Copying contents of a file using pipe() and fork()

Question

Similar questions has already been asked, but their solutions aren't helping me much

Program that read file and send it to parent process with pipe

Read/writing on a pipe, accomplishing file copying in C

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I'm trying to read from a file test.txt ( which contains a single line of text), write it to a pipe, from where a child process will read from the pipe and write the contents to another file.

 /* Read the contents of a file and display it using pipe */

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

void main()
{
  char buffer[100];
  char childbuff[100];
  int fd[2], des, bytes, target;

  pipe(fd);

  if(fork()) {
    /* parent process closes the downstream */
    close(fd[0]);

    /* reads the file */
    des = open("test.txt", O_RDONLY);
    bytes = read(des, buffer, sizeof(buffer));

    /* puts data in pipe */
    write(fd[1], buffer, bytes);
  } else {
    /* Child process closes the upstream */
    close(fd[1]);

    /* reads from the pipe */
    read(fd[0], childbuff, sizeof(childbuff));
    close(fd[0]);

    /* output the received string */
    printf("\nReceived string is -- %s", childbuff);
    target = open("copy.txt", O_CREAT, 00777);
    write(target, childbuff, (strlen(childbuff)-1));
  }
}

Problem is printf() prints the string on terminal, a file named copy.txt also gets created, but nothing is getting copied to it (it seems there is a problem with the write() function )

however, if I change

write(target, childbuff, (strlen(childbuff)-1));

to

write(1, childbuff, (strlen(childbuff)-1));

string is simply getting written on my terminal.

So what possibly am I doing wrong while writing to file?

Answer 1

You also need O_WRONLY to write to the file:

target = open("copy.txt", O_CREAT |O_WRONLY, 00777);

Note that you can't use strlen() or %s to print it as a C-string. read(2) doesn't return a NUL terminated string.

Instead get the number of bytes read from read() and use it in write():

    ssize_t num_bytes = read(fd[0], childbuff, sizeof(childbuff));

    write(target, childbuff, num_bytes);

You should check the return of all system calls for failure.