ReadFile byte count discrepancy

[Slowpoke]

Hello, all,

I am trying to write data to a file that will be read by another, older program.  The data to be written has the following structure:

typedef struct {

     double dd;

     int ii;

     float array1[10];

     unsigned char array2[59];

} STRUCT;

When in my program I step through the ReadFile function in debug mode, ReadFile gives me a bytes-read count of 112.  But when I do the same in the older code, it gives me a bytes-read count of 111, which might explain why the older program cannot read my file correctly.  The older code uses a SetFilePtr statement (SetFilePtr (file, 0, 0)) prior to the ReadFile, but when I do the same in my code, there's no change in the byte count.  What might cause or account for such a discrepancy? 

Thanks,

Slowpoke

[Slowpoke]

Forgot to mention the file in question above was in binary format.  Sorry!

Slowpoke

[Alex D]

My guess would be that your old program makes different assumptions about structure packing than CVI does. CVI reports sizeof(STRUCT) in your example to be 112 when I try it. If you want to force no packing in your CVI program, simply wrap the structure in #pragma pack directives, like this.

#pragma pack(1) // 1-byte packing

typedef struct {
     double dd;
     int ii;
     float array1[10];
     unsigned char array2[59];
} STRUCT;

#pragma pack() // reset to default packing

With this change, I get sizeof(STRUCT) to be 111, as you expected.

Hope this helps,

-alex

[Slowpoke]

Help?  It saved my sanity!  It's just what I needed.  Thanks!

Slowpoke