How do I Calculate the MD5 Hash of a File in C ?

Getting MD5 Hash from File in C : A Step-by-Step Guide

In C , computing the MD5 hash of a file is a crucial task for data integrity verification and secure transmission. To simplify this process, let's dive into a practical implementation.

Step 1: Link to OpenSSL Library

For this implementation, you'll need the OpenSSL library, which provides the MD5 hash algorithm. Use the following line during compilation:

gcc md5.c -o md5 -lssl

Step 2: Define the MD5 Hash

Declare an array to store the MD5 hash value:

unsigned char result[MD5_DIGEST_LENGTH];

Step 3: Open the File and Get Its Size

int file_descript = open(argv[1], O_RDONLY);
unsigned long file_size = get_size_by_fd(file_descript);

Step 4: Map the File into Memory

To efficiently process large files, map the file contents into memory using mmap():

file_buffer = mmap(0, file_size, PROT_READ, MAP_SHARED, file_descript, 0);

Step 5: Compute the MD5 Hash

Use the MD5() function to compute the hash value of the mapped file buffer:

MD5((unsigned char*) file_buffer, file_size, result);

Step 6: Print the MD5 Hash

Display the computed MD5 hash as hexadecimal digits:

print_md5_sum(result);
printf("  %s\n", argv[1]);

Complete Code Snippet:

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <openssl/md5.h>

unsigned char result[MD5_DIGEST_LENGTH];

void print_md5_sum(unsigned char* md){ ... }
unsigned long get_size_by_fd(int fd){ ... }

int main(int argc, char *argv[]){ ... }

This code gives you a comprehensive implementation to calculate the MD5 hash of files in C .

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