trouble pipeline three commands "dmesg|sort|more" c++
I have successfully piped the output of one command into the input of another and then show the output of the second command to the screen.
I want to do this with three successive commands. (actually eventually I want to do it with N commands passed into the program at run time.
Thi开发者_JAVA技巧s is my attempt at pipelining three commands together.
UPDATED: i updated my question to reflect my latest try.
#include <string.h>
#include <fstream>
#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/types.h>
using namespace std;
int main(int argc, char * argv[])
{
pid_t pid;
int pfd[2];
char* prgname = NULL;
if(pipe(pfd) == -1)
{
perror("error on pipe call");
return(1);
}
for(int j = 0;j<numberOfCommands;j++)
{
cout<<commands[j]<<"_"<<endl;
}
pid = fork();
if(pid == 0){//child process
close(pfd[0]); //close read end of pipe
dup2(pfd[1],1);//connect the pipes
close(pfd[1]);//close extra file descriptors
prgname = (char*)"dmesg"; //commands[0];//first command
execlp(prgname, prgname, 0);//Load the program
}
else
{
int pfd2[2];
if(pipe(pfd2) == -1)
{
perror("error on pipe call 2");
return(1);
}
pid = fork();
if(pid == 0)//child
{
close(pfd[1]);
dup2(pfd[0],0);
close(pfd[0]);
close(pfd2[0]);
dup2(pfd2[1],1);
close(pfd2[1]);
prgname = (char*)"sort";
execlp(prgname,prgname,0);
}
else
{
close(pfd2[1]); //close the write end of the pipe
dup2(pfd2[0],0);//connect the pipes
close(pfd2[0]); //close extra file descriptor
prgname = (char*)"more"; //commands[1];//now run the second command
execlp(prgname, prgname, 0);//Load the program
}
}
return 0;
}
i have hard coded all values for simplicity. The program displays what should be the output of "dmesg|more" but does not do the sort part and then freezes. I see the begging of dmesg and more in the bottom left but i can not view any more.
Any ideas?
pipe(2)
only provides 2 file descriptors for 1 pipe. The 3rd file descriptor (pfd[2]
) is junk and is never getting initialized. If you want to create a pipeline with 3 commands in it, you need to call pipe()
twice to get two pipes: one for connecting the first and second processes, and one for connecting the second and third processes.
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