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How to write a struct to a file?

Posted on 2006-07-13
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Last Modified: 2010-04-01
Hi!

I would like to know if there is any way to write a entira struct to a file.
I have a struct that i have got from a TCP connection it looks like this.

TCPConn2->ReadBuffer(VMes,sizeof(*mes));
mes = static_cast<Message*>(VMes);

The mes is the structure and it looks like this

typedef struct{
        __int32 Param_code;
        unsigned __int32 Time;
        __int32 Measure_Value;
}S_record;

typedef struct{
        //--------HEADERS-------
        unsigned __int16 STX;
      unsigned __int16 CRC;
        unsigned __int16 MachID;
        unsigned __int16 ClinicalSWID;
        unsigned __int32 MsgCounter;
        unsigned __int32 CmdCode;
        unsigned __int32 MsgInfo;
        unsigned __int32 Flags;
        unsigned __int32 PatId;
        //----------------------
        S_record records[412];  
}Message;

I would now like to write mes to a file. I know that i can use ifstream but then i have to convert each element to string if i have't missunderstod it.
Is there a way to write the data without converting it?
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Question by:mikrodidakt
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bastibartel earned 250 total points
ID: 17098220
Hi mikrodidakt,

You can write the entire struct to file in a binary format. That is, if you are sure the struct will not change in the near future ;-)


#include <fstream>
Message Msg;

std::ofstream oStream;
oStream.open("c:\\full path\\out.txt",std::fstream::out|std::ios::binary);
oStream.write(Msg, sizeof(Msg));
oStream.close()

Please do verify that sizeof(Msg) returns the size of your struct, as I am not absolutely sure the embdeded space for S_record[412] will be reflected properly by sizeof()
Other than that, since you have no dynamic objects in your struct - you are all set.

Cheers!
Sebastian
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Expert Comment

by:bastibartel
ID: 17098223
Uups, it's

oStream.open("c:\\full path\\out.txt",std::fstream::out|std::ios<wbr/>s::binary)<wbr/>;
                                                                                               ^
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by:bastibartel
ID: 17098230
aaahh forget the last post - (screw QuickPost !)
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Expert Comment

by:nafis_devlpr
ID: 17098253
use ofstream and use the binary mode to write into the file, but in case you change the allignment or add anything else in the structure you will have to change the old files.

Nafis
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Expert Comment

by:nafis_devlpr
ID: 17098254
sorry bastibartel, i was a bit late than you.
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Author Comment

by:mikrodidakt
ID: 17098296
thanks
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Author Comment

by:mikrodidakt
ID: 17098334
But why would there be a problem if the size of the struct is changed then the sizeof(mes) will also be change?
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Author Comment

by:mikrodidakt
ID: 17098375
sorry you have alreadt answered the question forget the last one thanks.
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Expert Comment

by:nafis_devlpr
ID: 17098379
when you change the struct, then the size is changed. You wont see any difference if you replace a variable with the same size. But the interpretation of the struct might change because of that
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Expert Comment

by:bastibartel
ID: 17098585
Hi again,

> But why would there be a problem if the size of the struct is changed then the sizeof(mes) will also be change?
Yes that is correct - you just won't be able to import your 'old' structs anymore.

Owing to this potential problem, Microsoft prefixes many structs with a variable holding the struct's size.
Thus you can later determine (by importing the first say 4 bytes), whether a stored struct is compatible with the current version of the struct.

Cheers,
Sebastian
 

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Author Comment

by:mikrodidakt
ID: 17098629
This is for programmers that might have the same problem as me in the future.
Her is my conde.

std::ofstream oStream;
AnsiString path = "C:\\Program\\Borland\\CBuilder6\\Projects\\Prismaflex\\Samples\\Sample";
path += IntToStr(messageNbr);

oStream.open(path.c_str(),std::fstream::out|std::ios::binary);
char* c_msg = (char*) msg;
oStream.write(c_msg, sizeof(msg));
oStream.close();
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Author Comment

by:mikrodidakt
ID: 17099217
Forget the comment above it dont work!!!!!!!!
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