How to pass the contents of a file to another process through a named pipe? And output to a file?

Question

linux

I created a named pipe and passed a string to it using these 2 programs:

#include <stdio.h>
#include <sys/stat.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>

#define FIFO_NAME "myfifo"
#define BUF_SIZE 512
int main (void)
{
  FILE * fifo;
  char * buf;
  if (mkfifo ("myfifo", 0640) == -1) {
    fprintf (stderr, "Can't create fifo\n");
    return 1;
  }
  fifo = fopen (FIFO_NAME, "r");
  if (fifo == NULL) {
    fprintf (stderr, "Cannot open fifo\n");
    return 1;
  }
  buf = (char *) malloc (BUF_SIZE);
  if (buf == NULL) {
    fprintf (stderr, "malloc () error\n");
    return 1;
  }
  fscanf (fifo, "%s", buf);
  printf ("%s\n", buf);
  fclose (fifo);
  free (buf);
  unlink (FIFO_NAME);
  return 0;
}

#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>

#define FIFO_NAME "myfifo"

int main (int argc, char ** argv)
{
  int fifo;
  
  if (argc < 2) {
    fprintf (stderr, "Too few arguments\n");
    return 1;
  }
  fifo = open (FIFO_NAME, O_WRONLY);
  if (fifo == -1) {
    fprintf (stderr, "Cannot open fifo\n");
    return 1;
  }
  if (write (fifo, argv[1], strlen (argv[1])) == -1) {
    fprintf (stderr, "write() error\n");
    return 1;
  }
  close (fifo);
  return 0;
}

I understand how to output a line from the terminal to the pipe, but I don’t understand how to read the contents of a file, transfer it to a FIFO and then write it to another file. How to do it?