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Full Discussion: Computing an MD5Sum in C
Top Forums Programming Computing an MD5Sum in C Post 302411808 by Corona688 on Friday 9th of April 2010 12:56:54 PM
Old 04-09-2010
Your example doesn't sound like C code at all, can you show your program?

There is indeed a library you can include, see 'man md5'. You use it by feeding MD5_Update arrays of data, which you can read from any source.
Code:
#include <openssl/md5.h>
#include <unistd.h>
int main()
{
        int n;
        MD5_CTX c;
        char buf[512];
        ssize_t bytes;

        MD5_Init(&c);
        bytes=read(STDIN_FILENO, buf, 512);
        while(bytes > 0)
        {
                MD5_Update(&c, buf, bytes);
                bytes=read(STDIN_FILENO, buf, 512);
        }

        MD5_Final(out, &c);

        for(n=0; n<MD5_DIGEST_LENGTH; n++)
                printf("%02x", out[n]);

        return(0);        
}

It reads only from stdin in this example. It outputs the same hash as the commandline md5sum command for the same data. You must link the program with -lssl.

Note that MD5 has been cracked; people can generate disparate strings with the same MD5 hash pretty much on command. For simple checksums this may not be important, for things where security is important MD5 is no good, SHA1 is better.
 

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LEARN ABOUT REDHAT

md5sum

MD5SUM(1)								FSF								 MD5SUM(1)

NAME

       md5sum - compute and check MD5 message digest

SYNOPSIS

       md5sum [OPTION] [FILE]...
       md5sum [OPTION] --check [FILE]

DESCRIPTION

       Print or check MD5 (128-bit) checksums.	With no FILE, or when FILE is -, read standard input.

       -b, --binary
	      read files in binary mode (default on DOS/Windows)

       -c, --check
	      check MD5 sums against given list

       -t, --text
	      read files in text mode (default)

   The following two options are useful only when verifying checksums:
       --status
	      don't output anything, status code shows success

       -w, --warn
	      warn about improperly formated checksum lines

       --help display this help and exit

       --version
	      output version information and exit

       The  sums  are computed as described in RFC 1321.  When checking, the input should be a former output of this program.  The default mode is
       to print a line with checksum, a character indicating type (`*' for binary, ` ' for text), and name for each FILE.

AUTHOR

       Written by Ulrich Drepper and Scott Miller.

REPORTING BUGS

       Report bugs to <bug-coreutils@gnu.org>.

COPYRIGHT

       Copyright (C) 2002 Free Software Foundation, Inc.
       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
       LAR PURPOSE.

SEE ALSO

       The  full documentation for md5sum is maintained as a Texinfo manual.  If the info and md5sum programs are properly installed at your site,
       the command

	      info md5sum

       should give you access to the complete manual.

md5sum (coreutils) 4.5.3					   February 2003							 MD5SUM(1)
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