c - 在 C 中不使用 fseek 移动文件位置

Question

如何在 C 中将文件位置后移 x 字节？

我不想使用 fseek()，因为我正在使用 open(fd, buf, buflength) 读取文件行，而不是 fopen().

我非常想避免将整个文件读入内存。

我想做的基本上是:

1. 读取 X + A = Y 字节。
2. 对前 X 个字节做一些事情。
3. 将文件指针向后移动A字节。
4. 从那里读取 Y 个字节。

除了第 3 步，我知道如何做所有事情。有人知道怎么做吗？

Answer 1

根据 http://rabbit.eng.miami.edu/info/functions/unixio.html你可以使用 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.