UNIX环境高级编程APUE练习3.2-不用fcntl实现dup2的功能

题面

编写与dup2功能相同的函数,要求不调用fcntl函数,并且要有正确的出错处理。

基本思路

不能用fcntl,能够返回一个文件描述符的只有opendup。而open会创建一个新的文件表项,返回的fd指向新的文件表项,与dup2的表现不符。dup基本能满足要求,但是返回的是最小的可用fd,需要进一步操作满足要求。另外需要自己添加错误处理,以及处理oldfd与newfd相等的情况等。具体地,

  1. 当dup返回出错时,直接返回出错
  2. 当dup返回值等于newfd时,直接返回
  3. 当dup返回值小于newfd时,记录返回值,循环调用dup直到返回值等于newfd。关闭前面记录的所有fd,返回newfd
  4. 当dup返回值大于newfd时,关闭返回值的fd。如果oldfd等于newfd,直接返回newfd;如果不相等,关掉newfd,然后再dup(因为不是原子的,返回值需要再判断)

出错处理

  1. oldfd的出错处理可以直接交给dup
  2. newfd的出错处理,需要判断是否超出文件描述符范围(RLIMIT_NOFILE in getrlimit)
  3. 对于dup返回EMFILE的情况,newfd如果没超过进程可打开的最大文件数,则不影响
  4. 另外还有一个判断顺序问题,是先判断参数是否合法还是oldfd==newfd, 这个可以根据dup2函数实测来确定

4 测试用例

进程打开的文件数没满的情况下

  1. 都超出范围,相同(MAX+1,MAX+1)
  2. 未打开描述符,相同 (100, 100)
  3. newfd超出范围 (1, MAX+1)
  4. newfd正好没超出 (1, MAX)
  5. oldfd和newfd相同 (2, 2)

进程打开的文件数满的情况下

  1. newfd正好超出范围 (1, MAX+1)
  2. newfd正好没超出 (1, MAX)
  3. oldfd和newfd相同 (2, 2)

5 开始撸码实测

5.1 先验证dup2的判断顺序问题

  • 测试源码
点击展开代码
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
#include <stdio.h>
#include <unistd.h>
#include <sys/resource.h>

int main()
{
int r;
struct rlimit old_rlim={0};
getrlimit(RLIMIT_NOFILE, &old_rlim);
printf("NOFILE limits: soft=%lld; hard=%lld\n",
(long long) old_rlim.rlim_cur, (long long) old_rlim.rlim_max);

r = dup2(10000, 10000);
if(r == -1) {
perror("dup2(10000, 10000) fail: ");
}
else {
printf("dup2(10000, 10000) success return %d\n", r);
}

r = dup2(100, 100);
if(r == -1) {
perror("dup2(100, 100) fail: ");
}
else {
printf("dup2(100, 100) success return %d\n", r);
}

r = dup2(1, 10000);
if(r == -1) {
perror("dup2(1, 10000) fail: ");
}
else {
printf("dup2(1, 10000) success return %d\n", r);
}

r = dup2(1, 100);
if(r == -1) {
perror("dup2(1, 100) fail: ");
}
else {
printf("dup2(1, 100) success return %d\n", r);
}

return 0;
}
  • MAC OSX下运行结果
1
2
3
4
5
6
^_^$ ./a.out
NOFILE limits: soft=7168; hard=9223372036854775807
dup2(10000, 10000) fail: : Bad file descriptor
dup2(100, 100) fail: : Bad file descriptor
dup2(1, 10000) fail: : Bad file descriptor
dup2(1, 100) success return 100

可见是参数出错判断是先于oldfd == newfd判断的

5.2 测试进程打开的最大文件数到上限时,dup2是否能成功

  • 测试源码
点击展开代码
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
#include <stdio.h>
#include <unistd.h>
#include <sys/resource.h>

int main()
{
int r;
int max_fd = 0;
struct rlimit old_rlim={0};
getrlimit(RLIMIT_NOFILE, &old_rlim);
printf("NOFILE limits: soft=%lld; hard=%lld\n",
(long long) old_rlim.rlim_cur, (long long) old_rlim.rlim_max);
while((r = dup(0))!= -1)
{
max_fd = r;
}
perror(NULL);
printf("max fd is %d\n", max_fd);

r = dup2(1, 10000);
if(r == -1) {
perror("dup2(1, 10000) fail: ");
}
else {
printf("dup2(1, 10000) success return %d\n", r);
}

r = dup2(1, 100);
if(r == -1) {
perror("dup2(1, 100) fail: ");
}
else {
printf("dup2(1, 100) success return %d\n", r);
}

return 0;
}
  • MAC OSX下运行结果
1
2
3
4
5
6
^_^$ ./a.out
NOFILE limits: soft=7168; hard=9223372036854775807
Too many open files
max fd is 7167
dup2(1, 7168) fail: : Bad file descriptor
dup2(1, 7167) success return 7167

可见在进程打开文件数达到上限时,dup2替换已经打开的文件是可以的

5.3 实现dup2的功能

  • 源码
点击展开代码
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
#include <stdio.h>
#include <unistd.h>
#include <sys/resource.h>
#include <sys/errno.h>

/*dup实现dup2的功能*/
int dup2_(int oldfd, int newfd) {
int ret;
int stack[7168];
int count = 0;
struct rlimit old_rlim={0};
getrlimit(RLIMIT_NOFILE, &old_rlim);
if (newfd < 0 || newfd > old_rlim.rlim_cur - 1) {
errno = EBADF;
return -1;
}
while(1) {
ret = dup(oldfd);
if(ret == -1 && errno != EMFILE) {
break;
}
else if(ret == -1 && errno == EMFILE) {
if(oldfd == newfd) {
return newfd;
}
printf("close(newfd)\n");
close(newfd);
}
else {
if(oldfd == newfd) {
close(ret);
return newfd;
}
if(ret == newfd) {
break;
}
else if(ret < newfd) {
stack[count++] = ret;
}
else {
close(ret);
printf("close(newfd)\n");
close(newfd);
}
}
}
while(count) {
close(stack[--count]);
}
return ret;
}

int main()
{
int r, max_fd;
struct rlimit old_rlim={0};
getrlimit(RLIMIT_NOFILE, &old_rlim);
printf("NOFILE limits: soft=%lld; hard=%lld\n",
(long long) old_rlim.rlim_cur, (long long) old_rlim.rlim_max);

r = dup2_(7168, 7168);
if(r == -1) {
perror("dup2_(7168, 7168) fail: ");
}
else {
printf("dup2_(7168, 7168) success return %d\n", r);
}

r = dup2_(100, 100);
if(r == -1) {
perror("dup2_(100, 100) fail: ");
}
else {
printf("dup2_(100, 100) success return %d\n", r);
}

r = dup2_(1, 7168);
if(r == -1) {
perror("dup2_(1, 7168) fail: ");
}
else {
printf("dup2_(1, 7168) success return %d\n", r);
}

r = dup2_(1, 7167);
if(r == -1) {
perror("dup2_(1, 7167) fail: ");
}
else {
printf("dup2_(1, 7167) success return %d\n", r);
}

r = dup2_(2, 2);
if(r == -1) {
perror("dup2_(2, 2) fail: ");
}
else {
printf("dup2_(2, 2) success return %d\n", r);
}
while((r = dup(0))!= -1)
{
max_fd = r;
}
perror(NULL);
printf("max fd is %d\n", max_fd);

r = dup2_(1, 7168);
if(r == -1) {
perror("dup2_(1, 7168) fail: ");
}
else {
printf("dup2_(1, 7168) success return %d\n", r);
}

r = dup2_(1, 7167);
if(r == -1) {
perror("dup2_(1, 7167) fail: ");
}
else {
printf("dup2_(1, 7167) success return %d\n", r);
}

r = dup2_(2, 2);
if(r == -1) {
perror("dup2_(2, 2) fail: ");
}
else {
printf("dup2_(2, 2) success return %d\n", r);
}

return 0;
}
  • MAC OSX下的运行结果
1
2
3
4
5
6
7
8
9
10
11
12
13
14
NOFILE limits: soft=7168; hard=9223372036854775807
dup2_(7168, 7168) fail: : Bad file descriptor
dup2_(100, 100) fail: : Bad file descriptor
dup2_(1, 7168) fail: : Bad file descriptor
dup2_(1, 7167) success return 7167
dup2_(2, 2) success return 2
Too many open files
max fd is 7167
dup2_(1, 7168) fail: : Bad file descriptor
close(newfd)
dup2_(1, 7167) success return 7167
close(newfd)
dup2_(1, 100) success return 100
dup2_(2, 2) success return 2

结果都符合预期

-------------本文结束感谢您的阅读-------------

欢迎关注我的其它发布渠道