Using OpenSSL's MD5 library



On the Linux machine, there is a command to calculate the MD5 value of the file, that is md5sum, if not, you need to install the RPM package: Coreutils.

Now we can also easily compute the MD5 value of the file using the OpenSSL library. The main function to use is

int Md5_init (Md5_ctx *c);
int Md5_update (md5_ctx *c, const void *data, size_t len);
int md5_final (unsigned char *md, md5_ctx *c);

Before using the library, you need to install the OpenSSL RPM package first. These three functions do not need to be explained, see the following example to know:

#include <openssl/md5.h> #include <iostream> #include <cstdio> #include <iomanip> using


namespace Std;
        int main () {FILE *fd=fopen ("test", "R");
        Md5_ctx C;
        unsigned char md5[17]={0};
                if (fd = NULL) {cout << "open failed" << Endl;
        return-1;
        int Len;
        unsigned char *pdata = (unsigned char*) malloc (1024*1024*1024);
                if (!pdata) {cout << "malloc failed" << Endl;
        return-1;
        } md5_init (&AMP;C);
        while (0!= (len = fread (PData, 1, 1024*1024*1024, FD)) {md5_update (&c, PData, Len);
        } md5_final (MD5,&AMP;C); for (int i = 0; i < i++) cout << hex << setw (2) << Setfill (' 0 ') << (int) md5[
        I];
        cout << Endl;
        Fclose (FD);
        Free (pData);
return 0; }

The compile link options are:

g++ Test.cpp-o Testmd5-lcrypto-lssl

After generating the testmd5, we put a test file of more than 300 m into a directory with TESTMD5. And comparing the results generated by TESTMD5 with the results of the Linux command "md5sum test", you will find the same.

int GetFileMD5 (std::string strfile, std::string& strMD5) {int nlen = 0;
	unsigned int nbuffersize = 1024 * 1024;
	Char sztemp[4];
	unsigned char szmd5[20];
	
	Md5_ctx CTX;
	Set buffer size by file size Ace_stat stfileinfo;
	memset (&stfileinfo, 0, sizeof (ACE_STAT));
	Nret = Ace_os::lstat (Strfile.c_str (), &stfileinfo);
		if (nret!= 0) {printf ("Stat file error[%s].\n", Strfile.c_str ());
	return-1;
	} if (Stfileinfo.st_size < nbuffersize) {nbuffersize = stfileinfo.st_size + 64;
	}//Open file File *FP = fopen (Strfile.c_str (), "R");
		if (fd = NULL) {printf ("fopen error:%s.\n", Strfile.c_str ());
	return-1;
	} unsigned char *pdatabuffer = (unsigned char*) malloc (nbuffersize);
		if (NULL = = Pdatabuffer) {printf ("malloc error:%d.\n", nbuffersize);
	Fclose (FP) return-1;
	}//Get MD5 md5_init (&AMP;CTX);
		while (true) {memset (pdatabuffer, 0, nbuffersize);
		Nlen = Fread (Pdatabuffer, 1, nbuffersize, FP);
		if (Nlen = = 0) {break; } md5_upDate (&ctx, Pdatabuffer, Nlen);
	} md5_final (SzMD5, &ctx);
	MD5 to string strMD5 = "";
		for (int i = 0; i < i++) {memset (sztemp, 0, sizeof (sztemp));
		snprintf (sztemp, sizeof (sztemp), "%02x", (int) szmd5[i]);
	StrMD5 + = sztemp;
	Fclose (FP);
		if (NULL!= pdatabuffer) {free (pdatabuffer);
	Pdatabuffer = NULL;
return 0;
 }