IPC

Inter-Process Communication
-> Mechanism for communication between process
-> Signals, I/O redirection, Anonymous pipe, Named pipe (FIFO), Shared Memory, Message Queue ...

int main(void){
	char c;
    int fd = open(“temp.txt", O_RDONLY);
    if (fork() == 0) {
    	read(fd, &c, 1);
        exit(0);
    }
    else {
    	wait(NULL);
        read(fd, &c, 1);
        printf(“c=%c\n”,c); //c=y
        }
    return 0;
}

Q. How does a shell implement I/O redirection?
A. dup2(oldfd, newfd)
-> Copies (per-process) descriptor table entry oldfd to entry newfd

int main()
{
	printf("ls > out.txt example\n");
    int f = open("out.txt", O_WRONLY|O_CREAT, 0666);
    
    if(fork() == 0){
    	dup2(f, 1);
        execl("/bin/ls","/bin/ls",NULL);
    }
    wait(NULL);
    
    return 0;
}

int pipe (int fd[2]);
fd[0]: open for reading
fd[1]: open for writing
-> The output of fd[1] is the input for fd[0]

Anonymous pipe
parent -> child:
parent closes fd[0];
child closes fd[1];

or

parent <- child:
parent closes fd[1];
child closes fd[0];

#include <unistd.h>
#include <stdlib.h>
#define MAXLINE 80
int main(void)
{
	int n, fd[2];
    pid_t pid;
    char line[MAXLINE];
    if(pipe(fd) < 0) exit(1);
    if((pid = fork()) < 0) exit(2);
    
    if (pid> 0) { /* parent */
    	close(fd[0]);
        write(fd[1], "hello world\n", 12);}
    else { /* child */
    	close(fd[1]);
        n = read(fd[0], line, MAXLINE);
        write(1, line, n);
    }
    
    exit(0);
}

Make mini shell which supports I/O redirection & pipes

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include <sys/msg.h>
#include <sys/types.h>

int main()
{
	char *cmd = NULL;
	size_t size = 0;
	char *arg[11];

	while(1){
		int i = 0;
		getline(&cmd, &size, stdin);
		cmd[strlen(cmd) - 1] = '\0';
		if(strlen(cmd) == 0)
			continue;

		char *ptr = strtok(cmd, " ");
		while(ptr != NULL){
			arg[i++] = ptr;
			ptr = strtok(NULL, " ");
		}
		arg[i] = NULL;
		if(strcmp("quit", arg[0]) == 0) //"quit"
			break;

		char path[100];
		sprintf(path, "/bin/%s", arg[0]);

		for(int x = 0; x < i; x++){
			if(strcmp(">", arg[x]) == 0){ //output redirection
				int fd = open(arg[x+1], O_RDWR|O_CREAT|O_TRUNC, 0644);
				
				if(fork() == 0){
					dup2(fd, STDOUT_FILENO);
					arg[x] = NULL;
					execv(path, arg);
				}
				wait(NULL);
				close(fd);
				break;
			}
			else if(strcmp(">>", arg[x]) == 0){ //output redirection (appending)
				int fd = open(arg[x+1], O_RDWR|O_APPEND, 0644);
				
				if(fork() == 0){
					dup2(fd, STDOUT_FILENO);
					arg[x] = NULL;
					execv(path, arg);
				}
				wait(NULL);
				close(fd);
				break;
			}
			else if(strcmp("<", arg[x]) == 0){ //input redirection
				int fd = open(arg[x+1], O_RDONLY);

				if(fork() == 0){
					dup2(fd, STDIN_FILENO);
					arg[x] = NULL;
					execv(path, arg);
				}
				wait(NULL);
				close(fd);
				break;
			}
			else if(strcmp("|", arg[x]) == 0){ //pipeline
				int fd[2];
				pid_t pid1, pid2;

				if(pipe(fd) < 0) exit(1);
				if((pid1 = fork()) == 0){ //parent
					close(fd[0]);
					dup2(fd[1], STDOUT_FILENO);
					arg[x] = NULL;
					execv(path, arg);
				}
				wait(NULL);

				if((pid2 = fork()) == 0){ //child
					close(fd[1]);
					char **arg2 = &arg[x + 1];
					sprintf(path, "/bin/%s", arg2[0]);
					dup2(fd[0], STDIN_FILENO);
					execv(path, arg2);
				}
				close(fd[0]);
				close(fd[1]);
				wait(NULL);
				break;
			}
			else if(strcmp("exit", arg[x]) == 0){ //exit
				if(arg[x+1] != NULL){
					int no = atoi(arg[x+1]);
					printf("%d\n", no);
					exit(no);
				}
				else{
					printf("%d\n", 0);
					exit(0);
				}
			}
	
		} //for loop end
	}