Pipex

This project will let you discover in detail a UNIX mechanism that you already know by using it in your program.

• Unix logic

• Imperative programming
• Unix

$> make all && make bonus

$> ./pipex infile "grep a" "tr a b" outfile

$> < infile grep a | tr a b > outfile

This function basically create a new process when it is called. Everything before the fork() is executed once, then it create new process, resulting 2 process, where the child process is always 0, here is an exemple;

#include <unistd.h>

int	main(void){
	int id = fork();
	if (id == 0)
		printf("Hello, child process id: %d\n", id);
	else
		printf("Hello, parent process id: %d\n", id);
	return 0;
}

Hello, child process id: 0
Hello, parent process id: 42

There are another functions called getpid() that return the process id of the calling process, and the getppid() that thet return the parent process the current proccess. Every process has its parent process, in the case of child process, 0, its parent is 42, and the parent of the parent process it might be 41.

The pipe() will create two file descriptor, so we declare as int fd[2]. One of them, fd[0], we will use to read() from a file, and the other, fd[1], we will write(). Here is an exemple how its works:

int main(void){
	int fd[2];
	pipe(fd);
	int pid = fork();
	if (pid == 0){
		close(fd[0]);
		int x = 42;
		write(fd[1], &x, sizeof(int));
		close(fd[1]);
		printf("writed %d in child process\n", x);
	} else {
		close(fd[1]);
		int y;
		read(fd[0], &y, sizeof(int));
		close(fd[0]);
		printf("readed %d in parent process\n", y);
	}
}

writed 42 in child process
readed 42 in parent process

As you see, we need to close four files descriptor, it's because the fds are inheriteded when we do fork, because that they are used to redirect the mensage between process from fd[1] to fd[0].

This function is a trick that allow us to print instead of the standart output (fd = 1), but to another file descriptor, printing in a file. It because when we use dup2(), we are duplicating our fd to another fd, that is usually to the STDOUT or STDIN, allowing to read from a file.

int main(void){
	printf("before dup\n");
	int fd = open("outfile", O_WRONLY | O_TRUNC | O_CREAT, 0777);
	if (fd == -1)
		return (1);
	int fd2 = dup2(fd, STDOUT_FILENO);
	close(fd);
	printf("after dup\n");
	close(fd2);
	return (0);
}

before dup

1	after dup
2

As you see, before dup we print in the terminal, and after dup we print in the file, as we have 2 fds we need to close both.

This family of functions are used to execute commands in the shell, once it is called without an error, the process is finished, if not, we return the error 127, command not found.