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名字

Name : lseek - reposition read/write file offset

lseek函数的作用是用来重新定位文件读写的位移。

头文件以及函数声明

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

offset为正则向文件末尾移动（向前移），为负数则向文件头部（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()函数会重新定位被打开文件的位移量，根据参数offset以及whence的组合来决定：

SEEK_SET:
　　从文件头部开始偏移offset个字节。
SEEK_CUR：
　　从文件当前读写的指针位置开始，增加offset个字节的偏移量。
SEEK_END：
　　文件偏移量设置为文件的大小加上偏移量字节。

测试代码：

#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"
//根据传入的参数来设置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权限：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，这里对offset取了相反数
  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;
}

结果解析

观察offset以及dest_file和src_file文件的大小不难看出：程序通过lseek函数将src_file文件指针重新定位到文件末尾 + offset(注意，本程序对offset取了相反数，即文件末尾 + （-offset）)处，然后从文件末尾 + offset处开始向前复制文件到dest_file中。

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