Question

I’m attempting to write a structure consisting of a an array of characters, an integer value, and a pointer to a pipe. The structure represents a node in a singly linked list.

//Define a linked-list node object
typedef struct node{
    char word[128];
    int frequency;
    struct node *next;
} NODE;

The object of the program is to use a pipe to pass a node from several concurrent child processes to the parent. The pipe structure I’ve implemented seems to work fine with regular character arrays but won’t work if I try to pass the entire node object.

    //for each file argument, create a child process
        for (i = 1; i < argc; i ++)
        {
            pipe(p[i-1]);
            pid = fork();

            if (pid == 0)
            {
                //child process
                close(p[i-1][0]);
                NODE *tmp;
                NODE *out = freqCheck(argv[i], tmp);
                write(p[i-1][1], out, sizeof(NODE));
                exit(0);
            }
        }
if (pid > 0){
                //parent process
                int j;
                for (j = 0; j < argc-1; j++)
                {
                    close(p[j][1]);
                    NODE *tmp;
                    read(p[j][0], tmp, sizeof(NODE));

                    printf("%s\n", tmp->word);
                }


            }

When the parent process attempts to read from the pipe and interpret one of the attributes of the structure, I just get null values back.

I’m using an array of 2 element integer arrays to keep track of each child processes’ pipe. The printf statement above returns null revery time.

Any idea what I’m doing wrong?

Code for the FrqCheck method:

//Method for determining the most occuring word
NODE * freqCheck(char *file, NODE *max)
{
    int i; 
    FILE *f;
    char str[128];

    //Set up head and tail nodes
    NODE *head = NULL;
    NODE *tail = NULL;

    if ((f = fopen(file, "r")) == NULL)
        {
            //sprintf(output, "%s could not be opened.", file);
        }else
        {
            //scan each word of the input file
            while(fscanf(f, "%s ", str) != EOF)
            {
                //if the linked-list has no nodes, create one
                if (head == NULL)
                {
                    NODE *n;
                    n = (NODE *)malloc(sizeof(NODE));
                    strcpy(n->word, str);
                    n->frequency = 1;
                    n->next = NULL;
                    head = n;
                    tail = n;

                }else{  //search the linked list for the found word.

                    NODE *current = head;
                    int found = 0;

                    while((current != NULL) && (found == 0))
                    {
                        //if the word is found increment the frequency
                        if (strcmp(current->word, str) == 0)
                        {
                            current->frequency ++;
                            found = 1;
                        }else
                        {
                            current = current->next;
                        }
                    }

                    //if the word is not found, create a node and add to the liked-list
                    if (found == 0)
                    {
                        NODE *new;
                        new = (NODE *)malloc(sizeof(NODE));
                        strcpy(new->word, str);
                        new->frequency = 1;
                        new->next = NULL;
                        tail->next = new;
                        tail = new;
                    }
                }
            }
            //traverse the linked-list and find the word with the maximum frequency
            NODE *tmp = head;
            max = tmp;

            while (tmp != NULL)
            {
                if (tmp->frequency > max->frequency)
                {
                    max = tmp;
                }else
                {
                    tmp = tmp->next;
                }
            }
            //sprintf(output, "%s %s %d", file, max->word, max->frequency);
        }
    //printf("%s\n", max->word);
    return max;
}

Answer 1

Where are you allocating storage for your NODE structures? Does freqCheck allocate storage? In the parent process, when you call read() and then printf, you pass in an uninitialized NODE pointer, so of course you’re getting undefined behavior.