How to check if a string is in a binary file

Question

i am making a program that gets two binary files, and checks if the second file (string) is in the first file. I tried to use strstr function but it doesnt work. This is that part of my code: Am i reading the files right?

    fseek(fileToCheckv, 0 , SEEK_END);
    size = ftell(fileToCheckv);
    rewind(fileToCheckv);
    fseek(virusSignit, 0L, SEEK_END);
    vsize = ftell(virusSignit);
    rewind(virusSignit);
    buffer = (char*)realloc(buffer, size+1 * sizeof(char));
    virusSig = (char*)realloc(virusSig, vsize+1 * sizeof(char));
    buffer[size] = 0;
    virusSig[vsize] = 0;
    fread(buffer,1 , size, fileToCheckv);
    fread(virusSig,1 ,vsize, virusSignit);
    result = strstr(buffer, virusSig);
    if (result != NULL)
    {
        printf("\nVirus was found in file: %s\n", fileToOpen);
    }
    else
    {
        printf("The virus was not found\n");
    }

_size+1 * sizeof(char)_ == size + sizeof(char)... I guess that is not what you meant — CIsForCookies, Jun 01 '17 at 11:36
Of course `strstr` wont work because it operates on NUL terminated strings. You need to write your own "binbin" function having e.g. this signature: `char *binbin(const char *needle, const char *haystack, int length)`. I didn't check for other issues though. — Jabberwocky, Jun 01 '17 at 11:41
Since the fread will copy the data into a char array that has a 0 in it's end, won't this be just as NUL terminated strings? — CIsForCookies, Jun 01 '17 at 11:43
@CIsForCookies the binary data itself could contain NUL bytes anywhere. — Jabberwocky, Jun 01 '17 at 11:44
@MichaelWalz I see... but in case with strings not containing NUL bytes, this will work, wouldn't it? — CIsForCookies, Jun 01 '17 at 11:46
@MichaelWalz There is the memmem() function, which is basically strstr(), but allowing NULs in the "strings". It has **four** arguments, two pointers and two sizes. NOTE: *This function is a GNU extension.* — joop, Jun 01 '17 at 12:02
@joop `memmem` is a gnu extension. It doesn't exist on Windows, but it's not very hard to rewrite. — Jabberwocky, Jun 01 '17 at 12:11
@user8097385 [this SO question](https://stackoverflow.com/questions/2188914/c-searching-for-a-string-in-a-file) may help. — Jabberwocky, Jun 01 '17 at 12:23
In C a _string_ is a sequence of characters up to and including a ending _null chracter_. With _binary_ files, "checks if the second file is in the first file." does not make sense as the second files contents may not constitute a _string_. — chux - Reinstate Monica, Jun 01 '17 at 15:44

CIsForCookies · Answer 1 · 2017-06-01T11:59:15.300

You are opening the files correctly, but you have some other minor issues:

buffer = (char*)realloc(buffer, size+1 * sizeof(char));. You probably meant to do (size+1) * sizeof(char) of simply size+1 as sizeof(char) will be always 1. This problem occurs twice in your code
In the same line' you use realloc without checking if the pointer is NULL. That could prove problematic if you allocation fails
As @Michael Walz said, strstr() is for NUL-terminated strings, so for binary you should either create your own strstr-like function for binary, or verify there aren't NUL-bytes in your strings

How to check if a string is in a binary file

1 Answers1