client.c

#include "mesg.h"

void client(int readfd, int writefd) {
  size_t len;
  ssize_t n;
  struct mymesg mesg;
  
  Fgets(mesg.mesg_data, MAXMESGDATA, stdin);
  len = strlen(mesg.mesg_data);
  
  if (mesg.mesg_data[len - 1] == '\n') {
    len--;
  }
  
  mesg.mesg_len = len;
  mesg.mesg_type = 1;
  
  Mesg_send(writefd, &mesg);
  
  while ((n = Mesg_recv(readfd, &mesg)) > 0) {
   Write(STDOUT_FILENO, mesg.mesg_data, n); 
  }
  
}

mesg.h

#include "unpipc.h"

#define MAXMESGDATA (PIPE_BUF - 2*sizeof(long))

#define MESGHDRSIZE (sizeof(struct mymesg) - MAXMESGDATA)

struct  mymesg {
  long mesg_len;
  long mesg_type;
  char mesg_data[MAXMESGDATA];
};

ssize_t mesg_send(int, struct mymesg *);
void Mesg_send(int, struct mymesg *);
ssize_t mesg_recv(int, struct mymesg *);
ssize_t Mesg_recv(int, stuct mymesg *);

mesg_recv.c

#include "mesg.h"

ssize_t mesg_recv (int fd, struct mymesg *mptr) {
  
  size_t len;
  ssize_t n;
  
  if ((n = Read(fd, mptr, MESGHDRSIZE)) == 0) {
    return (0);
  }
  
  else if (n != MESGHDRSIZE) {
    err_quit("message header: expected %d, got %d", MESGHDRSIZE, n);
  }
  
  if ((len = mptr->mesg_len) > 0) {
    if ((n = Read(fd, mptr->mesg_data, len)) != len) {
      err_quit("message data: expected %d, got %d",len, n);
    }
  }
  
  return len;
}

mesg_send.c

#include "mesg.h"

ssize_t mesg_send (int fd, struct mymesg *mptr) {
  
  return (write(fd, mptr, MESGHDRSIZE + mptr->mesg_len));
  
}

server.c

void server(int readfd, int writefd) {
  FILE *fp;
  ssize_t n;
  struct mymesg mesg;
  
  mesg.mesg_type = 1;
  if ((n = Mesg_recv(readfd, &mesg)) == 0) {
    err_quit("pathname missing");
  }
  
  mesg.mesg_data[n] = '\0';
  
  if ((fp = fopen(mesg.mesg_data, "r")) == NULL) {
    snprintf(mesg.mesg_data + n, sizeof(mesg.mesg_data) - n, "can't open %s\n", strerror(errno));
    mesg.mesg_len = strlen(mesg.mesg_data);
    Mesg_send(writefd, &mesg);
  } else {
    while (Fgets(mesg.mesg_data, MAXMESGDATA, fp) != NULL) {
      mesg.mesg_len = strlen(mesg.mesg_data);
      Mesg_send(writefd, &mesg);
    }
    Fclose(fp);
  }
  
  mesg.mesg_len = 0;
  Mesg_send(writefd, &mesg);
}