How to Calculate an MD5 Hash from a File in C ?

Calculating MD5 Hash from Files in C

Getting the MD5 hash of a file in C is essential for data security and integrity checks. In this article, we'll explore a straightforward C implementation that can compute and display the MD5 hash of a file.

The implementation leverages the OpenSSL library to perform the MD5 calculations. It includes functions for:

• Reading a file into a buffer
• Using the MD5 function to calculate the hash
• Printing the result as hex-digits

The code starts by opening the specified file and mapping it into memory using mmap(). This allows the application to work with the file as a buffer in memory, facilitating efficient MD5 computation. The MD5() function then calculates the hash and stores it in a predefined array.

Finally, the print_md5_sum() function converts the MD5 hash into hex-digits and prints them alongside the file name. This allows for easy verification and comparison of file integrity.

Here's the code:

#include <openssl/md5.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

unsigned char result[MD5_DIGEST_LENGTH];

// Print the MD5 sum as hex-digits.
void print_md5_sum(unsigned char *md) {
    int i;
    for (i = 0; i < MD5_DIGEST_LENGTH; i++) {
        printf("%02x", md[i]);
    }
}

// Get the size of the file by its file descriptor
unsigned long get_size_by_fd(int fd) {
    struct stat statbuf;
    if (fstat(fd, &statbuf) < 0) exit(-1);
    return statbuf.st_size;
}

int main(int argc, char *argv[]) {
    int file_descript;
    unsigned long file_size;
    char *file_buffer;

    if (argc != 2) {
        printf("Must specify the file\n");
        exit(-1);
    }
    printf("using file:\t%s\n", argv[1]);

    file_descript = open(argv[1], O_RDONLY);
    if (file_descript < 0) exit(-1);

    file_size = get_size_by_fd(file_descript);
    printf("file size:\t%lu\n", file_size);

    file_buffer = mmap(0, file_size, PROT_READ, MAP_SHARED, file_descript, 0);
    MD5((unsigned char *)file_buffer, file_size, result);
    munmap(file_buffer, file_size);

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

    return 0;
}

The above is the detailed content of How to Calculate an MD5 Hash from a File in C ?. For more information, please follow other related articles on the PHP Chinese website!