read data through parent and send it to child using pipe in c

Question

I am trying to pass data from parent to child using pipe. But the parent process is getting closed before the child process gets executed.

I am providing the code please let me know where I am getting wrong.

    int main(void)
   {   
    int     fd[2], nbytes;
    pid_t   childpid;
    char    string[50];
    char    readbuffer[100];
    FILE *fp;
    pipe(fd);
    char var[100];

    if((childpid = fork()) == -1)
    {
            perror("fork");
            exit(1);
    }

    if(childpid != 0)
    {
    printf("PID of parent %u\n",getpid());                
    /* Child process closes up input side of pipe */
            close(fd[0]);
    fp = fopen("test.txt","r");     

    //scanning from file
    while(fscanf(fp,"%s\n",string)!=EOF)
    {
        strcat(var,string);
    }

    //printf("%s",string);
            /* Send "string" through the output side of pipe */
            write(fd[1], var, (strlen(var)+1));
            close(fd[1]);
           // exit(0);
    }
    else
    {
            printf("PID of child %u\n",getppid());
            /* Parent process closes up output side of pipe */

    close(fd[1]);

    //open a file
    //fp = fopen("test.txt","r");       

    //scanning from file
    //fscanf(fp,"%s\n",string);
    //printf("%s",string);
    //printinf from file
    //fprintf(fp,string);
            /* Read in a string from the pipe */
            nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
            printf("Received string: %s", readbuffer);
            //exit(0);
            close(fd[0]);
    } 
    return(0);
    }

and the output I am getting is like this:

PID of parent 7432

xyz@ubuntu:~/Desktop$ PID of child 4686

Received string: Canyoucanacan.

I do not want the parent process to close before the child process execution is over.

score 1 · Accepted Answer · answered Sep 16 '16 at 18:44

In the parent, you must wait for the child to complete with waitpid [or equivalent].

So, the last line of the parent part of the if/else should be:

waitpid(childpid,NULL,0);

Here's the updated code showing placement:

int
main(void)
{
    int fd[2],
     nbytes;
    pid_t childpid;
    char string[50];
    char readbuffer[100];
    FILE *fp;

    pipe(fd);
    char var[100];

    if ((childpid = fork()) == -1) {
        perror("fork");
        exit(1);
    }

    if (childpid != 0) {
        printf("PID of parent %u\n", getpid());
        /* Child process closes up input side of pipe */
        close(fd[0]);
        fp = fopen("test.txt", "r");

        // scanning from file
        while (fscanf(fp, "%s\n", string) != EOF) {
            strcat(var, string);
        }

        // printf("%s",string);
        /* Send "string" through the output side of pipe */
        write(fd[1], var, (strlen(var) + 1));
        close(fd[1]);
        // exit(0);

#if 1
        waitpid(childpid,NULL,0);
#endif
    }

    else {
        printf("PID of child %u\n", getppid());
        /* Parent process closes up output side of pipe */

        close(fd[1]);

        // open a file
        // fp = fopen("test.txt","r");

        // scanning from file
        // fscanf(fp,"%s\n",string);
        // printf("%s",string);
        // printinf from file
        // fprintf(fp,string);
        /* Read in a string from the pipe */
        nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
        printf("Received string: %s", readbuffer);
        // exit(0);
        close(fd[0]);
    }

    return (0);
}

You're welcome. If you'd like to see a more advanced usage of pipes (i.e. where the child reads from an input pipe and writes to an output pipe), see my recent answer here: http://stackoverflow.com/questions/39497736/pipe-function-not-executing-properly/39499414#39499414 It's about creating a shell-like program that does pipes, so the exact child actions differ from yours, but it may be useful for the future. — Craig Estey, Sep 16 '16 at 18:58

read data through parent and send it to child using pipe in c

1 Answers1