Newbie on forking child process and I/O redirection

neo_in_matrix · 09-14-2005, 10:19 AM

Hi, I am new to child process concept under Linux. I want a simple but complete example code that demonstrates the following basics of forking child process:

1. Forking a child process.
2. Redirecting its stdin and stdout.
3. The parent process writes to stdout and at the same time reads child stdout.

Thanks for any information.

kees-jan · 09-14-2005, 10:47 AM

"simple but complete example code"...

People get more and more demanding these days....

Off the top of my head, the best I can do is some pseudocode. It'll be simple, but you have to complete it :-)

Code:

// Pseudocode. This will not compile and/or contain severe syntax errors

// Omitted error checking for simplicity. Don't try this at home

int main ()
{
    int fd[2]; // file descriptors

    // Create pipe
    pipe(fd);

    // Fork child process
    if (fork() == 0)
    {
        char buffer [80];
        
        // We're in the child process

        // close stdin
        close(0);

        // connect pipe to stdin
        dup(fd[0]);
        close(fd[0]);

        // read from stdin
        gets(buffer);

        // Prove that it worked
        printf("Parent said %s", buffer);
    }
    else
    {
        // We're in the parent process

        // close stdout
        close(1);

        // connect pipe to stdout
        dup(fd[1]);
        close(fd[1]);

        // Write to standard out
        printf ("Hello world!\n");
    }
    // the end
    return 0;
}

For details, refer to the manpages of the used functions. Or ask more specific questions

Groetjes,

Kees-Jan

neo_in_matrix · 09-15-2005, 02:03 AM

Thanks for replying.

I tested the code, it worked. But it's not exactly what I want.

What I want is bascially the same as what Eclipse does with gdb. It spawns gdb as its child process and writes commands to the child process and retrieves gdb's output and displays or parses the captured output.

Have I made my problem clear?

neo_in_matrix · 09-15-2005, 04:49 AM

Oops, I have found a solution myself after 3 hours of struggling. The program runs fine, but I wonder if I am making minor mistakes. See my code below:

Code:

void test_pipe()
{
	int		pipe1[2];
	int		pipe2[2];
	pid_t	pid;

	pipe(pipe1);
	pipe(pipe2);

	pid = fork();
	if(pid == 0)
	{
		puts("In child process");
		close(pipe1[1]);
		dup2(pipe1[0], STDIN_FILENO);

		close(pipe2[0]);
		dup2(pipe2[1], STDOUT_FILENO);

		puts("Spawning 'sort'");
		execlp("sort", "sort", 0);
	}
	else
	{
		puts("In parent process");
		close(pipe1[0]);

		puts("Writing to child input");
		FILE* fpw = fdopen(pipe1[1], "w");
		srand(time(0));
		puts("How many random numbers?");
		char* str = safegets();
		int n = atoi(str);
		for(int i = 0; i < n; i++)
		{
			fprintf(fpw, "%d\n", rand());
		}
		fclose(fpw);
		close(pipe1[1]);

		puts("Reading child output");
		close(pipe2[1]);
		char buf[1000];
		FILE* fpr = fdopen(pipe2[0], "r");
		n = 0;
		while(fgets(buf, 1000, fpr))
		{
			printf("#%d:%s", n, buf);
			n++;
		}
		fclose(fpr);
		close(pipe2[0]);
		puts("End of child output");

		puts("Waiting for child close");
		waitpid(pid, 0, 0);
	}
}

kees-jan · 09-16-2005, 03:05 AM

Wow... This is most impressive...

Two minor points:

After executing, for example

Code:

		close(pipe1[1]);
		dup2(pipe1[0], STDIN_FILENO);

you still have filedescriptor pipe1[0] open, but you no longer need it, so you might as well close it, like so:

Code:

		close(pipe1[1]);
		dup2(pipe1[0], STDIN_FILENO);
		close(pipe1[0]);

Secondly, in the parent, you first write a lot of stuff to the client process, and then proceed to read its output. In this case, this is fine, because 'sort' doesn't output anything before reading the last line of input.

Other programs (grep or cat being examples) may want to start writing output before reading the complete input. If these can't write output, they may stop reading input, causing your parent program to block on the writes. That would cause a deadlock.

Pipes do include some buffering, so this will only happen for "large" inputs and outputs, but it is still something worth considering.

Groetjes,

Kees-Jan