gggrrr, unknown problem with <fstream>

k, i'm gonna paste a bunch of code in here cause i'm not sure what is causing this to generate errors. my problem apears to be in
void pkrEdit::export(string file)

if i compile and run exfile gets written to 0 bytes, however if i single step the program it produces and access violation at the line of code

ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);

hope someone can help with this.


main.cpp

Code:

#include "pkr.h"


int main()
{
	pkrEdit pk;
	string *dirs;
	string *files;
	ofstream test;
	test.open("test.txt", ios::binary | ios::app);
	files = new string[pk.numfiles];
	dirs = new string[pk.numdirs];
	pk.GetDirs(dirs);
	pk.GetFiles(files);
    pk.export(files[20]);
}

pkr.cpp

Code:

#include "pkr.h"

pkrEdit::pkrEdit()
{

	ifAllPkr.open("ALL.PKR", ios::binary | ios::ate);

	ifAllPkr.seekg(0, ios::beg);

	ifAllPkr.read((char *)&HStruct, sizeof(HStruct));

	DStruct = new PK_DIR_STRUC[HStruct.num_dirs];    //create an array of dir_structs and file_structs
	FStruct = new PK_FILE_STRUC[HStruct.num_files];  //= the number of dirs and files

	for(int i = 0; i < HStruct.num_dirs; i++)        //and populate said arrays
	{
		ifAllPkr.read((char *)&DStruct[i], sizeof(PK_DIR_STRUC));
	}

	for(i = 0; i < HStruct.num_files; i++)
	{
		ifAllPkr.read((char *)&FStruct[i], sizeof(PK_FILE_STRUC));
	}
	numfiles = HStruct.num_files;
	numdirs = HStruct.num_dirs;

//	ofAllPkr.open("ALL.PKR", ios::binary | ios::ate);
//	ofAllPkr.seekp(0, ios::beg);
}

string * pkrEdit::GetDirs(string *dirs)
{

	for(int i = 0; i < HStruct.num_dirs; i++)
	{
	
		dirs[i] = DStruct[i].path;

	}
	return(dirs);

}

string * pkrEdit::GetFiles(string *files)
{
	for(int i = 0; i < HStruct.num_files; i++)
	{
		files[i] = FStruct[i].name;
	}
	return(files);
}

void pkrEdit::import(char *oldFile, char *newfile)
{}

void pkrEdit::export(string file)
{
	char *buffer;
	for(int i = 0; i < HStruct.num_files; i++)
	{
		if(file == FStruct[i].name)
		{
            ifAllPkr.seekg(FStruct[i].data_offset);
			buffer = new char[FStruct[i].data_len1];
			ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);
			exFile.open(file.c_str(), ios::binary | ios::ate);
			exFile.write(buffer, FStruct[i].data_len1);
			exFile.close();
			delete[] buffer;
		}
	}
}

pkr.h

Code:

#include <fstream>
#include <string>
#include <iostream>

using namespace std;

typedef struct PK_HEAD_STRUC
{
	char ident[4];
	int unknown;// always 1
	int num_dirs;
	int num_files;
}
PK_HEAD;

typedef struct PK_DIR_STRUC
{
	char path[32];
	int file_offset;
	int file_count;
}
PK_DIR;

typedef struct PK_FILE_STRUC
{
	char name[32];
	int unknown; // always -2
	int data_offset;
	int data_len1;
	int data_len2; // always == data_len2
}
PK_FILE;


class pkrEdit 
{
public:
	int numfiles;
	int numdirs;
    pkrEdit();
	string *GetDirs(string *dirs);
	string *GetFiles(string *files);
	void import(char *oldFile, char *newFile);
	void export(string file);

protected:
	PK_HEAD_STRUC HStruct;
	PK_DIR_STRUC *DStruct;
	PK_FILE_STRUC *FStruct;
private:
	ofstream exFile;
	ifstream imFile;
	ofstream ofAllPkr;
	ifstream ifAllPkr;

	
};

ps. thanks in advance for any help.
btw, i know i still need to go back and cleanup a lot of the memory i allocated just havn't got around to it yet

>>ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);
Try:
>>ifAllPkr.read((char *)buffer, FStruct[i].data_len1);
Note the lack of &

still the same, after single steping some more and shuffling some code around i've found that the problem is partialy realated to

FStruct[i].data_len1

as in

ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);

i have verified that FStruct[I].data_len1 is a valid integer
and is being read properly in the class constructor.
so why it should generate an error is beyond me.

however, i have noticed that if i replace all the references
to FStruct[i].data_len1 and replace them with integers such as

ifAllPkr.read((char *)&buffer, 500);

then the read operation is succesfull but
i then get another access violation from
the exfile.open() statement.

hope that made some sense , this has been driving me mad all day

[edit]
after playing around with it a bit more i found that removing the & does kind of work however, the first 120,000 someodd bytes of the output file are blank, after that everything is hunky dorey.
any ideas?
[/edit]

success!!! but i'm at a loss as to explain why.
so here's my revised code if anyone cares to take a crack at
telling me why one works and not the other i'd really aprieciate it.

new code

Code:

void pkrEdit::export(string file)
{
	char *buffer;
	for(int i = 0; i < HStruct.num_files; i++)
	{
		if(file == FStruct[i].name)
		{
		         ifAllPkr.seekg(FStruct[i].data_offset, ios::beg);
			buffer = new char[FStruct[i].data_len1];
			ifAllPkr.read((char *)buffer, FStruct[i].data_len1);
			cout<<FStruct[i].data_len1;
			exFile.open("file.c_str()", ios::binary | ios::ate);
			exFile.write((char *)buffer, FStruct[i].data_len1);
                           exFile.close();
			delete[] buffer;
		}
	}
}

old code

Code:

void pkrEdit::export(string file)
{
     char *buffer;
     for(int i = 0; i < HStruct.num_files; i++)
    {
          if(file == FStruct[i].name)
         {
                ifAllPkr.seekg(FStruct[i].data_offset);
                buffer = new char[FStruct[i].data_len1];
                ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);
                exFile.open(file.c_str(), ios::binary | ios::ate);
                exFile.write(buffer, FStruct[i].data_len1);
                exFile.close();
                delete[] buffer;
           }
      }
}

1) This was wrong, the & makes it mean something completely different.

ifAllPkr.read((char *)&buffer, FStruct[i].data_len1);

it should be

ifAllPkr.read(buffer, FStruct[i].data_len1);

buffer itself is a pointer to a character array -- that is, it holds the address of the array. &buffer is the ADDRESS of the address of the array (a char **). You cast it to a char *, but when it tries to write there, it's not writing into the buffer, it's writing onto the stack where the address of the buffer lived. You corrupt your stack and kill your program.

The very fact it wouldn't compile without a cast should make you seriously question whether you were doing the right thing. Your compiler was smart enough to inform you that you were using a char** where a char* was needed, but you basically went over its head and told it to do it anyway.

2) This line doesn't do what you think it does:

exFile.open("file.c_str()", ios::binary | ios::ate);

This does not open the file whose name is stored in the string file, it opens a file that is actually named "file.c_str()" in the current directory. If it crashed before, it might be because the string within file was garbage.

3) This is a useless cast:

exFile.write((char *)buffer, FStruct[i].data_len1);

buffer's type is already char *, there's no need to convert it. Just do:

exFile.write(buffer, FStruct[i].data_len1);

hey, thanks for the help


grrr, gotta start looking over my code more carefully before i post though

exFile.open("file.c_str()", ios::binary | ios::ate);

for a minute there i was thinking that fstream couldn't accept
file.c_str so i just quickly enclosed it in quotes to see if it would would accept a normal string or if there was something else wrong there. but i forgot to get rid of the quotes when i was done. embarassing

anyway thanks once again to both of you for helping me out.