How can i check if binary file's content is found in other binary file?

Since the strstr function won't work here for an arbitrary binary data (it is working only for strings with \0. termination), I can see three approaches here:
1) Naive approach: iterate over one array of bytes, and use memcmp with the other array starting at different positions each time. Easy, but consumes O(k*n) time (k, n - sizes of the data).
2) Using the KMP algorithm. Requires some work on understanding and coding, but giving the best time complexity O(k+n).
3) If the performance is not important, and you don't want to mess with ANY somewhat non-trivial algorithms:
-- Convert your binary datas to strings, representing each byte with it's two digits HEX value.
-- Use strstr.

Update: After a little thinking about the third approach, there might be a case when it won't work right. Consider that you want to find the data represented by AA AA inside 1A AA A1. It shouldn't be found, since it is not there. But, if you represent the data as concatenated characters without delimiters, it will be like find AAAA in 1AAAA1, which will succeed. So adding some delimiter would be a good idea here.

Do it yourself (notify me if there's a bug):

/* Returns location of substring in string. If not found, return -1.
 * ssize_t is defined by POSIX. */
ssize_t bin_strstr(void* data, size_t len, void* subdata, size_t sublen) {
    len -= sublen;
    for ( ; len >= 1; --len)
        if (memcmp(data + len, subdata, sublen) == 0)
            return len;
    return memcmp(data, subdata, sublen) ? 0 : -1;
}