Move the file position without fseek in C -


how can move file position x bytes in c?

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

i'd avoid reading entire file memory.

what want basically:

  1. read x + = y bytes.
  2. do first x bytes.
  3. move file pointer backwards bytes.
  4. read y bytes there.

i know how except step 3. know how?

according http://rabbit.eng.miami.edu/info/functions/unixio.html use lseek:

int lseek(int fd, int position, int startpoint)   include: <unistd.h>   sets file position effective next read or write operation.   fd = file descriptor returned open   position = position within file:     number of bytes between startpoint , desired position, may negative.   startpoint = location in file position relative to, 1 of:     seek_set        position number of bytes beginning of file     seek_end        position number of bytes after end of file     seek_cur        position number of bytes after current position   returns <0 error,     or resulting file position relative beginning of file.    moving beyond end of file permitted; file extended in length   if write occurs beyond current end of file. moving before   beginning of file not permitted. write operation after move   overwrites number of bytes written.

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