problem with char* block[5000] and binary reading

[ishmael4]

hello everyone!
i have a problem with reading from binary file. i was googling and
searching, but i just cant understand, why isnt this code working. i could
use any help. here's the source code:

--cut here--

typedef struct pkg_ {
short int info;
char* data[5000];
} pkg;

TFileStream* File;

void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);

while(File->Position<Fil e->Size)
{
char *block[5000];
pkg* newPkg=new pkg;
File->Read(block,500 0);
memcpy(*(newPkg->data),*(block) ,5000); //this line
newPkg->info=0;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
}
pkg* newPkg=new pkg;
newPkg->info=status;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
delete File;
return;
}

--cut here--

i was trying
memcpy(*(newPkg->data),*(block) ,5000); //this line

and getting "access violation" message, as well as
memcpy(newPkg->data,block,500 0); //this line
but in this case "newPkg->data" appears to be a simple pointer, not an array
of pointers, as "block". And during debugging the following information
about contents of these variables are shown:
newPkg->data:=009056 8C
block={:57200A0 D, :6D756C6F, etc. etc. (the actual content of a binary
file)}
juest after instruction "memcpy(new Pkg->data,block,500 0); //this line".
Please help me.

[mlimber]

ishmael4 wrote:

hello everyone!
i have a problem with reading from binary file. i was googling and
searching, but i just cant understand, why isnt this code working. i could
use any help. here's the source code:

--cut here--

typedef struct pkg_ {
short int info;
char* data[5000];
You realize this is an array of 5000 pointers-to-char, not 5000 bytes,
right?
} pkg;

TFileStream* File;

void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);

while(File->Position<Fil e->Size)
{
char *block[5000];
Same thing here.
pkg* newPkg=new pkg;
File->Read(block,500 0);
memcpy(*(newPkg->data),*(block) ,5000); //this line
newPkg has only 5000 pointers allocated, no data. Your dereferencing of
the pointer is certainly an error. Dereferencing block may also be an
error depending on what happens in TFileStream::Re ad().
newPkg->info=0;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
}
pkg* newPkg=new pkg;
newPkg->info=status;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
delete File;
return;
}
You may want to consider using a smart pointer like std::auto_ptr
instead of all your news and deletes. It gives you exception safety at
no performance cost, and it means less for you to remember.
--cut here--

[snip]

Done.

Cheers! --M

[mlimber]

mlimber wrote:

ishmael4 wrote:

[snip]

pkg* newPkg=new pkg;
File->Read(block,500 0);
memcpy(*(newPkg->data),*(block) ,5000); //this line

newPkg has only 5000 pointers allocated, no data. Your dereferencing of
the pointer is certainly an error. Dereferencing block may also be an
error depending on what happens in TFileStream::Re ad().

[snip]

Allow me to clarify my statement. *(newPkg->data) is the same as
newPkg->data[0], but this location is not a char but a pointer-to-char
and that pointer is clearly uninitialized. Either you meant to have an
array of chars or you need to allocate some space for each of those
5000 pointers to point to, e.g.,

pkg::pkg()
{
for( int i=0; i < 5000; ++i )
data[i] = new char[ 30 ];
}

pkg::~pkg()
{
for( int i=0; i < 5000; ++i )
delete data[i];
}

Cheers! --M

[mlimber]

mlimber wrote:
[snip]

pkg::~pkg()
{
for( int i=0; i < 5000; ++i )
delete data[i];
}

That should be:

delete [] data[i];

Cheers! --M

[Bryan Donlan]

ishmael4 wrote:

hello everyone!
i have a problem with reading from binary file. i was googling and
searching, but i just cant understand, why isnt this code working. i could
use any help. here's the source code:

--cut here--

typedef struct pkg_ {
short int info;
char* data[5000];

This creates an array of 5000 char pointers, not a buffer of 5000 chars.
Try: char data[5000];

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(nzi)i)(λz.λi .i(iz)))zi)))(( λn.λz.λi.n(n zi)i)(λz.λi.i (iz)))zi))

[John Harrison]

ishmael4 wrote:

hello everyone!
i have a problem with reading from binary file. i was googling and
searching, but i just cant understand, why isnt this code working. i could
use any help. here's the source code:

--cut here--

typedef struct pkg_ {
short int info;
char* data[5000];
} pkg;

TFileStream* File;

void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);

while(File->Position<Fil e->Size)
{
char *block[5000];
pkg* newPkg=new pkg;
File->Read(block,500 0);
memcpy(*(newPkg->data),*(block) ,5000); //this line
newPkg->info=0;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
}
pkg* newPkg=new pkg;
newPkg->info=status;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( newPkg));
delete newPkg;
delete File;
return;
}

--cut here--

i was trying
memcpy(*(newPkg->data),*(block) ,5000); //this line

and getting "access violation" message, as well as
memcpy(newPkg->data,block,500 0); //this line
but in this case "newPkg->data" appears to be a simple pointer, not an array
of pointers, as "block". And during debugging the following information
about contents of these variables are shown:
newPkg->data:=009056 8C
block={:57200A0 D, :6D756C6F, etc. etc. (the actual content of a binary
file)}
juest after instruction "memcpy(new Pkg->data,block,500 0); //this line".
Please help me.

Almost certainly you want

char block[5000];

not

char *block[5000];

and

typedef struct pkg_ {
short int info;
char data[5000];
} pkg;

not

typedef struct pkg_ {
short int info;
char* data[5000];
} pkg;

john

[ishmael4]

> Almost certainly you want

char block[5000];

not

char *block[5000];

and

typedef struct pkg_ {
short int info;
char data[5000];
} pkg;

not

typedef struct pkg_ {
short int info;
char* data[5000];
} pkg;

john

fixed sourcecode:

--cut here--

typedef struct pkg_ {
short int info;
char data[5000];
} pkg;

void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);

while(File->Position<Fil e->Size)
{
pkg* newPkg=new pkg;
File->Read(newPkg->data,5000);
newPkg->info=0;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( pkg));
delete newPkg;
}
pkg* newPkg=new pkg;
newPkg->info=status;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( pkg));
delete newPkg;
delete File;
return;
}

--cut here--

this one works correctly with txt files, but not with binary. For some
reason,
File->Read(newPkg->data,5000); reads data from binary only when data is
char* data[5000]; (but not inside the structure[?!?]). Here's info about
File->Read():

--begin here--

virtual int __fastcall Read(void *Buffer, int Count);

Description

Use Read to read data from the resource associated with the handle stream
when the number of bytes in the file is not known. Read transfers up to
Count bytes from the resource, starting at the current position, and then
advances the current position in the resource by the number of bytes
actually transferred. Read returns the number of bytes actually transferred,
which may be less than Count if the end of file marker is encountered.

All other data-reading methods of a handle stream (ReadBuffer,
ReadComponent) call Read to do the actual reading.

--end here--

I've read a few times, but i can't figure out, why Buffer has to be char*
buffer[count], and for some reason it has to be outside the structure (see
the previous code). But maybe my english isnt well enough to understand the
description of this function. Summing up, i need to load a block of binary
data and it works only with char* bufer[5000] that is outside the structure.
i cant copy it to similar variable inside the structure. char buffer[5000]
works with textfiles (debug info: newPkg->data:="blahbla hblah"), but doesnt
with binary. sorry if it seems a bit messed up, but i am working on this
project since a week 10 hours a day and i grow depressed about it. thanks in
advance for any help.

ishmael4

[mlimber]

ishmael4 wrote:

fixed sourcecode:

--cut here--

typedef struct pkg_ {
short int info;
char data[5000];
} pkg;
This typedef method is C-style and unnecessary in C++. Just write:

struct pkg {
short int info;
char data[5000];
} pkg;
void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);
Why is File a file-scope variable if you always new and delete it here.
Why not just create an object on the stack like this:

TFileStream File( FName, fmOpenRead );

[snip]
this one works correctly with txt files, but not with binary. For some
reason,
File->Read(newPkg->data,5000); reads data from binary only when data is
char* data[5000]; (but not inside the structure[?!?]). Here's info about
File->Read():

--begin here--

virtual int __fastcall Read(void *Buffer, int Count);

Description

Use Read to read data from the resource associated with the handle stream
when the number of bytes in the file is not known. Read transfers up to
Count bytes from the resource, starting at the current position, and then
advances the current position in the resource by the number of bytes
actually transferred. Read returns the number of bytes actually transferred,
which may be less than Count if the end of file marker is encountered.

All other data-reading methods of a handle stream (ReadBuffer,
ReadComponent) call Read to do the actual reading.

--end here--

I've read a few times, but i can't figure out, why Buffer has to be char*
buffer[count], and for some reason it has to be outside the structure (see
the previous code). But maybe my english isnt well enough to understand the
description of this function. Summing up, i need to load a block of binary
data and it works only with char* bufer[5000] that is outside the structure.
i cant copy it to similar variable inside the structure. char buffer[5000]
works with textfiles (debug info: newPkg->data:="blahbla hblah"), but doesnt
with binary. sorry if it seems a bit messed up, but i am working on this
project since a week 10 hours a day and i grow depressed about it. thanks in
advance for any help.

You might need to pass a flag into the TFileStream constructor to tell
it to operate in binary mode, but this class isn't standard C++, is
unfamiliar to me, and is, strictly speaking, off-topic in this forum.
Could you dump that TFileStream and use the standard <fstream> instead.
It would be much easier for us to help you get it to work in binary or
text mode.

Cheers! --M

[mlimber]

mlimber wrote:

This typedef method is C-style and unnecessary in C++. Just write:

struct pkg {
short int info;
char data[5000];
} pkg;

Correction: delete the "pkg" after the "}".

[Ali Çehreli]

"ishmael4" <is******@wp.pl > wrote in message
news:dg******** **@atlantis.new s.tpi.pl...

typedef struct pkg_ {
short int info;
char data[5000];
Don't use magical constants define the buffer size somewhere before the
definition of pkg. For example:

size_t const bufferSize = 5000;

Use only bufferSize in the code.
} pkg;
Also, you don't need the 'typedef struct' idiom in C++. Just do

struct pkg { /* ... */};
void SendFile(char* FName, int status)
{
File=new TFileStream(FNa me,fmOpenRead);

while(File->Position<Fil e->Size)
Himmm... You are not incrementing File->Position by one in the loop. Your
test above is not safe. There is a danger of attempting to read more than
there is in the file.
{
pkg* newPkg=new pkg;
File->Read(newPkg->data,5000);
There you go! What if File->Position was one less than File->Size? You would
attempt to read more. The documentation below for Read says that what it
returns is the number of bytes actually transfered. Don't ignore that value.

Also, chances are, if 0-based, Position will probably never equal Size. It's
range will be [0,Size-1].
newPkg->info=0;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( pkg));
delete newPkg;
}
pkg* newPkg=new pkg;
newPkg->info=status;
Form1->SS1->Socket->Connections[0]->SendBuf((void* )newPkg,sizeof( pkg));
delete newPkg;
delete File;
return;
}

--cut here--

this one works correctly with txt files, but not with binary. For some
reason,
File->Read(newPkg->data,5000); reads data from binary only when data is
char* data[5000]; (but not inside the structure[?!?]). Here's info about
char vs. char* has nothing directly related to the problem. It seems to work
for you probably because sizeof(char*) is likely 4 times sizeof(char) on
your system. Allocating more than what you need is hiding the bug.

Here is the bit about the return value of Read:
Read returns the number of bytes actually transferred, which may be less
than Count if the end of file marker is encountered.

Ali