chained pipe example

xeon123 · 10-20-2007, 02:12 PM

Hi,

I would like to program a C example that execute the following command:

command 1 | command 2 | command n

I don't know how to do it. Can anyone give me an example?

Thanks,

henk1420 · 10-20-2007, 02:46 PM

Quote:

Originally Posted by pedrosacosta

Hi,

I would like to program a C example that execute the following command:

command 1 | command 2 | command n

The trick is that command 1 writes to stdout and command 2 reads from stdin and writes to stdout. Etc.

fprintf(stdout, format, data...); // In command 1

fgets(line_of_text, length_of_line_of_text, stdin); or fscanf(stdin, format, pointers_to_data); // In command 2.

Regards,
Henk

xeon123 · 10-20-2007, 05:31 PM

But the problem is that, I want to execute to commands.

command1 | command2

To execute them I need to use some function like execl. But, I don't know how to use the stdout from command one and pass to stdin from command2.

Any help?

paulsm4 · 10-20-2007, 07:20 PM

Hi, Pedrosacosta:

1. In Unix and Linux, each file you open gets a consecutively numbered file descriptor. The first three descriptors come automatically when the program is created: "stdin" is "0", "stdout" is "1", and "stderr" is "2". The first file or socket you open after that will be "3", and so on.

2. The way I/O redirection works is to *close* some file (e.g. "stdin") and *open* another file. The new file will get the old number.

3. "dup()" lets you "clone" an existing descriptor.

"dup2()" saves you the trouble of doing a separate "close()" (the "close" and "open" become part of the same, atomic action.

4. Here's a link that might make it all a bit clearer (look down in the presentation for item #3: I/O redirection):

http://www.comp.nus.edu.sg/~cs2281/2...ls9.colour.pdf

xeon123 · 10-21-2007, 05:39 AM

paulsm4, I've saw the example, but I'm still very confusing about it.

Code:

1void piping(char* cmd[], int nr)
2{
3  int i;
4  int thisin;
5  int pid[2];
6
7  thisin = fileno( stdin );// get stdin descriptor
8
9  for ( i=0; i<nr; i++)
10    {
11     pipe( pid );
12
13      if ( fork() == 0 )
14        {
15          dup2( thisin, fileno( stdin ));
16
17          if ( i != (nr-1) )// i isn't the last example
18            dup2( pid[1], fileno ( stdout ));
19
20          close(pid[0]);
21          close(pid[1]);
22
23         execlp ( cmd[i], cmd[i], NULL);
24        }
25      else
26        {
27          dup2( pid[0], thisin);
28
29          close(pid[0]);
30          close(pid[1]);
31        }
32    }
33}

- why line 17 is needed? The last command don't have to write in the stdout?
- Why this example only works to pipes with 3 programs execution? Eg. "ls -la | sort | wc". And doesn't work with 2, 4, 5, ..., n program executions?

I'm really confused about the functioning of the pipes. I've already googled up, read books ("Advanced Programming in Unix environments", and I'm still very mixed up.

I don't have any friend that program C at this level, so I can't get any help from them.