# 系統程式設計 - `llseek(3)`

[TOC]

## 課程影片

### 系統程式設計 2018 - W3 1

{%youtube KhnUbPf44_w %}

## 例子

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

#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
#define FILE_FLAG (O_CREAT | O_WRONLY | O_TRUNC)

char	buf1[] = "abcdefghij";
char	buf2[] = "ABCDEFGHIJ";

int main(void)
{
	int fd;
	if ((fd = open("file.hole", FILE_FLAG, FILE_MODE)) < 0)
		perror("creat error");

	if (write(fd, buf1, 10) != 10)
		perror("buf1 write error");
	/* offset now = 10 */

	if (lseek(fd, 40, SEEK_SET) == -1)
		perror("lseek error");
	/* offset now = 40 */

	if (write(fd, buf2, 10) != 10)
		perror("buf2 write error");
	/* offset now = 50 */

	exit(0);
}

### `lseek`

1d...    0.889 us   |  post_ttbr_update_workaround();
1d...               |  do_el0_svc() {
1d...               |    el0_svc_common.constprop.0() {
1....               |      invoke_syscall() {
1....               |        __arm64_sys_lseek() {
1....               |          __fdget_pos() {
1....    2.093 us   |            __fget_light();
1....    6.112 us   |          }
1....               |          ext4_llseek() {
1....    2.056 us   |            generic_file_llseek_size();
1....    6.222 us   |          }
1....  + 18.796 us  |        }
1....  + 23.000 us  |      }
1d...  + 27.111 us  |    }
1d...  + 28.482 us  |  }