Question

I working my way through a C++ and Operating Systems book and I’ve come upon an assignment that requires creation, writing, and reading from pipes. However my program stalls on reading from the second pipe. My program is to accept input and parse out a space delimited string into tokens and classifying those tokens accordingly. My code is bellow with my problem area marked. Any help is as always very appreciated.

edit: This is supposed to have two children. One for processing the space delimited tokens and the other for determining the type of delimited tokens. As far as debugging goes I only have access to cout as a debugger. So I inserted a cout before the read and after the one before the read appeared but the one after did not.

#include <iostream>
#include <fstream>
#include <string>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

using namespace std;

//declaring the pipes
int pipeOne[2];
int pipeTwo[2];

struct inputStruct {
char str[256]; /* one extra spot for \n */
int  len;      /* length of str         */
int  flag;     /*  0 for normal input, 1 to indicate “done” */
};

struct tokenStruct {
char token[256]; /* tokens can be 255 max */
int  flag;       /* same as inputStruct   */
int  tokenType;  /* a code value          */
};

void dataProcess(){

//new input struct to contain the the input from the parent
inputStruct input;

//the intial read from the pipe to populate the input stuct
read( pipeOne[0], (char*)&input, sizeof(inputStruct));

//set the flag
int flag = input.flag;

while (flag != 1){

    int size = 0;

    //get the size of the array up until the null character
    while (input.str[size] != '\0'){
        size++;
    }

    //Here's the parsing of each token
    for (int i=0; i<size; i++) {

        int tokenLength;

        tokenStruct token;

        //while the char isn't white space or null increment through it
        while (input.str[i] != ' ' && input.str[i] != '\0') {

            //a is the index of the string token 
            int a = 0;

            //write the parsed string
            token.token[a] = input.str[i];

            a++;
            i++;

        }

        //write to process 2
        write(pipeTwo[1], (char*)&token, sizeof(tokenStruct));

    }

    //read again and store the results
    read(pipeOne[0], (char*)&input, sizeof(inputStruct));
    flag = input.flag;

}

tokenStruct token;

token.flag = flag;

//final write to the second child to tell it to commit suicide
write(pipeTwo[1], (char*)&token, sizeof(tokenStruct));

exit(0);
}

void tokenClassifer(){

tokenStruct token;

//Problem area is here on ****************************************************

//the initial read
read(pipeTwo[0], (char*)&token, sizeof(tokenStruct));

while (token.flag != 1){

    int size = 0;

    //get the size of the array up until the null character
    while (token.token[size] != '\0'){
        size++;
    }

    if (size == 1) {
        //check for the one char things first
        switch (token.token[0])
        {
            case '(':
                token.tokenType = 0;
                break;
            case ')':
                token.tokenType = 0;
                break;
            case ';':
                token.tokenType = 0;
                break;
            case '+':
                token.tokenType = 1;
                break;
            case '-':
                token.tokenType = 1;
                break;
            case '/':
                token.tokenType = 1;
                break;
            case '*':
                token.tokenType = 1;
                break;
            default:
                if (isdigit(token.token[0])) {
                    token.tokenType = 2;
                } else {
                    token.tokenType = 3;
                }
                break;
        }

    } else {

        bool isStr;

        int i = 0;
        //check for the more than one character
        while (token.token[i] != '\0'){

            //check if it's a string or digits
            if (isdigit(token.token[0])) {
                isStr=false;
            } else{
                //set up the bools to show it is a string
                isStr=true;
                break;
            }
        }

        //if it is a string token type 3
        if (isStr) {
            token.tokenType = 3;
        } else {
            //if not then it's digits and token type 2
            token.tokenType = 2;
        }

    }

    //print out the token and token type
    cout << "Token type is: " << token.tokenType << "Token value is: " << token.token << "\n";

    //read the pipe again and start the process all over
    read(pipeTwo[0], (char*)&token, sizeof(tokenStruct));
}

exit(0);
}

int main()
{

//create the pipes for reading and writing between processes
pipe(pipeOne);
pipe(pipeTwo);

//fork off both processes
int value = fork();
int value2 = fork();

//do the process for the first fork
if(value == 0){

    //fork one
    dataProcess();

} else {

    wait(0);

}

//do the process for the second fork
if (value2 == 0) {
    //fork two

    //the token classifer function for the second fork
    tokenClassifer();

} else {
    cout << "Type some tokens (or just press enter to quit) \n";
    //this is all of the parent functions        
    for (string line; getline(cin, line); )
    {

        inputStruct input;
        if (line.empty())
        {
            // if the line is empty, that means the user didn't
            // press anything before hitting the enter key
            input.flag = 1;
            write( pipeOne[1], (char*)&input, sizeof(inputStruct));
            break;
        } else {

            //else copy the string into an array
            strcpy(input.str, line.c_str());

            //set the flag to zero to show everthing is ok
            input.flag = 0;
        }

        //write the stuct to the pipe
        write( pipeOne[1], (char*)&input, sizeof(inputStruct));

        cout << "Type some tokens (or just press enter to quit) \n";
    }

    wait(0);
}
}

Answer 1

One problem that is evident:

//fork off both processes
int value = fork();
int value2 = fork();

This will fork 3 new processes. The initial fork will leave you with two processes, each of which go on to fork a new process.

EDIT:

Proper forking:

int value = fork();
if (value == 0) {
    // do child stuff
    exit(0);
} else if (value == -1) {
    //fork failed
}

int value2 = fork();
if (value2 == 0) {
    //do child stuff
    exit(0);
} else if (value2 == -1) {
    //fork failed
}

I’m actually not quite clear about how data goes through your program, so I’ll leave it to you to add the waits. I’d actually change the names of value and value2, but that’s just me. Also, I’m only addressing the forking issue here so there may be other problems with your code (which I kind of suspect since you have two pipes).

EDIT 2:

Another issue that I see is that you’re not closing the ends of the pipes that you don’t use. If you never close the write end of a pipe, your reads will block until the pipe has data (or there are no more writers to the pipe, that is, the write end is not open). This means that the write end of the pipe should be closed in all processes when you are not using it or are finished with it.