How to Calculate the MD5 Hash of a File using C ?

Obtaining the MD5 Hash of a File in C

To determine the MD5 hash of a file, one can utilize the following steps:

Step 1: Establish File Descriptor and Determine File Size

Using the open() function, acquire a file descriptor for the target file. Subsequently, utilize the fstat() function to determine the file's size.

Step 2: Map File into Memory

Employ the mmap() function to map the file into memory, providing read-only access. This mapping facilitates direct access to the file's contents.

Step 3: Compute MD5 Hash

Utilizing the OpenSSL library, invoke the MD5() function to compute the MD5 hash of the file's contents.

Step 4: Unmap File from Memory

Once the MD5 hash has been computed, employ the munmap() function to unmap the file from memory. This step releases the file mapping.

Step 5: Print MD5 Hash with File Name

Display the computed MD5 hash along with the corresponding file name for reference.

The following C code snippet demonstrates this process:

#include <openssl/md5.h>

void computeMD5(const char* filename) {
  // Establish file descriptor and determine file size
  int fd = open(filename, O_RDONLY);
  struct stat statbuf;
  fstat(fd, &statbuf);
  size_t file_size = statbuf.st_size;

  // Map file into memory
  char* file_buffer = (char*)mmap(0, file_size, PROT_READ, MAP_SHARED, fd, 0);

  // Compute MD5 hash
  unsigned char md5[MD5_DIGEST_LENGTH];
  MD5((unsigned char*)file_buffer, file_size, md5);

  // Unmap file from memory
  munmap(file_buffer, file_size);

  // Print MD5 hash with file name
  printf("MD5 hash of '%s': ", filename);
  for (int i = 0; i < MD5_DIGEST_LENGTH; i++) {
    printf("%02x", md5[i]);
  }
  printf("\n");
}

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