trying to read a file

Question

basically trying to make an anti virus but all I get when trying to read the infected file into a buffer is EOF... it's a jpg and I have no idea how to fix this

about the file functions I'm allowed to use: fread/fwrite fgets fputs fclose fopen fgetc fputc fscanf fprintf

int fullScan(FILE* sign, FILE* infected);
char* getFile(FILE* file);

int main(int argc, char** argv)
{
    FILE* sign = fopen("KittenVirusSign", "rb");
    FILE* infected = fopen("kitten_frog.jpg", "rb");
    int j = 0;
    if (infected == NULL)
    {
        printf("couldn't open the file (suspicious file)");
        return -1;
    }
    if (sign == NULL)
    {
        printf("couldn't open the file (virus signature)");
        return -1;
    }

    j = fullScan(sign, infected);
    return 0;
}

int fullScan(FILE* sign, FILE* infected)
{
    char* sign_c = NULL;
    char* infec_c = NULL;
    int infect_res = -1;
    int sign_len = 0;
    int infec_len = 0;
    int i = 0;
    int j = 0;
    sign_c = getFile(sign);
    infec_c = getFile(infected);
    while (1)
    {

        if (*(infec_c + i) == *(sign_c + j))
        {
            infect_res = 1;
            if (*(sign_c + j) == EOF)
            {
                break;
            }
            else if (*(infec_c + i) == EOF)
            {
                infect_res = -1;
                break;
            }
            i++;
            j++;
            continue;
        }
        else if (*(infec_c + i) != *(sign_c + j))
        {
            if (*(infec_c + i) == EOF || *(sign_c + j) == EOF)
            {
                break;
            }
            i++;
            j = 0;
            infect_res = -1;
        }

    }
    fclose(infected);
    free(sign_c);
    free(infec_c);
    return infect_res;
}


char* getFile(FILE* file)
{
    char* buffer;
    long filelen;
    int i;
    fseek(file, 0, SEEK_END);
    filelen = ftell(file);
    fseek(file, 0, SEEK_SET);
    buffer = (char *)malloc((filelen + 1)*sizeof(char));
    for (i = 0; i < filelen; i++)
    {
        fread(buffer + i, sizeof(char), 1, file);
    }

    return buffer;
}

what actually happens when you run the program? what kind of output are you expecting, and what do you get instead? — Erik Nyquist
- Erik Nyquist I'm expecting to get the binary values of a jpg image (using getFile) and then check to see if the sign (virus signature) exists in the file, the virus signature goes perfectly fine but the image doesn't. — teggr234
when calling any of the memory allocation family of functions: (malloc, calloc, realloc) 1) do not cast the returned value. 2) the expression: sizeof(char) is defined in the C standard as 1 and multiplying by 1 has absolutely no effect on the parameter passed. 3) always check (!=NULL) the returned value to assure the operation was successful — user3629249
when calling the system functions: fseek(), fread(), always check the returned value to assure the operation was successful. — user3629249

John Bollinger John Bollinger · Accepted Answer · 2016-05-21T20:39:02

EOF is a special integer value returned by some input functions to indicate that the end of the file has been reached, but it is not part of the file data. Your fread() will therefore never store an EOF character into the input buffer you provided. However, if your C implementation features signed default chars, as many do, then there is a char value that is numerically equal to EOF (usually -1).

If either file happens to contain that byte, then your code will misinterpret it as designating the end of that file. If it happens to be the first byte in either file then the program will misinterpret the file as being empty.

Since you are analyzing binary files,

I recommend using buffers of unsigned char rather than default char.
All possible byte values can appear in the file data, so you cannot identify the end of the data by the value of any byte within.

Probably, getFile() should return a struct that contains both a pointer to the buffer and its size.

trying to read a file

2 Answers