Thread: setting a position in a binary file

Apparently, you haven't read the documentation for fseek() all that closely. Either use fseek(file, 56, SEEK_CUR) after reading the data in position 37-40, or fseek(file, 97, SEEK_SET) to position relative to the start of the file.

If you're reading multiple data structures, each struct will be sizeof(your structure) bytes in length. So add an appropriate multiple and use positioning relative to the start of the file to position for the data in position 37-40.

Your description "manual position" is meaningless.

Note that if the structure contains pointers, the file will contain the value of the pointers, not whatever those pointers point at.

> The struct holds various data char *, ints and floats.
Therein lies the problem.

You can't just fwrite the contents of a struct, if it contains pointers.

Code:

struct foo {
   char *a;
} bar;
bar.a = malloc(100);
strcpy(bar.a,"this is a really long string");
fwrite(&bar,sizeof(bar),1,fp);

It will fwrite the pointer, but you won't see your string in the file.

So exactly how are you serialising the struct then?

If you have (as a result) variable length records because you're outputting variable length strings, then using fseek() to go to a particular record suddenly becomes a far less easy thing to do.

Originally Posted by generaltso78

That is actually not a problem, I have no problems storing the Struct in the binary file and retrieving it. The string name gets displayed fine for all my entries.

Really? That means you are using arrays rather than pointers (which is fine) or you have a critically flawed program with symptoms that have yet to emerge.

Yes, The structs are stored in a array and then written to binary file. The Struct contains a char[20], 4 ints and 6 floats. Every time a struct is stored the file size increments by 60 bytes. This is still uncharted territory for me, so bare with my terminology. I made a variable to contain my byte location for fseek. That way I can increment the position through a loop as I run through the file. I haven't figured out the exact increment size Im looking for yet, so I need to keep playing with it. My big concern is that when I increment my byte location variable than eventually fseek will move beyond the end of file and return a value that I am not looking for (would this happen before my EOF condition is triggered?)

Originally Posted by generaltso78

Yes, The structs are stored in a array and then written to binary file. The Struct contains a char[20], 4 ints and 6 floats. Every time a struct is stored the file size increments by 60 bytes. This is still uncharted territory for me, so bare with my terminology. I made a variable to contain my byte location for fseek. That way I can increment the position through a loop as I run through the file. I haven't figured out the exact increment size Im looking for yet, so I need to keep playing with it. My big concern is that when I increment my byte location variable than eventually fseek will move beyond the end of file and return a value that I am not looking for (would this happen before my EOF condition is triggered?)

You need to use the sizeof() operator to get the size of your struct. Manually adding the field sizes might not give the right size because of padding. You should then read a whole struct in and out of the binary file. Whilst you can read a field, it's easy to get it subtly wrong.
The equation is simply pos = sizeof(the_struct) * index;

Originally Posted by generaltso78

This is where my program pushes my mental limits.

So your mental limits are pushed by the need to retrieve a particular field from a repeated series of structs? I've heard of pretty low limits in my time .....

Originally Posted by generaltso78

The positions I want are at 36, 96, 156, 216, 276, etc..... Obviously, my sad attempt at += 20 only works for the next position. I'm thinking I may need to reset the binary position back to 0 after every fseek and then increment my bP(36) by 60.

If you really want to unnecessarily reduce performance of your program when reading, go for it.

Your basic problem is that you're not taking into account the fact that, after fread(), the file pointer will be incremented by the amount successfully read. Either that or you don't understand that relative and absolute positioning in a file are related by a subtraction.

If positioning with fseek() is beyond you, AndiPersti's approach will also work.