md5.c

#include <stdio.h>
#include <string.h>
#include <openssl/md5.h>

// gcc -lssl md5.c -o md5

int main(int argc, char *argv[]) {
  unsigned char buffer[1024];
  int i;
  unsigned char digest[16];
  MD5_CTX ctx;

  MD5_Init( &ctx );

  switch(argc) {
    // read from stdin
    case 1:
      while(fgets(buffer, sizeof(buffer), stdin)!=NULL) MD5_Update( &ctx, buffer, strlen(buffer)-1 );
      break;
    case 2:
      MD5_Update( &ctx, argv[1], strlen(argv[1]) );
      break;
    default:
      printf("Usage:\n\t%s string [or pipe]\n", argv[0]);
      return -1;
  }

  MD5_Final(digest, &ctx);
  for(i = 0; i < 16; i++) printf("%02x", digest[i]);
  printf("\n");
  return 0;
}