CODEWITHSUNDEEP
clinuxfileoffset
Lipika Deka
Lipika Deka

Reputation: 3884

Calculating file offset after a write

I am writing a program where I log all writes to a file and just before process termination I write all writes in place i.e from log to actual file. Now, in the log each record of type something like :

 offset, length, data

After a write I need to calculate the offset so that I can lseek to that position when i finally write in the actual file so as to write the next write or for read.

Will current offset position+bytes written give me the file offset after a write.

Thanks

Upvotes: 0

Views: 2076

Answers (2)

likern
likern

Reputation: 3954

There are several ways to achieve that:

  1. First Way (In this case position will be updated automatically to how much you have read.)

    1. Read all log content into memory
    2. Sort them by (offset + length) growth
    3. Using consecutive read calls

  2. Second Way (most portable)

    1. Using fgetpos(), fgetpos64() and fsetpos(),fsetpos64() set of functions.

  3. Third Way

    1. Using ftello(), fseeko() and ftello64(), fseeko64() set of functions.

  4. Fourth Way (less portable)

    1. Using ftell(), fseek() and ftell64(), fseek64() set of functions.

The difference between func() and func64() is that func64() uses correct 64-bit offset which eliminates problem with files more than 4Gb, while func() uses int type which could be 32 or 64 bits, depending on the architecture.

The fseeko() and ftello() functions are identical to fseek() and ftell() except that the offset argument of fseeko() and the return value of ftello() is of type off_t instead of long.

As a conclusion the best practice is to use fgetpos64()/fsetpos64() or ftello64()/fseeko64(). As much as possible try to avoid any using of ftell() and fseek().

Upvotes: 0

David Harris
David Harris

Reputation: 2340

Use fgetpos to get the file offset and fsetpos to set the file offset.

Upvotes: 1

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