how to skip a file inside the tar file to get a particular file

Question

i am tring to get the contents of a html file which is present inside the tar file(i am using visual c++ to accomplish my task). my approach is to store the tar in a buffer

Answer 1

Finally i have made the solution for this question the code must be as follow-

char* StartPosition;
size_t skip= 0;
    char HtmlFileContents [200000];
    char contents [8000];
    do
    { 
            int SizeOfFile = CreateOctalToInteger(&buffer[skip+124],11);
            size_t distance= ((SizeOfFile%512) ? SizeOfFile + 512 - (SizeOfFile%512) : SizeOfFile );
            skip += distance + 512;
            memcpy(contents,&buffer[skip],100);
            if (StartPosition=strstr(contents,".html"))
            {
                MessageBox(m_hwndPreview,L"finally string is copied",L"BTN WND6",MB_ICONINFORMATION);
                int SizeOfFile = CreateOctalToInteger(&buffer[skip+124],11);
                memcpy(HtmlFileContents,&buffer[skip+512],SizeOfFile);
                break;
            }


    }
    while(strcmp(contents,".html") != NULL);

I guess its self explantory . and If not ?? Do not hesitate to ask me.

Answer 2

Doesn't look too bad except for the errors :-)

1. you set skip = ... instead of skip += .., so your position in buffer is only correct for the second file
2. You don't check the first file (because it's do { ... } while() and the first time you call strstr(), contents is already filled with buffer at some poition skip > 0).
3. You should also add a 'break' condition to stop looping when you find a 'file name' "".

EDIT and we should of course also check for the tar file size.

I would try it like that:

// I assume size_t bufsize to be the tar file size

size_t skip = 0;
while( bufsize > skip && strcmp( buffer+skip, "" ) != 0 && strstr( buffer+skip, ".html" ) != 0 ) {
     int SizeOfFile = CreateOctalToInteger(&buffer[skip+124],11);
     size_t distance= ((SizeOfFile%512) ? SizeOfFile + 512 - (SizeOfFile%512) : SizeOfFile );
     skip += distance +512;  
}

if( bufsize > skip && strstr( buffer+skip, ".html" ) == 0 ) {
    // hooray
    int SizeOfHTML = CreateOctalToInteger(&buffer[skip+124],11);
    char *htmlData = buffer+skip+512;

    // do stuff with htmlData
}