Thread: Communication pipe

You mean these errors?

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘main’:
foo.c:23:11: error: ‘filedes’ undeclared (first use in this function)
       dup(filedes[0]); /* duplicate the new input file desired*/
           ^
foo.c:23:11: note: each undeclared identifier is reported only once for each function it appears in
foo.c:29:16: warning: format not a string literal and no format arguments [-Wformat-security]
         printf(tab); /* print the message */
                ^
foo.c:43:16: warning: format not a string literal and no format arguments [-Wformat-security]
         printf(tab); /* writing in the new output file (communication pipe input) */
                ^
foo.c:10:23: warning: variable ‘s’ set but not used [-Wunused-but-set-variable]
   char tab[TABSIZE], *s;

Let's start with this.
> int pipe(int filedes[2]); /* create a communication pipe */
You just made another prototype for pipe.
You didn't declare an array called filedes, and you didn't call pipe either.

int filedes[2];
pipe(filedes);

> printf(tab);
All someone has to do is type in a % character, and you've lost.
Never pass user input as the format string for printf.

Originally Posted by Salem

You mean these errors?

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘main’:
foo.c:23:11: error: ‘filedes’ undeclared (first use in this function)
       dup(filedes[0]); /* duplicate the new input file desired*/
           ^
foo.c:23:11: note: each undeclared identifier is reported only once for each function it appears in
foo.c:29:16: warning: format not a string literal and no format arguments [-Wformat-security]
         printf(tab); /* print the message */
                ^
foo.c:43:16: warning: format not a string literal and no format arguments [-Wformat-security]
         printf(tab); /* writing in the new output file (communication pipe input) */
                ^
foo.c:10:23: warning: variable ‘s’ set but not used [-Wunused-but-set-variable]
   char tab[TABSIZE], *s;

Let's start with this.
> int pipe(int filedes[2]); /* create a communication pipe */
You just made another prototype for pipe.
You didn't declare an array called filedes, and you didn't call pipe either.

int filedes[2];
pipe(filedes);

> printf(tab);
All someone has to do is type in a % character, and you've lost.
Never pass user input as the format string for printf.

First, thank you for your remarks.
I declared the communication pipe properly now, and I used printf correctly.
Here is the new program:

Code:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
 
#define TABSIZE 512
 
int main(){
  int pid;
  int filedes[2];
  pipe(filedes); /* create a communication pipe */
  char tab[TABSIZE];
 
  pid = fork(); /* create son process */
   
  switch(pid){
   
    case -1:
      break;
 
    case 0: /* son process */
       
      /* REDIRECTION */
      close(STDIN_FILENO); /* close standard input file */
      dup(filedes[0]); /* duplicate the new input file desired*/
      close(filedes[0]); /* the communication pipe is now the standard input file */
       
      /* listening in the the pipe and printing the received message in the terminal*/
      for (;;) {
        fgets(tab, TABSIZE, stdin); /* getting the text contained in the input file (that corresponds to the pipe) and putting it in the tab */
        printf("%s", tab); /* print the message */
      }
 
    default: /* father process */
       
      /* REDIRECTION */
      close(STDOUT_FILENO);
      dup(filedes[1]); 
      close(filedes[1]);
 
      /* listening in the the pipe and printing the received message in the new output file (which is the communication pipe) */
      for (;;) {
        fgets(tab, TABSIZE, stdin); /* getting the text written in the terminal and putting it in a tab */
        printf("%s", tab); /* writing in the new output file (communication pipe input) */
      }
  }  
  exit(EXIT_SUCCESS);
}

I also deleted the prompt displaying in the father's code because I think that doesn't have sense since the output file of the father is no longer the screen.
the program runs now with errors now. But it doesn't work as expected, the son does not printf the messages he receives from the father on the screen (if he receives something of course)

An example.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/select.h>

const int ReadEnd = 0, WriteEnd = 1;

void die(const char *msg) {
  fprintf(stderr,"Grrr:%s:%d:%s\n",strerror(errno),errno,msg);
  exit(1);
}

void childScreenWriter(int readfd) {
  fprintf(stderr,"childScreenWriter enter\n");
  char buff[BUFSIZ] = { 0 };
  FILE *fp = fdopen(readfd,"r");
  setvbuf(fp,NULL,_IOLBF,0); // make it line buffered, pipe ends start fully buffered
  while ( fgets(buff,BUFSIZ,fp) ) {
    printf("C:%s",buff); // tag as being received by child
  }
  fprintf(stderr,"childScreenWriter exiting\n");
  fclose(fp);  // also closes associated descriptor
}

void parentInputReader(int writefd) {
  fprintf(stderr,"parentInputReader enter\n");
  char buff[BUFSIZ];
  FILE *fp = fdopen(writefd,"w");
  setvbuf(fp,NULL,_IOLBF,0); // make it line buffered, pipe ends start fully buffered
  while ( fgets(buff,BUFSIZ,stdin) ) {
    fprintf(fp,"P:%s",buff); // tag as being sent from parent to child
  }
  fprintf(stderr,"parentInputReader exiting\n");
  fclose(fp);  // also closes associated descriptor
}

#define CHECK(op,expected,msg) \
  if ( op != expected ) die(msg)

int main ()
{
  int parent2child[2];
  CHECK(pipe(parent2child),0,"pipe1");
  pid_t p = fork();
  if ( p == 0 ) {
    printf("Child=%d\n",getpid());
    CHECK(close(parent2child[WriteEnd]),0,"close1");
    childScreenWriter(parent2child[ReadEnd]);
  } else {
    printf("Parent=%d\n",getpid());
    CHECK(close(parent2child[ReadEnd]),0,"close3");
    parentInputReader(parent2child[WriteEnd]);

    int status;
    fprintf(stderr,"Waiting...\n");
    CHECK(waitpid(p,&status,0),p,"wait");
    fprintf(stderr,"Exit status=%d\n",status);
  }
  return 0;
}

$ ./a.out 
Parent=5186
parentInputReader enter
Child=5187
childScreenWriter enter
hello
C:P:hello
world
C:P:world
bye
C:P:bye
parentInputReader exiting
Waiting...
childScreenWriter exiting
Exit status=0

Here's an example that redirects the standard streams like you were trying to do.
Note that you can use stderr to print the parent's prompt since stderr is still going to the terminal.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
 
#define TABSIZE 512
enum {INPUT_END, OUTPUT_END};
 
int main() {
    char tab[TABSIZE];
 
    int fd[2];
    if (pipe(fd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }
 
    pid_t pid = fork();
 
    switch (pid) {
    case -1:
        perror("fork");
        exit(EXIT_FAILURE);
        break;
 
    case 0: // child
        dup2(fd[INPUT_END], STDIN_FILENO);
        close(fd[INPUT_END]);
        close(fd[OUTPUT_END]);
 
        while (fgets(tab, TABSIZE, stdin))
            printf("child: %s", tab);
 
        close(STDIN_FILENO);
        break;
 
    default: // parent
        dup2(fd[OUTPUT_END], STDOUT_FILENO);
        close(fd[INPUT_END]);
        close(fd[OUTPUT_END]);
 
        while (fputs(">>> ", stderr), fgets(tab, TABSIZE, stdin)) {
            printf("%s", tab);
            fflush(stdout);
        }
        fputs("\n", stderr);
 
        close(STDOUT_FILENO);
        wait(NULL);
    }
 
    return 0;
}