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名字

Name : lseek - reposition read/write file offset

lseek函數的作用是用來重新定位文件讀寫的位移。

頭文件以及函數聲明

#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);

offset為正則向文件末尾移動（向前移），為負數則向文件頭部（向后移）。

描述

lseek() repositions the file offset of the open file description associated with the file descriptor fd to the argument offset according to the directive whence as follows:
SEEK_SET The file offset is set to offset bytes.
SEEK_CUR The file offset is set to its current location plus offset bytes.
SEEK_END The file offset is set to the size of the file plus offset bytes.

lseek() allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a “hole”) return null bytes (‘\0') until data is actually written into the gap.

lseek()函數會重新定位被打開文件的位移量，根據參數offset以及whence的組合來決定：

SEEK_SET:
　　從文件頭部開始偏移offset個字節。
SEEK_CUR：
　　從文件當前讀寫的指針位置開始，增加offset個字節的偏移量。
SEEK_END：
　　文件偏移量設置為文件的大小加上偏移量字節。

測試代碼：

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

#define BUFFER_SIZE 1024
#define SRC_FILE_NAME "src_file"
#define DEST_FILE_NAME "dest_file"
//根據傳入的參數來設置offset
#define OFFSET (atoi(args[1])) 

int main(int argc, char*args[]) {
  int src_file, dest_file;
  unsigned char buff[BUFFER_SIZE];
  int real_read_len, off_set;
  if (argc != 2) {
    fprintf(stderr, "Usage: %s offset\n", args[0]);
    exit(-1);
  }
  src_file = open(SRC_FILE_NAME, O_RDONLY);
  dest_file = open(DEST_FILE_NAME, O_WRONLY | O_CREAT, S_IREAD | S_IWRITE );//owner權限：rw
  if (src_file < 0 || dest_file < 0) {
    fprintf(stderr, "Open file error!\n");
    exit(1);
  }
  off_set = lseek(src_file, -OFFSET, SEEK_END);//注意，這里對offset取了相反數
  printf("lseek() reposisiton the file offset of src_file: %d\n", off_set);
  while((real_read_len = read(src_file, buff, sizeof(buff))) > 0) {
    write(dest_file, buff, real_read_len);
  }
  close(dest_file);
  close(src_file);
  return 0;
}

結果解析

觀察offset以及dest_file和src_file文件的大小不難看出：程序通過lseek函數將src_file文件指針重新定位到文件末尾 + offset(注意，本程序對offset取了相反數，即文件末尾 + （-offset）)處，然后從文件末尾 + offset處開始向前復制文件到dest_file中。

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