This is a comprehensive guide to interprocess communication.
Today's lesson is on piping. You have process A, and process B, created by using the fork() command. You want the output of process A to be the input to process B. This is called piping. The following code excerpt demonstrates Inter Process Communication between two child processes, Process A, and Process B. Process A and Process B are running shell commands.
Code:
int pipeID[2]; /* Pipes Have two values, one for the process A */
/* And one for process B. Think of each value */
/* as an end to the pipe */
/* pipeid[0] is process A, pipeid[1] is process B */
pipe(pipeID); /* Create Pipe. */
/* Fork current process to create process A and process B. */
/*Both child processes. Process A is created by the first fork(), */
/*process B by the second fork(). */
// Process A code
if (fork()==0){ /* If the new process A is a child */
// child
close(pipeID[0]); //write stdout
dup2(pipeID[1],1); // to pipe using dup
fillArgs(sPipe[0],&args); /* Create agrc and argv */
execvp(args.argv[0],args.argv); // Execute
close(pipeID[1]);// stdout goes to pipe.
}
// Process B code
if (fork()==0){ /* If the new process is a child */
close(pipeID[1]); //read from pipe 1 stdout
dup2(pipeID[0],0); //as stdin.
fillArgs(sPipe[1],&args2); /* Create argc and argv */
execvp(args2.argv[0],args2.argv); // Execute
close(pipeID[0]);// process B reads stdin from pipe
}
close(pipeID[1]); /* Close pipes else, we don't want to have stdin, and stdout */
close(pipeID[0]); /* permanently redirected. */
I know my example isn't easy. At least I gave it a try
Last edited by flyingpenguin; 11-02-2005 at 10:51 PM..
Reason: Indenting Code
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LEARN ABOUT BSD
fork
FORK(2) System Calls Manual FORK(2)NAME
fork - create a new process
SYNOPSIS
pid = fork()
int pid;
DESCRIPTION
Fork causes creation of a new process. The new process (child process) is an exact copy of the calling process except for the following:
The child process has a unique process ID.
The child process has a different parent process ID (i.e., the process ID of the parent process).
The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for
instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child
process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard
input and output for newly created processes as well as to set up pipes.
The child processes resource utilizations are set to 0; see setrlimit(2).
RETURN VALUE
Upon successful completion, fork returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
indicate the error.
ERRORS
Fork will fail and no child process will be created if one or more of the following are true:
[EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-
dependent.
[EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would be
exceeded.
[ENOMEM] There is insufficient swap space for the new process.
SEE ALSO execve(2), wait(2)3rd Berkeley Distribution May 22, 1986 FORK(2)
11-02-2005
