title: EVP_MD_CTX "error: storage size of ‘ctx’ isn’t known"
body: <p>My system is Ubuntu16.04 LTS. when I use the OpenSSL <code>EVP_MD_CTX</code>, this error appeared. Can anyone help me?</p>
  
  <hr>
  
  <p><strong><em>CODE</em></strong>:</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;openssl/evp.h&gt;
  #include &lt;openssl/rsa.h&gt;
  int main()
  {
      int ret,inlen,outlen=0;
      unsigned long e=RSA_3;
      char data[100],out[500];
      EVP_MD_CTX md_ctx, md_ctx2;
      ...
  }
  </code></pre>
  
  <p><strong><em>ERROR</em></strong>:</p>
  
  <pre><code>root@ubuntu:/work/test# gcc evp_openssl_test.c -I/usr/local/include -L/usr/local/lib -lssl -lcrypto -o
  evptestevp_openssl_test.c: In function ‘main’:
  evp_openssl_test.c:13:19: error: storage size of ‘md_ctx’ isn’t known
      EVP_MD_CTX md_ctx,md_ctx2;
  </code></pre>