Same content, different MD5 - File and String

Question

1. I have a file testfile and a string teststring.

2. In a shell I wrote:
   echo "a" > testfile

3. then xxd testfile
   so I can see the hexadecimal values of my filecontent
   output:

   0000000: 610a               a.
   

4. see my code:

   int file;
   struct stat s;
   unsigned long size;
   char* buffer;
   char md5[MD5_DIGEST_LENGTH]
   
   file = open("testfile", O_RDONLY);
   if (file < 0)
       return false;
   
   if (fstat(file, &s) < 0)
   {
       close(file);
       return false;
   }
   
   size = s.st_size;                       //GET FILE SIZE
   printf("filesize: %lu\n", size);        //PRINT FILESIZE FOR DEBUGGING
   buffer = (char*)mmap(0, size, PROT_READ, MAP_SHARED, file, 0); //MAP FILE CONTENT TO BUFFER
   MD5((unsigned char*)buffer, size, md5); //GENERATE MD5
   munmap(buffer, size);                   //UNMAP BUFFER
   close(file);
   
   for (int i = 0; i < MD5_DIGEST_LENGTH; i++)
       printf("%02x", md5[i]);
   printf("\n");
   
   
   unsigned char* teststring = "\x61\x0a"; //SAME STRING AS IN THE FILE
   
   MD5((unsigned char*)teststring, 2, md5);
   for (int i = 0; i < MD5_DIGEST_LENGTH; i++)
       printf("%02x", md5[i]);
   printf("\n");
   

5. it prints:

   filesize: 2  
   60b725f10c9c85c70d97880dfe8191b3  
   e29311f6f1bf1af907f9ef9f44b8328b  
   

   two completely different md5 hashes.
   i tried writing the buffer into a file
   and writing the teststring into a file they are the same!
   by why?
   isn't the buffer the same as the teststring?

Answer 1

The correct hash is your first hash, 60b725f10c9c85c70d97880dfe8191b3.

$ echo "a" | md5
60b725f10c9c85c70d97880dfe8191b3

Your second hash happens to be the hash of "\x64\x0a", or the character 'd' followed by a newline:

$ echo "d" | md5
e29311f6f1bf1af907f9ef9f44b8328b

Are you sure the code you posted is what you are compiling/running? Did you forget to recompile? Are you executing an old binary?