strstr not able to search the string inside the data in octal format

Question

I am trying to find a string in the given file (actually the file is tar file(please pay attention here) and i opened the file in notepad++ and took randomly a string from that opened file) and i stored that full tar file in a buffer and now i want to find the position of the string i copied using strstr function in the stored buffer.

The code to do is this(which is absolutely correct)-

char *compare= "_.png"; //suppose this is the copied string
//which is to be find out in buffer using strstr
            char * StartPosition;
            StartPosition = strstr (buffer,compare);
            __int64 count=0; 
            MessageBox(m_hwndPreview,L"before the while loop",L"BTN WND6",MB_ICONINFORMATION);
            while (StartPosition!=NULL)
            {
                MessageBox(m_hwndPreview,L"hurr inside the while loop",L"BTN WND6",MB_ICONINFORMATION);
                MessageBoxA(m_hwndPreview,strerror(errno),"BTN WND4", MB_ICONINFORMATION);
                count=StartPosition-buffer+1;
                return 0;
            }

and suppose if i have content of tar file in notepad like as below from where i copied this string stored in compare-

3_VehicleWithKinematicsAndAerodynamics_.000.png  IHDR (here is some strange data which can't be copied and also there are lot of NULL but we have to find out the position of "_.png" so not so difficult in this case ).

The question is my code works fine until i store the data before the .png then i am able to find its position using strstr the problem is when i try to find out the string postion which is appearing after

`3_VehicleWithKinematicsAndAerodynamics_.000.png  IHDR ...suppose here we have strange data (which is data block if we see the tar parser)...after this we have another file  like..."3_VehicleWithKinematicsAndAerodynamics_.html"`

and if i want to find this "3_VehicleWithKinematicsAndAerodynamics_.html" using strstr then i am not able to find it due to strange data in between them.(because i think that those data are not recognized by the compiler and dut to that i am not able to access the file which is located after the strange data) to make more clear see the location of file in tar file is as follows-

3_VehicleWithKinematicsAndAerodynamics_.000.png  IHDR ............(its data blocl-which is strange contents if you open in tar file)....3_VehicleWithKinematicsAndAerodynamics_.000.html

i have to access the .html file using strstr . why it is not accessing it ?? any ideas ?? *

PLEASE GIVE THE ALTERNATIVES TO ACHIEVE IT..I am sure what i try won't work..

Answer 1

A C style string is a number of characters terminated by a zero-character (NUL character - the value zero, not the character '0'). This means that strstr will stop as soon as it hits such a byte.

One solution that is quite plausible is to simply write a function that searches through binary data based on it's length, not on a "terminating character".

Something like this (this still assumes that the str is a C style string):

 char *find_str_in_data(const char *str, const char *data, int length)
 {
    int pos = 0;
    int slen = strlen(str);
    while(pos < length-slen)
    {
       int i = 0;
       while(i < slen && str[i] = data[pos+i])
       {
           i++;
       }
       if (i == slen)
          return data + pos;
   }
   return NULL;
}

Answer 2

If you really want to use strstr then you need to escape the string contained in buffer with '\0'. If you know the size of data that was put into the buffer (let's say, sizeOfData), than you could do something like this before you use strstr:

buffer[sizeOfData] = '\0';

Warning: if sizeOfData is equal to the size of buffer, then you will either need a greater buffer or overwrite the last character with '\0' (in the second case you should check the buffer tail manually, because the character you've overwritten could be one of the characters of sequence you are looking for).