C++

Forum

 
Piping with 2 processes in c++

Xilonian (27)
I am writing a program that is supposed to run 2 unix commands using 2 separate processes and using pipes i send the output of 1 into the input of another. I have the program running through but for some reason it shows output for the first command and does not seem to pipe the output to the second command and run it. Hers is my code (I tried to comment to make it easier to go through):

I am using a copy of linux mint and geany is my editor.

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
 
#include <iostream>
#include <iomanip>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <unistd.h>

using namespace std;

int main()
{
	char *cmd1[] = { "/bin/ls", "-l", "/", 0 }; 
	char *cmd2[] = { "usr/bin/wc", "-w", "/", 0 };
	
	int pipefd[2], rs;
	
	rs = pipe(pipefd);
	if (rs < 0) 
	{
		perror("pipe");
		exit(EXIT_FAILURE);
	}
	
	if(rs == 0) 
	{	// Child process
		// close write end of pipe
		close(pipefd[1]);
		// close standard input
		close(0);
		// duplicate read end of pipe
		// into standard input
		dup(pipefd[0]);
		// close read end of pipe
		close(pipefd[0]);
		// run wc
		rs = execvp(cmd1[0], cmd1);
		if (rs < 0) 
		{
			perror("execl");
			exit(EXIT_FAILURE);
		}
	}
	else 
	{ 	// Parent process
		// close read end of pipe,
		// keep write end open
		close(pipefd[0]);
		// close standard output
		close(1);
		// duplicate write end of pipe
		// into standard output
		dup(pipefd[1]);
		// close write end of pipe
		close(pipefd[1]);
		// run ls
		rs = execvp(cmd2[0], cmd2);
		if(rs < 0) 
		{
			perror("execl");
			exit(EXIT_FAILURE);
		}
	}
	return 0;
	}
Last edited on
ne555 (10692)
`pipe()' does not `fork()'


`geany' is not a compiler.
ShodanHo (110)
Typo:
char *cmd2[] = { "usr/bin/wc", "-w", "/", 0 };
to:
char *cmd2[] = { "/usr/bin/wc", "-w", "/", 0 };
Last edited on
Xilonian (27)
added the forks but this time it runs but will not end. It shows the output of the ls command. I am going based on notes I have. Do I need to take the output out of the pipe into a buffer and pass it to the wc command as an argument?

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
 
#include <iostream>
#include <iomanip>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <unistd.h>

using namespace std;

int main()
{
	char *cmd1[] = { "/bin/ls", "-l", "/", 0 }; 
	char *cmd2[] = { "/usr/bin/wc", "-w", "/", 0 };

	int pipefd[2], rs;
	
	rs = pipe(pipefd);
	if (rs < 0) 
	{
		perror("pipe");
		exit(EXIT_FAILURE);
	}
	
	rs = fork();
	if(rs < 0) 
	{
		perror("fork");
		exit(EXIT_FAILURE);
	}
	
	if(rs == 0) 
	{	// Child process
		// close write end of pipe
		close(pipefd[1]);
		// close standard input
		close(0);
		// duplicate read end of pipe
		// into standard input
		dup(pipefd[0]);
		// close read end of pipe
		close(pipefd[0]);
		// run wc
		rs = execvp(cmd1[0], cmd1);
		if (rs < 0) 
		{
			perror("execl");
			exit(EXIT_FAILURE);
		}
	}
	
	rs = fork();
	if(rs < 0) 
	{
		perror("fork");
		exit(EXIT_FAILURE);
	}
	
	if(rs==0)

	{ 	// Parent process
		// close read end of pipe,
		// keep write end open
		close(pipefd[0]);
		// close standard output
		close(1);
		// duplicate write end of pipe
		// into standard output
		dup(pipefd[1]);
		// close write end of pipe
		close(pipefd[1]);
		// run ls
		rs = execvp(cmd2[0], cmd2);
		if(rs < 0) 
		{
			perror("execl");
			exit(EXIT_FAILURE);
		}
	}
	return 0;
	}
ShodanHo (110)
If you check out the man page on execvp it has the line: "The exec() family of functions replaces the current process image with a new process image." So your program is no longer running after the execvp() call.
I think you have to fork() to make two processes, and then execvp() in one, and proceed in the other to fork() again etc.

Refer:
http://stackoverflow.com/questions/5631229/using-exec-to-execute-a-system-command-in-a-new-process
mik2718 (347)
not sure what you are doing but have you considered popen?

You can put a pipe separated string of commands into popen, anything you can put on the command line

http://www.sw-at.com/blog/2011/03/23/popen-execute-shell-command-from-cc/
Topic archived. No new replies allowed.