一、
[lingyun@localhost lseek]$ vim lseek.c
+ lseek.c/*********************************************************************************
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 01:33:51 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define ERR_EXIT(m)\
do\
{\
perror(m);\
exit(EXIT_FAILURE);\
}while(0)
int main(void)
{
int fd;
if(fd = open("file.hole", O_WRONLY|O_CREAT|O_TRUNC, 0644) == -1)
ERR_EXIT("open error");
write(fd, "hello", 5);
/* offset now = 5 */
if(lseek(fd, 2*1024, SEEK_CUR) == -1)
ERR_EXIT("lseek error");
/* offset now = 2*1024 + 5 */
write(fd, "world\n", 5);
/* offset now = 2*1024 + 10 */
close(fd);
return 0;
}
[lingyun@localhost lseek]$ gcc lseek.c
[lingyun@localhost lseek]$ ls
a.out lseek.c
[lingyun@localhost lseek]$ ./a.out
hellolseek error: Illegal seek
[lingyun@localhost lseek]$ sudo ./a.out
hellolseek error: Illegal seek
[lingyun@localhost lseek]$ ls
a.out file.hole lseek.c
[lingyun@localhost lseek]$ cat file.hole
原因是上面的函数中的红色语句有问题：
因为"<"的优先级大于"="，因此你open那一句实际上把0赋给了fd，所以改成if((fd = open("2.txt",O_RDWR|O_CREAT)) < 0) 就对了
[lingyun@localhost lseek]$ vim lseek.c
+ lseek.c
/*********************************************************************************
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 01:33:51 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define ERR_EXIT(m)\
do\
{\
perror(m);\
exit(EXIT_FAILURE);\
}while(0)
int main(void)
{
int fd;
if((fd = open("file.hole", O_WRONLY|O_CREAT|O_TRUNC, 0644)) == -1)
ERR_EXIT("open error");
write(fd, "hello", 5);
/* offset now = 5 */
if(lseek(fd, 2*1024, SEEK_CUR) == -1)
ERR_EXIT("lseek error");
/* offset now = 2*1024 + 5 */
write(fd, "world\n", 5);
/* offset now = 2*1024 + 10 */
close(fd);
return 0;
}
[lingyun@localhost lseek]$ gcc lseek.c
[lingyun@localhost lseek]$ ./a.out
[lingyun@localhost lseek]$ cat file.hole
helloworld
[lingyun@localhost lseek]$ du -h file.hole
4.0K file.hole
[lingyun@localhost lseek]$ od -c file.hole
0000000 h e l l o \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
*
0004000 \0 \0 \0 \0 \0 w o r l d
0004012
[lingyun@localhost lseek]$
二、
[lingyun@localhost lseek_2]$ vim lseek.c
if(write(fd, buf1, 10) != 10)
/*********************************************************************************
+ lseek.c
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 03:36:38 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
char buf1[] = "abcdefghij";
char buf2[] = "ABCDEFGHIJ";
int main(void)
{
int fd;
if((fd = creat("file.hole",0644)) < 0)
{
perror("creat error");
exit(EXIT_FAILURE);
}
if(write(fd, buf1, 10) != 10)
{
perror("write buf1 error");
exit(EXIT_FAILURE);
}
/* offset now = 10 */
if(lseek(fd, 16384, SEEK_SET) == -1)
{
perror("lseek error");
exit(EXIT_FAILURE);
}
/* offset now = 16384 */
if(write(fd, buf2, 10) != 10)
{
perror("write error");
exit(EXIT_FAILURE);
}
/* offset now = 16394 */
exit(0);
}
~/apue/lseek/lseek_2/lseek.c[+] CWD: /usr/local/src/lingyun/apue/lseek/lseek_2 Line: 54/56:12
"lseek.c" [New] 56L, 1258C written
[lingyun@localhost lseek_2]$ ls
lseek.c
[lingyun@localhost lseek_2]$ gcc lseek.c
[lingyun@localhost lseek_2]$ ./a.out
[lingyun@localhost lseek_2]$ du -h file.hole
8.0K file.hole
[lingyun@localhost lseek_2]$ od -c file.hole
0000000 a b c d e f g h i j \0 \0 \0 \0 \0 \0
0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
*
0040000 A B C D E F G H I J
0040012
[lingyun@localhost lseek_2]$ cat file.hole
abcdefghijABCDEFGHIJ[lingyun@localhost lseek_2]$
三、
下面是把SEEK_CUR改为了SEEK_END，并且offset = -5,得到的结果就是buf2覆盖了buf1后面的5个字符。如下：
[lingyun@localhost lseek_3]$ vim lseek.c
+ lseek.c
/*********************************************************************************
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 03:36:38 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
char buf1[] = "abcdefghij";
char buf2[] = "ABCDEFGHIJ";
int main(void)
{
int fd;
if((fd = creat("file.hole",0644)) < 0)
{
perror("creat error");
exit(EXIT_FAILURE);
}
if(write(fd, buf1, 10) != 10)
{
perror("write buf1 error");
exit(EXIT_FAILURE);
}
/* offset now = 10 */
if(lseek(fd, -5, SEEK_END) == -1)
{
perror("lseek error");
exit(EXIT_FAILURE);
}
/* offset now = 16384 */
if(write(fd, buf2, 10) != 10)
{
perror("write error");
exit(EXIT_FAILURE);
}
/* offset now = 16394 */
exit(0);
~/apue/lseek/lseek_3/lseek.c[+] CWD: /usr/local/src/lingyun/apue/lseek/lseek_3 Line: 40/56:19
"lseek.c" 56L, 1255C written
[lingyun@localhost lseek_3]$ gcc lseek.c
[lingyun@localhost lseek_3]$ ./a.out
[lingyun@localhost lseek_3]$ od -c file.hole
0000000 a b c d e A B C D E F G H I J
0000017
[lingyun@localhost lseek_3]$
四、
[lingyun@localhost lseek_3]$ vim lseek.c
if(lseek(fd, 0, SEEK_CUR) == -1)
/*********************************************************************************
+ lseek.c
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 03:36:38 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
char buf1[] = "abcdefghij";
char buf2[] = "ABCDEFGHIJ";
int main(void)
{
int fd;
if((fd = creat("file.hole",0644)) < 0)
{
perror("creat error");
exit(EXIT_FAILURE);
}
if(write(fd, buf1, 10) != 10)
{
perror("write buf1 error");
exit(EXIT_FAILURE);
}
/* offset now = 10 */
if(lseek(fd, 0, SEEK_CUR) == -1)
{
perror("lseek error");
exit(EXIT_FAILURE);
}
/* offset now = 16384 */
if(write(fd, buf2, 10) != 10)
{
perror("write error");
exit(EXIT_FAILURE);
}
/* offset now = 16394 */
exit(0);
}
~/apue/lseek/lseek_3/lseek.c[+] CWD: /usr/local/src/lingyun/apue/lseek/lseek_3 Line: 56/56:1
"lseek.c" 56L, 1254C written
[lingyun@localhost lseek_3]$ ls
a.out lseek.c
[lingyun@localhost lseek_3]$ gcc lseek.c
[lingyun@localhost lseek_3]$ ./a.out
[lingyun@localhost lseek_3]$ od -c file.hole
0000000 a b c d e f g h i j A B C D E F
0000020 G H I J
0000024
[lingyun@localhost lseek_3]$
五、
[lingyun@localhost lseek_3]$ vim lseek.c
+ lseek.c
* Copyright: (C) 2013 fulinux<fulinux@sina.com>
* All rights reserved.
*
* Filename: lseek.c
* Description: This file
*
* Version: 1.0.0(07/28/2013~)
* Author: fulinux <fulinux@sina.com>
* ChangeLog: 1, Release initial version on "07/28/2013 03:36:38 PM"
*
********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
char buf1[] = "abcdefghij";
char buf2[] = "ABCDEFGHIJ";
int main(void)
{
int fd;
if((fd = creat("file.hole",0644)) < 0)
{
perror("creat error");
exit(EXIT_FAILURE);
}
if(write(fd, buf1, 10) != 10)
{
perror("write buf1 error");
exit(EXIT_FAILURE);
}
/* offset now = 10 */
if(lseek(fd, 0, SEEK_SET) == -1)
{
perror("lseek error");
exit(EXIT_FAILURE);
}
/* offset now = 16384 */
if(write(fd, buf2, 10) != 10)
{
perror("write error");
exit(EXIT_FAILURE);
}
/* offset now = 16394 */
exit(0);
}
~/apue/lseek/lseek_3/lseek.c[+] CWD: /usr/local/src/lingyun/apue/lseek/lseek_3 Line: 40/56:28
"lseek.c" 56L, 1254C written
[lingyun@localhost lseek_3]$ gcc lseek.c
[lingyun@localhost lseek_3]$ ./a.out
[lingyun@localhost lseek_3]$ od -c file.hole
0000000 A B C D E F G H I J
0000012
[lingyun@localhost lseek_3]$