[SOLVED] fifo special characters

doru · 06-26-2010, 12:23 PM

I use fifos to communicate between two simple programs, a client and a server. The client runs, sends three characters to the server on a fifo, reads three characters from the server on another fifo, and ends. When I use write and read with size (3'rd parameter) 3 or 4 I get strange characters when I read the fifos both in client and server. When I use write and read with size 64, everything looks right. As a beginner it took me a while to figure this out, and I would like to know what happens. I know that the client sends an EOF when it exits, but this does not seem to be a problem with size 64. Please throw to me some suggestions, as of what could cause this behaviour.

Here are the programs, in case you need them:

Code:

//client 
#include <stdio.h>
#include <string.h>
#include <fcntl.h>

#define CP "c6"
#define BUFSIZE 3

int main(void){

  int a, fdi, fdo;
  char *fifo1="fifo1", *fifo2="fifo2", s[BUFSIZE]="123", t[BUFSIZE];

  fdo = open(fifo1, O_WRONLY);
  printf("%s - fdo: %i\n", CP, fdo);

  fdi = open(fifo2, O_RDONLY);
  printf("%s - fdi: %i\n", CP, fdi);

  a = write(fdo, s, BUFSIZE);
  printf("%s - write: %i\n", CP, a);

  a = read(fdi, t, BUFSIZE);
  printf("%s - read: %i\n", CP, a);

  printf("%s - read: %s\n", CP, t);

}

//server
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
//#include <stdlib.h>

#define CP "s6"
#define BUFSIZE 3

int main(void){

  int a, fdo, fdi;
  char *fifo1="fifo1", *fifo2="fifo2", s[BUFSIZE]="321", t[BUFSIZE];

  fdi = open(fifo1, O_RDWR);
  printf("%s - fdi: %i\n", CP, fdi);

  fdo = open(fifo2, O_RDWR);
  printf("%s - fdo: %i\n", CP, fdo);

  for(;;){

    a = read(fdi, t, BUFSIZE);
    printf("%s - read: %i\n", CP, a);

    printf("%s - read: %s\n", CP, t);

    a = write(fdo, s, BUFSIZE);
    printf("%s - write: %i\n", CP, a);

//    read(0, t, BUFSIZE);

  }

}

harry edwards · 06-27-2010, 06:19 AM

BUFSIZE should be four as each char array should allow room to be NUL terminated, thus, a char's size is always one more than you want. The reason is due to how the C programming language handles strings. The length of a C char is found by searching for the (first) NUL byte. Samples of C string handling code can be seen in strlen and strcpy. If there is no NUL character you'll see erroneous output.

doru · 06-27-2010, 09:08 AM

Thank you for your help.
You are right, it works with four.

doru · 06-28-2010, 03:09 AM

When I read from console a short string like "12 34":

Code:

char t[11];
read(0, t, 10);

I get t = "12 34\n". read does not add '\0'. One solution is to add it myself:

Code:

t[5]='\0';

Is there a better solution which I overlook? Or this is how it should work? Of course I could replace the first '\n' in t with '\0' with some code.

harry edwards · 06-28-2010, 04:08 PM

The preferred method I would recommend is to NUL terminate based on the length of data read from the console. The read function returns the length of the data read, thus, you can use this value to terminate e.g.

Code:

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

int main(void)
{
char t[11];
int len = 0;
len = read(0, t, 10);

  t[len-1]=0;
  printf("[%s]", t);
}

Of course, you should also check the value of len, as it could be zero.

doru · 06-29-2010, 02:51 AM

Thank you again. Now it makes sense.

doru · 07-08-2010, 04:08 AM

I am sorry that I have to return.

I used this short program to test how can I write to a fifo from bash.

Code:

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

#define BFOUT 12

int main(void){
  int fdo, fdi, a;
  char *fifo1="fifo1", *fifo2="fifo2", *s, t[BFOUT];

  if((fdi = open(fifo2, O_RDONLY)) == -1) err_sys("client fifo2 open error");

  if(read(fdi, t, BFOUT) == -1) err_sys("client fifo2 read error");

  write(1, t, strlen(t)); // javascript arrays do not use "\0", so no need for + 1
  }

int err_sys(const char *s){
  printf("%s\n", s);
  exit(1);}

I do:

Code:

mkfifo fifo2;
./rdfifo&
echo abc$'\0' > fifo2

and again I get the special characters after abc. Can you please point me in the right direction as of why does this happen?

in man bash, search \\n and you will find that $'\0' is supposed to write a character with ASCII code 0 - the null character.

Still:

Code:

cat fifo2&
echo abc$'\0' > fifo2

works well.

Writing to fifo from C and reading from bash goes well, too.

I also wonder what happens to stdin and stdout of a process which was run in the background from bash, and then the bash was stopped. Such a process becomes a child of init, and I believe that its stdin and stdout are no more connected to any terminal.

Wim Sturkenboom · 07-08-2010, 05:08 AM

My guess: strlen(t) will return 3, not 4, as it does not count the string terminator

Furher: when running something in the background from bash and killing bash will also kill the background application (as far as I know).

doru · 07-09-2010, 04:56 AM

Quote:

Originally Posted by Wim Sturkenboom

My guess: strlen(t) will return 3, not 4, as it does not count the string terminator

This is irrelevant, as write will only print 3 characters to the terminal, without any garbage.

Quote:

Originally Posted by Wim Sturkenboom

Furher: when running something in the background from bash and killing bash will also kill the background application (as far as I know).

My textbook says that any child process of a process that ends automatically becomes a child of init, and:

Code:

pstree | grep process

before and after you close the current terminal (in my case, putty) clearly show that this is the case.

Wim Sturkenboom · 07-09-2010, 11:49 AM

You were right on both counts.

doru · 07-10-2010, 05:18 AM

Quote:

Originally Posted by Wim Sturkenboom

You were right on both counts.

Please give me any idea of why

Code:

echo abc$'\0' > fifo2

does not print the right '\0'.

doru · 07-20-2010, 04:39 AM

Quote:

Originally Posted by doru

Please give me any idea of why

Code:

echo abc$'\0' > fifo2

does not print the right '\0'.

I discovered that echo abc$'\0' > fifo does not really send $'\0' to the pipe. Probably bash thinks that this is silly, and it just does not send it. $'\0' is probably expanded in '\0', as it should, but then I don't see it in the pipe.

The solution is to use:

Code:

printf "abc\0" > fifo

printf is also a builtin and it works like a charm. I can see the '\0' character in the fifo.