Question

I’ve a binary data which contains a text. The text is known. What could be a fast method to search for that text:

As an eg.

This is text 1—

!@##$%%#^%&!%^$! <= Assume this line is 3 MB of binary data

Now, This is text 2 —

!@##$%%#^%&!%^$! <= Assume this line is 2.5 MB of binary data

This is text 3 —

How can I search for text This is text 2.

Currently I’m doing like:

size_t count = 0;
size_t s_len = strlen("This is text 2");

//Assume data_len is length of the data from which text is to be found and data is pointer (char*) to the start of it.
for(; count < data_len; ++count)
{
    if(!memcmp("This is text 2", data + count, s_len)
    {
         printf("%s\n", "Hurray found you...");
    }
}

• Is there any other way, more efficient way to do this
• Will replacing ++count logic with memchr('T') logic help <= Please ignore if this statement is not clear
• what should be the average case big-O comlexity of memchr

Answer 1

There’s nothing in standard C to help you, but there is a GNU extension memmem() that does this:

#define TEXT2 "This is text 2"

char *pos = memmem(data, data_len, TEXT2, sizeof(TEXT2));

if (pos != NULL)
    /* Found it. */

If you need to be portable to systems that don’t have this, you could take the glibc implementation of memmem() and incorporate it into your program.