I am trying to create a csh clone, but am having problems implementing piped commands. Specifically, the below code simply hangs after input of ls | grep <text>
It does however filter the output and display it correctly, but it appears that grep hasn't exited and my shell never comes back to the waiting parent.
I am having doubts if I am using the pipe correctly.
Following is an extract of the relevant code(pseudo):
Code:
save stdin stdout and stderr using dup.
For every command
{
int fd[2];
if(not last command in pipe)
{
if(count ==0)
{
if(pipe(fd_pipe) < 0)
{
perror("pipe");
}
}
if(dup2(fd_pipe[1], STDOUT_FILENO) <0)
{
perror("pipe");
}
if(dup2(fd_pipe[0], STDIN_FILENO) <0)
{
perror("pipe");
}
++count;
}
}
else if(last command)
{
char line[100];
if(dup2(fd_pipe[0], STDIN_FILENO) <0)
{
perror("pipe");
}
if(dup2(bstdout, 1) <0)
{
perror("pipe");
}
}
Execute the command in subshell(fork and exec)
}
restore stdin out and stderr.
---------- Post updated at 03:37 AM ---------- Previous update was at 03:35 AM ----------
AArgh...Have cracked my head till 4: 30 am on this, but can't figure out what's wrong.
i tried using a separate file as an intermediate storage for the pipe output.
But then grep fails to filter the input from the file.
Moderator's Comments:
Moved to a more fitting forum
---------- Post updated at 12:14 PM ---------- Previous update was at 04:37 AM ----------
Anyone there?
To the moderator who moved the post...seems like moving it to a more "fitting" forum killed the chances of a reply
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LEARN ABOUT OSX
pipe
PIPE(2) BSD System Calls Manual PIPE(2)NAME
pipe -- create descriptor pair for interprocess communication
SYNOPSIS
#include <unistd.h>
int
pipe(int fildes[2]);
DESCRIPTION
The pipe() function creates a pipe (an object that allows unidirectional data flow) and allocates a pair of file descriptors. The first
descriptor connects to the read end of the pipe; the second connects to the write end.
Data written to fildes[1] appears on (i.e., can be read from) fildes[0]. This allows the output of one program to be sent to another pro-
gram: the source's standard output is set up to be the write end of the pipe; the sink's standard input is set up to be the read end of the
pipe. The pipe itself persists until all of its associated descriptors are closed.
A pipe whose read or write end has been closed is considered widowed. Writing on such a pipe causes the writing process to receive a SIGPIPE
signal. Widowing a pipe is the only way to deliver end-of-file to a reader: after the reader consumes any buffered data, reading a widowed
pipe returns a zero count.
The generation of the SIGPIPE signal can be suppressed using the F_SETNOSIGPIPE fcntl command.
RETURN VALUES
On successful creation of the pipe, zero is returned. Otherwise, a value of -1 is returned and the variable errno set to indicate the error.
ERRORS
The pipe() call will fail if:
[EFAULT] The fildes buffer is in an invalid area of the process's address space.
[EMFILE] Too many descriptors are active.
[ENFILE] The system file table is full.
SEE ALSO sh(1), fork(2), read(2), socketpair(2), fcntl(2), write(2)HISTORY
A pipe() function call appeared in Version 6 AT&T UNIX.
4th Berkeley Distribution February 17, 2011 4th Berkeley Distribution
10-07-2011
