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名字

Name : lseek - reposition read/write file offset

lseek函數(shù)的作用是用來重新定位文件讀寫的位移。

頭文件以及函數(shù)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(dòng)（向前移），為負(fù)數(shù)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(shù)會(huì)重新定位被打開文件的位移量，根據(jù)參數(shù)offset以及whence的組合來決定：

SEEK_SET:
　　從文件頭部開始偏移offset個(gè)字節(jié)。
SEEK_CUR：
　　從文件當(dāng)前讀寫的指針位置開始，增加offset個(gè)字節(jié)的偏移量。
SEEK_END：
　　文件偏移量設(shè)置為文件的大小加上偏移量字節(jié)。

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(jù)傳入的參數(shù)來設(shè)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(quán)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(shù)
  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(jié)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(shù)將src_file文件指針重新定位到文件末尾 + offset(注意，本程序?qū)ffset取了相反數(shù)，即文件末尾 + （-offset）)處，然后從文件末尾 + offset處開始向前復(fù)制文件到dest_file中。

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