CODEWITHSUNDEEP
cfile
user3475234
user3475234

Reputation: 1573

Move the file position without fseek in C

How can I move the file position back x bytes in C?

I don't want to use fseek(), because I'm reading lines of the file with open(fd, buf, buflength), not fopen().

I'd very much like to avoid reading the entire file into memory.

What I want to do is basically:

  1. Read X + A = Y bytes.
  2. Do something with the first X bytes.
  3. Move the file pointer backwards A bytes.
  4. Read Y bytes from there.

I know how to do everything except step 3. Anyone know how?

Upvotes: 0

Views: 2065

Answers (1)

Jerry Jeremiah
Jerry Jeremiah

Reputation: 9618

According to http://rabbit.eng.miami.edu/info/functions/unixio.html you could use lseek:

int lseek(int fd, int position, int startpoint)
  include: <unistd.h>
  Sets the file position effective for the next read or write operation.
  fd = file descriptor as returned by open
  position = position within file:
    number of bytes between startpoint and the desired position, may be negative.
  startpoint = location in file that position is relative to, one of:
    SEEK_SET        Position is number of bytes from beginning of file
    SEEK_END        Position is number of bytes after end of file
    SEEK_CUR        Position is number of bytes after current position
  returns <0 for error,
    or resulting file position relative to beginning of file.

  Moving beyond the end of the file is permitted; the file is extended in length
  only if a write occurs beyond the current end of file. Moving before the
  beginning of a file is not permitted. A write operation after a move only
  overwrites the number of bytes actually written.

Upvotes: 2

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