p_KeyPath);


    /*  Žò¿ªÃÜÔ¿ÎÄŒþ */

    if(NULL == (fp = fopen(p_KeyPath, "r")))

    {

        printf( "fopen[%s] failed \n", p_KeyPath);

        return NULL;
    }

    /*  »ñÈ¡ËœÃÜÔ¿ */

    priRsa = PEM_read_RSAPrivateKey(fp, NULL, NULL,NULL);

    if(NULL == priRsa)

    {
        ERR_print_errors_fp(stdout);

        printf( "PEM_read_RSAPrivateKey\n");

        fclose(fp);

        return NULL;

    }

    fclose(fp);


pOut = priRsa;

    return pOut;

}


/*¶ÁÈ¡¹«³×*/

RSA* ReadPublicKey(char* p_KeyPath)

{

FILE *fp = NULL;

    char szKeyPath[1024];

    RSA  *priRsa = NULL, *pubRsa = NULL, *pOut = NULL;



printf("PublicKeyPath[%s]\n", p_KeyPath);


    /*  Žò¿ªÃÜÔ¿ÎÄŒþ */

    if(NULL == (fp = fopen(p_KeyPath, "r")))

    {

        printf( "fopen[%s] \n", p_KeyPath);

        return NULL;

    }

    /*  »ñÈ¡ËœÃÜÔ¿ */

    if(NULL == (priRsa = PEM_read_RSA_PUBKEY(fp, NULL, NULL,NULL)))

    {

        printf( "PEM_read_RSAPrivateKey error\n");

        fclose(fp);

        return NULL;

    }

    fclose(fp);



    pOut = priRsa;

    return pOut;

}


int main()

{

    char *ct = "55dsd421fd4df1x21c1d4sd21sd51s5";

    char *buf;   

    char *buf2;

    RSA *pubKey;

    RSA *privKey;

    int len;

//  int nOutLen;



    buf = malloc(520);

    buf2 = malloc(520);





    SHA512_CTX c;

    unsigned char md[SHA512_DIGEST_LENGTH];




    sha512_init(&c);

    SHA512_Update(&c, ct, strlen(ct));


    SHA512_Final(md, &c); 

    OPENSSL_cleanse(&c, sizeof(c));


    printHash(md, SHA512_DIGEST_LENGTH);



              privKey = ReadPrivateKey(PRIVATE_KEY_PATH);

              if (!privKey) 

    {

        ERR_print_errors_fp (stderr);

        exit (1);

    }


    pubKey = ReadPublicKey(PUBLIC_KEY_PATH);

    if(!pubKey)

    {

       RSA_free(privKey);

       printf("Error: can't load public key");

       exit(1);

    }







    len = RSA_private_encrypt(SHA512_DIGEST_LENGTH, md, buf,

        privKey,RSA_PKCS1_PADDING);

    if (len != 256)

    {

        printf("Error: ciphertext should match length of key len = %d \n", len);

        exit(1);

    }



    printf("RSA_private_encrypt:");

    printHash(buf, strlen(buf));



    /*¹«Ô¿œâÃÜ*/

    RSA_public_decrypt(len, (const unsigned char*)buf, (unsigned char*)buf2, 

        pubKey,RSA_PKCS1_PADDING);

    printf("RSA_public_decrypt:");

    printHash(buf2, strlen(buf2));



    rsa_free(privKey);

    rsa_free(pubKey);

    free(buf);

    free(buf2);


     return 0;

}