dup and dup2 commands

Question

What I'm trying to do is that put the output of the ls command in a file, and then use grep command to read from that file and store it in a new file and based on the contents on that file, print something on the terminal.

So the following output redirections are there:

1) from standard output to file known as oioioi.txt (for ls command)
2) from oioioi.txt to grep.txt (for grep command)
3) from grep.txt back to terminal

Here's my code:

#include<stdio.h>
#include<sys/wait.h>
#include<unistd.h>
#include<errno.h>
#include<stdlib.h>
#include<string.h>
int main(void)
{
        //if(!fork())
                //execl("/bin/grep","grep","input","oioioi.txt",0);
        FILE *fp1,*fp2;
        int fd,fd2;
        fp1=fopen("oioioi.txt","w+");
        fp2=fopen("grep.txt","w+");
        fd=fileno(fp1);
        fd2=fileno(fp2);
        char filename[40],id[40],marks[40],filename2[40];
        scanf("%s %s %s",filename,id,marks);
        char *execarg2[]={"ls",0};
        dup2(fd,1); //1st redirection
        //close(1);
        if(!fork())
        {
                execv("/bin/ls",execarg2);
        }
        sleep(1);
        wait(NULL);
        //dup(fd2);
        close(fd);
        dup(1);
        dup2(fd2,1); //2nd redirection
        if(!fork())
        {
                //sleep(1);
                execl("/bin/grep","grep",filename,"oioioi.txt",0);
        }
        sleep(1);
        wait(NULL);
        close(fd2);
        dup2(1,fd2); //3rd one which is the incorrect one
        fscanf(fp2,"%s",filename2);
                printf("%s",filename2);
        if(strcmp(filename2,filename)==0)
                printf("FILE FOUND");

        return(0);
}

I believe that the 3rd one is incorrect. However I'm not really sure of the first 2 either. If you guys could have a look at my code and tell me where I'm going wrong or give me an exampple using dup for the following redirections, i'd really really be grateful

Answer 1

The problem begins at your first dup call: dup(fd, 1);. In that line, you lose your standard output to the terminal. If you wanted to secretly keep a reference to it, you'd dup(STDOUT_FILENO); beforehand.

It follows that subsequent calls to printf will actually be redirected to fd2 and not to stdout's original fd (1).

There are probably some other issues lurking here, but that's as far as I was able to grok it (see my comments on the question).

n.b.

The manpage of dup2 states that it closes the new fd (second arg) if necessary, making the close in

close(fd2);
dup(1, fd2);

redundant.