Thread: IPC using PIPES

parent.c

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>

/*
 *Decimal number is taken in through standard input and
 *output to a file called bin.txt
 */
int main(int argc, char* argv[]){
	pid_t pid;
	int rv;
	int	commpipe[2];		/* This holds the fd for the input & output of the pipe */
	char input[100];
	FILE *inputfile;
	FILE *outputfile;
	inputfile = fopen("dec.txt", "r");
	outputfile = fopen("bin.txt", "w");
	
	if(inputfile == NULL) fprintf(stderr, "dec.txt NOT opened successfully\n");
	else fprintf(stderr, "dec.txt opened successfully\n");
	
	if(outputfile == NULL) fprintf(stderr, "bin.txt NOT created successfully\n");
	else fprintf(stderr, "bin.txt created successfully\n");
	
	/* Setup communication pipeline first */
	if(pipe(commpipe)){
		fprintf(stderr,"Pipe error!\n");
		exit(1);
	}
	
	/* Attempt to fork and check for errors */
	if( (pid=fork()) == -1){
		fprintf(stderr,"Fork error. Exiting.\n");  /* something went wrong */
		exit(1);        
	}
	
	if(pid == 0){
		dup2(commpipe[0], 0);
		close(commpipe[1]);
		if(execl("child","child",NULL) == -1){
			fprintf(stderr,"execl Error!");
			exit(1);
		}
		
	}
	else{
		/* A positive (non-negative) PID indicates the parent process */
		dup2(commpipe[1],1);	/* Replace stdout with out side of the pipe */
		close(commpipe[0]);		/* Close unused side of pipe (in side) */
		setvbuf(stdout,(char*)NULL,_IONBF,0);	/* Set non-buffered output on stdout */
		while(fscanf(inputfile, "%s", &input) != EOF){
			printf("%s\n", input);
		}
		close(commpipe[1]);
		dup2(fileno(stdout), 1);
		printf("Pipe closed\n");
	}
	wait(&rv);				/* Wait for child process to end */

	return 0;
	
}

child.c

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
void dectohex(int decimal, char *hex){
	int index=0;
	
	while(decimal){
		hex[index] = (decimal%16) < 10 ? (decimal%16): 'A' + (decimal%16) - 10;
	}
	printf("%d\n", hex);
}
/*
 *Will take a decimal number from the stantard input and
 *output the hexidecimal version to hex.txt
 */
int main(int argc, char* argv[]){
	char input[256];
	char hex[256];	
	do{
		if(fgets(input, sizeof(input), stdin) != NULL)
			printf("Child got: %s", input);
	}while(!feof(stdin));
	
	return 0;
}

What I have here is the code from two processes.
The child is simple:
When it is run alone it will take input from the terminal and will re-output the input that was given separated by \n. It will stop running when EOF is encountered (user input ctrl D)

The Parent is a bit more complex:
The parent will get its input from a file "input.txt" "input.txt" will contain random words or numbers, one per line. The parent is forked, a child is created (child.c) which is then duped into taking its input from a pipe called commpipe.
The parent will feed the child input and the child will output what it was fed.

All of this is done well enough but what I need is a way for the child to be handed an EOF when the EOF of input.txt is encounter. What can I change in parent.c or child.c to get this.
Is there a way to just close the writing end of the pipe and have the child detect that the parent's writing end of the pipe is no longer open?

Originally Posted by MK27

Another possibility is to just communicate with the child (send "END" or something, and then close it, and the child closes when it recieves "END").

I do not want to send the child flags by having to type "END" or something similar at the end of every file. That does not achieve what I am trying to get.

Originally Posted by MK27

Have you tried that (just closing the end of the pipe)?

Am I not doing that with the

Code:

close(commpipe[1]);

If so, then I need to know a way to make the child process check if the writing end of the pipe has been closed.

This cant work because the child would end upon encountering something other than ctrl D.

If the child is run by itself then the user should be able to type words indefinitely until he or she hits ctrl D. Then the child will stop reading.

I could have easily made the loop break upon seeing an END or some other reserved word but that does not achieve what I am trying to do here.

I want they parent (in a manner of speaking) to either send a EOF, ctrl D, w/e through the pipe to the child or I want the parent to close its side of the pipe and have the child detect that the parent has closed its end of the pipe.

Originally Posted by MK27

Then do that (send an EOF or a ctrl-d to the child). The idea is exactly the same, just my idea is slightly more foolproof and requires a wooping 2 more lines of code (at most).

I know this is stupid but HOW do i send the EOF? Can i send it through the stdout using printf() and if so, how?

Like, I literally want to send ctrl D through the pipe to the child.

That seems as though it would work but my child program just spits out "Child got:?" and sits there waiting for more stuff to read through the pipe.

For some reason it is seeing that EOF as something that can only be represented as ?

Code:

while(fgets(input, sizeof(input), stdin)!=NULL){
		if(strchr(input, EOF)) break;
		else printf("Child got: %s", input);
	}

seems to work so far