Grep concatenation in C

I am trying to do a multiple grep using c language. Here is the code

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

int main(int argc, char** argv)
{
    int pid1, pid2;
    int stdout = 1;
    int pipefd[2];

    char* name = malloc(sizeof(char) * 100);
    char* year = malloc(sizeof(char) * 100);

    bzero(name, sizeof(name));
    bzero(year, sizeof(year));

    pid1 = fork();

    if(pid1 == 0){
        strcpy(name, "Jack");
        strcpy(year, "2018");
        close(1);

        if(pipe(pipefd) == -1){
            perror("pipe");
            exit(-1);
        }

        pid2 = fork();
        dup(pipefd[1]);
        close(pipefd[1]);
        execlp("grep", "grep", year, "file.txt", (char*)NULL);

        if(pid2 == 0){
            read(pipefd[0], &stdout, sizeof(stdout));
            int a = dup2(pipefd[0], 0);
            close(pipefd[0]);
            execlp("grep", "grep", name, a, (char*)NULL);
        }
    } else if (pid1 < 0){
        printf("pid error\n");
    }
}

So basically it gives me error in the first exec, saying that the file descriptor is invalid. I didn't find a solution to pass the result of the first exec to the second fork.

Solution

Your program behaves like this (I only look at fork and exec calls):

creation: process A
fork: you have 2 processes: A(parent) and B(child)
you do nothing more in A
in B, you make a fork: you have 2 processes B and C its child
in both B and C you exec a grep command.
your code is no longer executed in B and C : they have been replaced by grep processes.

But, you were not far to a solution.

What you have to do is:

create a pipe in the parent process
create the first child
- setup its i/o
- exec the child program
create the second child
- setup its i/o
- exec the child program
wait for child to end.

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

int main(int argc, char **argv)
{
    int pid1, pid2;
    int stdout = 1;
    int pipefd[2];

    /* patterns to search */
    char name[] = "Jack";
    char year[] = "2018";

    /* pipe creation */
    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(-1);
    }

    /* create first child */
    pid1 = fork();
    if (pid1 == 0) {
        close(pipefd[0]);
        dup2(pipefd[1], 1);
        close(pipefd[1]);
        execlp("grep", "grep", year, "file.txt", (char *) NULL);
    }

    /* second child */
    pid2 = fork();
    if (pid2 == 0) {
        close(pipefd[1]);
        dup2(pipefd[0], 0);
        close(pipefd[0]);
        execlp("grep", "grep", name, "-", (char *) NULL);
    }

    /* close pipe to prevent child hang up */
    close(pipefd[0]);
    close(pipefd[1]);

    while (1) {
        pid_t cid = wait(NULL);
        if (-1 == cid)
            break;
        printf("process %d terminated\n", cid);
    }

}