The first program of openssl under Linux

OpenSSL is open source, you can write test code on it, how to write it? Here is an example of this.

The first step: write a test program MD5test1.c, the code is as follows

1. #include<stdio.h>
2. #include<openssl/md5.h>
3. #include<string.h>
4. int main(int argc,char**argv )
5. {
6. MD5_CTX ctx;
7. unsignedchar*data="123";
8. unsignedchar md[16];
9. char buf[33]={'\0'};
10. char tmp[3]={'\0'};
11. int i;
12. MD5_Init(&ctx);
13. MD5_Update(&ctx,data, strlen (data));
14. MD5_Final(md,&ctx);
15. for( i=0; i<16; i++)
16. {
17. sprintf (tmp,"%02X",md[i]);
18. strcat (buf,tmp);
19. }
20. printf ("%s\n",buf);
21. return0;
22. }

Step 2: Put the code in the following directory:

/openssl-1.1.0c/test

Step 3: Compile MD5test1.c.

[root@localhost test]# gcc -o MD5test1 MD5test1.c

/tmp/cczP8S6Z.o: In function `main':

MD5test1.c:(.text+0x6b): undefined reference to `MD5_Init'

MD5test1.c:(.text+0x8d): undefined reference to `MD5_Update'

MD5test1.c:(.text+0xa0): undefined reference to `MD5_Final'

collect2: error: ld returned 1 exit status

Three errors occurred.

The reason for the error is

The library containing the md5 function is /usr/lib/libcrypto.a(.so), use -lcrypto when compiling.

Step 4: Recompile MD5test1.c.

[root@localhost test]# gcc -o MD5test1 MD5test1.c  -lcrypto

Compilation succeeded.

Step 5: Run MD5test1

[root@localhost test]# ./MD5test1 

202CB962AC59075B964B07152D234B70