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From: [EMAIL PROTECTED][SMTP:[EMAIL PROTECTED]]
Sent: Friday, October 09, 1998 12:02 PM
To: [EMAIL PROTECTED]
Subject: pipe blocking semantics
I've been looking at this small program which performs an "ls | wc",
trying to work out how the blocking semantics works.
I understand that fork() has resulted in two identical processes being
run simultaneously but I can't work out how the parent process is
being forced to execute before the child.
There is no guarantee that parent process will be executed before
the child only thing is "ls" and "wc" use write and read system calls
which are blocking in nature. So as long as there is no data in the pipe
"wc" will be waiting and whenever the process running "ls" is scheduled
"ls" will write the data in to pipe. Only when ls finish writing the EOF
the wc will terminate.
Hope you got the point.
-subbaraom
After the fork(), are there two separate copies of pfd[] ?
/* ls | wc */
#include stdio.h
#include unistd.h
int main(void)
{
int pfd[2];
int pid;
pipe(pfd);
pid=fork();
if (pid==0)
{
/* child */
close(pfd[1]);
dup2(pfd[0],0);
close(pfd[0]);
execlp("wc", "wc", (char *) 0 );
}
else
{
/* parent */
close(pfd[0]);
dup2(pfd[1],1);
close(pfd[1]);
execlp("ls", "ls", (char *) 0);
}
return(0);
}
