This is a two-part question, and all is moot if it is not possible to
do random access on a text file.
Part 1.
=====
I would like to be able to check the first few characters of a text
file, and then check the last few characters of a text file to
determine the type of file it is without having to do a line-by-line
check (it's an XML file). I have been unable to get seekp or seekg to
work. Here's a sample of what I've been trying:
string hdr; // varies depending on data
const string ender1("</root>\n")
const string ender2("</root><!-- incomplete -->\n")
fstream f; // will be used for in and out
f.open("f.xml", ios::in);
char c;
string buf;
for (unsigned i = 0; !f.eof() && i < hdr.size(); ++i) {
f.get(c);
buf += c;
}
if (buf != hdr)
; // do something
buf.clear()
f.seekg(ios::end - ender1.size());
for (unsigned i = 0; !f.eof() && i < ender1.size(); ++i) {
f.get(c);
buf += c;
}
if (buf != ender1)
; // do something
// etc.
The first part, the hdr check works fine, but the seek and subsequent
reads don't. I get one character, a newline, regardless of how much i
back up from the end.
What am I doing wrong? Do I need to specify the ios:;binary mode? Is
random seeking of text possible?
I have tried the readsome function instead of the get function but I
can't get it to work either. I have tried the relative form of seekg
with
streampos relpos = -1 * ender1.size()l
f.seekp(relpos, ios::end);
Still no success.
Part 2:
=====
If I could get part 1 to work, then I would like to do this:
f.close();
f.seep(ios::end - ender2.size());
f << data;
...
f << ender1;
f.close();
Essentially these two parts are so I can have a restart capability if
my xml-writing process is interrupted. I will leave the unfinished
xml in a well-formed state, but then I can pick up where I left
without rewriting or copying the file just to add data.
Any ideas or suggestions are very welcome.
Thanks so much.
-Tom
I
