how to read a particular string from a buffer

Question

i have a buffer

char buffer[size];

which i am using to store the file contents of a stream(suppose pStream here)

HRESULT hr = pStream->Read(buffer, size, &cbRead );

now i have all the contents of this stream in buffer which is of size(suppose size here). now i know that i have two strings

"<!doctortype html" and ".html>" 

which are present somewhere (we don't their loctions) inside the stored contents of this buffer and i want to store just the contents of the buffer from the location

"<!doctortype html" to another string ".html>"  

in to another buffer2[SizeWeDontKnow] yet.

How to do that ??? (actually contents from these two location are the contents of a html file and i want to store the contents of only html file present in this buffer). any ideas how to do that ??

Answer 1

You can use strnstr function to find the right position in your buffer. After you've found the starting and ending tag, you can extract the text inbetween using strncpy, or use it in place if the performance is an issue.
You can calculate needed size from the positions of the tags and the length of the first tag
nLength = nPosEnd - nPosStart - nStartTagLength

Answer 2

Look for HTML parsers for C/C++.

Another way is to have a char pointer from the start of the buffer and then check each char there after. See if it follows your requirement.

Answer 3

Are you limited to C, or can you use C++?

In the C library reference there are plenty of useful ways of tokenising strings and comparing for matches (string.h):

http://www.cplusplus.com/reference/cstring/

Using C++ I would do the following (using buffer and size variables from your code):

    // copy char array to std::string
    std::string text(buffer, buffer + size);

    // define what we're looking for
    std::string begin_text("<!doctortype html");
    std::string end_text(".html>");

    // find the start and end of the text we need to extract
    size_t begin_pos = text.find(begin_text) + begin_text.length();
    size_t end_pos = text.find(end_text);

    // create a substring from the positions
    std::string extract = text.substr(begin_pos,end_pos);

    // test that we got the extract
    std::cout << extract << std::endl;

If you need C string compatibility you can use:

char* tmp =  extract.c_str();

Answer 4

If that's the only operation which operates on HTML code in your app, then you could use the solution I provided below (you can also test it online - here). However, if you are going to do some more complicated parsing, then I suggest using some external library.

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

int main()
{
    const char* beforePrefix = "asdfasdfasdfasdf";
    const char* prefix = "<!doctortype html";
    const char* suffix = ".html>";
    const char* postSuffix = "asdasdasd";

    unsigned size = 1024;
    char buf[size];
    sprintf(buf, "%s%sTHE STRING YOU WANT TO GET%s%s", beforePrefix, prefix, suffix, postSuffix);

    cout << "Before: " << buf << endl;

    const char* firstOccurenceOfPrefixPtr = strstr(buf, prefix);
    const char* firstOccurenceOfSuffixPtr = strstr(buf, suffix);

    if (firstOccurenceOfPrefixPtr && firstOccurenceOfSuffixPtr)
    {
        unsigned textLen = (unsigned)(firstOccurenceOfSuffixPtr - firstOccurenceOfPrefixPtr - strlen(prefix));
        char newBuf[size];
        strncpy(newBuf, firstOccurenceOfPrefixPtr + strlen(prefix), textLen);
        newBuf[textLen] = 0;

        cout << "After: " << newBuf << endl;
    }

    return 0;
}

EDIT I get it now :). You should use strstr to find the first occurence of the prefix then. I edited the code above, and updated the link.