管道是进程间通信的有力的武器.记得初中时在写Windows的后门的时候,也是开了一个匿名的管道和cmd进行交互.今天终于在Linux上写了下管道.
在Linux下，两个程序之间传递数据的最简单的方法就是使用popen和pclose两个函数了。但它们都是需要启动一个shell。这个成本略高啊。
于是我们可以使用底层的pipe函数。通过这个函数，我们在两个程序之间传递数据而不需要启动一个shell来解释请求的命令。
原型是:
1
2
3
| #include <unistd.h>
int pipe(int file_descriptor[2]);
|
参数是一个由两个整型的文件描述符组成的数组的指针。成功返回0，失败返回-1，并设置errno来表面失败的原因。
两个文件描述符以特殊的方式连接起来。写到file_descriptor[1]的所有数据都可以从file_descriptor[0]中读回来。数据基于先进先出FIFO的原则。
我们使用read和write来访问数据。
管道有一些内置的缓冲区，它在write和read调用之间保存数据。
缓冲区满的时候，write可能会阻塞。
当没有数据可读时，read通常会阻塞，也就是被操作系统挂起了。
如果管道的另一端已经被关闭，也就是说没有进程打开这个管道冰箱它写入数据了，这时的对read的阻塞就是毫无意义了。于是对于对一个已经关闭写数据的管道做read调用将会返回0而不是阻塞。
下面是一个例子：
程序fork后多了一个子进程，此时管道的引用计数加1，所以我们要及时关闭不用的管道让引用计数及时的修正。
引用计数是程序设计中非常重要的思想啊~~~
cpp1
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
|
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
int data_processed;
int file_pipes[2];
char some_data[32];
char buffer[BUFSIZ + 1];
pid_t fork_result;
int counter = 30;
memset(buffer, 0, sizeof(buffer));
if (pipe(file_pipes) == 0)
{
fork_result = fork();
if (fork_result == -1)
{
fprintf(stderr, "Fork Failure");
exit(EXIT_FAILURE);
}
if (fork_result == 0)
{
close(file_pipes[1]);
while (counter--)
{
data_processed = read(file_pipes[0], buffer, BUFSIZ);
if (data_processed == 0)
break;
buffer[data_processed] = '\0';
printf("%d - Read %d bytes: %s\n", counter, data_processed, buffer);
}
close(file_pipes[0]);
printf("reader done\n");
}
else
{
close(file_pipes[0]);
while (counter--)
{
sprintf(some_data, "[%d] ", counter);
data_processed = write(file_pipes[1], some_data, strlen(some_data));
if (data_processed == 0)
break;
printf("%d - Wrote %d bytes\n", counter, data_processed);
}
close(file_pipes[1]);
printf("writer done\n");
}
exit(EXIT_SUCCESS);
}
exit(EXIT_FAILURE);
}
|
下面是在我的虚拟机的输出：（因为有两个屏幕，开个虚拟机让一个鼠标在两个系统两个屏幕间互相移动，这是相当的爽啊）
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
| cedricporter-virtual-machine 1 # ./1
29 - Wrote 5 bytes
28 - Wrote 5 bytes
27 - Wrote 5 bytes
26 - Wrote 5 bytes
25 - Wrote 5 bytes
24 - Wrote 5 bytes
23 - Wrote 5 bytes
22 - Wrote 5 bytes
21 - Wrote 5 bytes
20 - Wrote 5 bytes
19 - Wrote 5 bytes
18 - Wrote 5 bytes
29 - Read 60 bytes: [29] [28] [27] [26] [25] [24] [23] [22] [21] [20] [19] [18]
17 - Wrote 5 bytes
16 - Wrote 5 bytes
15 - Wrote 5 bytes
14 - Wrote 5 bytes
13 - Wrote 5 bytes
12 - Wrote 5 bytes
11 - Wrote 5 bytes
10 - Wrote 5 bytes
9 - Wrote 4 bytes
8 - Wrote 4 bytes
7 - Wrote 4 bytes
6 - Wrote 4 bytes
5 - Wrote 4 bytes
4 - Wrote 4 bytes
3 - Wrote 4 bytes
2 - Wrote 4 bytes
1 - Wrote 4 bytes
0 - Wrote 4 bytes
writer done
cedricporter-virtual-machine 1 # 28 - Read 80 bytes: [17] [16] [15] [14] [13] [12] [11] [10] [9] [8] [7] [6] [5] [4] [3] [2] [1] [0]
reader done
|