Problem in writing to an output file using pipes

Question

I am writing a C program on working of transport layer multiplexing using OS tools. There is 3 stream of bits, in file1, file2 and file3 respectively. I read 10 bits from file1, file2, file3 in sequence and combine them into a frame and store it in file multiplex.

Given, the only means of communication is pipe created by you.

Please Note: I didn't use fwrite function and wanted to explore more with pipes.

The corresponding code is

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/wait.h>


int main()
{
        int fd[3][2], pid,i, ch_count=10, count, status,f;
        FILE *fp, *fp1;
        fp1=fopen("multiplex.txt", "w");
        char ch, inp[10];

        for(i=0;i<3;i++)
                pipe(fd[i]);

        for(i=0;i<3;i++)
        {
                pid=fork();
                if(pid==0)
                {
                        printf("Inside Child process \n");
                        f=fileno(fp1);
                        close(fd[i][0]);
                        dup2(fd[i][1],f);
                        if(i==0)
                                        fp=fopen("file1.txt","r");
                        else if(i==2)
                                        fp=fopen("file2.txt","r");
                        else
                                        fp=fopen("file3.txt", "r");
                        while(1)
                        {
                                count=0;
                                while(( ch = fgetc(fp) ) != EOF && count<=ch_count)
                                        inp[count++]=ch;
                                if(ch==EOF)
                                        exit(0);
                                printf("The string read is %s \n", inp);
                                write(fd[i][1], inp, 10);
                                sleep(3-i);
                        }
                        close(fd[i][1]);
                }
                else
                        printf("Parent process \n");
        }
        return 0;
}

The problem is:

i) I can't write anything in multiplex file.

ii) I read alien characters when I print the inp string. Moreover the strings read aren't in sequence(i.e, file1 first, file2 second then file3) . The output is:

The string read is s not goldAR^�q�@�      //from file 2
                                     �7�U 
The string read is l that glitR^�q�@�      //from file 2 again
                                     �7�U 
The string read is s over the R^�q�@�      //from file 1
                                     �7�U 
The string read is well that eR^�q�@�      //from file 3
                                     �7�U 
The string read is ers is not R^�q�@�      //from file 2
                                     �7�U 
The string read is oldAll thatR^�q�@�
                                     �7�U 
The string read is ds well.AllR^�q�@�
                                     �7�U 
The string read is glitters isR^�q�@�
                                     �7�U 
The string read is azy dogThe R^�q�@�
                                     �7�U 
The string read is not goldAllR^�q�@�

What is the problem?

Edit: The contents of file1

The_quick_brown_fox_jumps_over_a_lazy_dog_The_quick_brown_fox_jumps_over_a_lazy_dog_`

The contents of file2

The_quick_brown_fox_jumps_over_a_lazy_dog_The_quick_brown_fox_jumps_over_a_lazy_dog_

The contents of file3

All's_well_that_ends_well_All's_well_that_ends_well_All's_well_that_ends_well_

Answer 1

i) I can't write anything in multiplex file.

by

        fp1=fopen("multiplex.txt", "w");
        /* ... */
                        f=fileno(fp1);
                        dup2(fd[i][1],f);

you disassociate fp1 from "multiplex.txt" (within the children). Using fp1 within the children would be risky/undefined because FILE is usually buffered and changes in buffer position can not be shared between processes.

You will have to redesign your program (write into pipe in the children, read within the parent) to implement the wanted functionality.

ii) I read alien characters when I print the inp string.

                                while(( ch = fgetc(fp) ) != EOF && count<=ch_count)
                                        inp[count++]=ch;
                                /* ... */
                                printf("The string read is %s \n", inp);

does not zero-terminate it. Either do a inp[count] = '\0' before the printf, or use

                                printf("The string read is %.*s \n", (int)count, inp);