redirecting stdout to pipe write end

Question

I'm writing a little program, and here is what it should do.

In the main process I have to create a new one and that one should execute another program which only does a printf("text"). I want to redirect the pipe write end on stdout and the main process should read from its pipe read and and print it on stdout. I wrote the code but again and again I get a segmentation fault when the parent process tries to read from the pipe.

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

void write_to(FILE *f){
  char buf[50];
  fprintf(f,"KOMA");
}

int main(){
  int cpPipe[2];
  int child1_fd;
  int child2_fd;

  if(pipe(cpPipe) == -1){

    fprintf(stderr,"ERROR PIPE creation");
    exit(1);

  }else{printf("pipe couldn't be created\n");}

  child1_fd = fork();

  if(child1_fd < 0){
    fprintf(stderr, " CHILD creation error");
    exit(1);
  }

  if(child1_fd == 0){
    printf("*CHILD*\n");
    char program[] = "./Damn";
    int dupK;
    printf("stdout %d \n", STDOUT_FILENO);
    printf("stdin %d \n", STDIN_FILENO);
    printf("pipe1 %d \n", cpPipe[1]);
    printf("pipe0 %d \n", cpPipe[0]);

    // closing pipe write
    close(cpPipe[0]);
    close(1);
    dup(cpPipe[1]);

    printf("and");

    close(cpPipe[1]);
    exit(0);
  }else{
    printf("*Parent*\n");
    char *p;
    char *buf;
    FILE *pipe_read;

    close(cpPipe[1]);
    pipe_read = fdopen(cpPipe[0],"r");

    while((buf = fgets(p,30,pipe_read)) != NULL){
      printf("buf %s \n", buf);
    }

    wait();
    printf("Child is done\n");
    fclose(pipe_read);

    exit(0);
  }
}

Do I have to close the pipe write end when I redirect stdout to it?

Answer 1

Uhm,... the reason for your segmentation fault is here:

buf = fgets(p,30,pipe_read);

p is a pointer to essentially nowhere of importance. It's content is whatever is in the stack at the time of execution, you never initialize it. You need it to point to a chunk of memory you can use! Assign the return of a malloc() call to it, or declare it as char p[LEN].

Edit: you are also reopening already open file descriptors. Check the documentation on fgets and pipe, I think you are confused as to how they work.

Now, that said, the flow of your function is kinda confusing. Try working on clarifying it! Remember, code is meant to express intentions, ideas of functionality. Try using pencil and paper to organize your program, and then write it as actual code :).

Cheers!

Answer 2

Do I have to close the pipe write end when I redirect stdout to it?

In general, yes, because while there is a process with the write end of the pipe open, the processes reading the pipe will not get EOF and will hang. It is also tidy to close file descriptors you aren't going to use, of course.

Your code also says "pipe could not be created" in the success path.