Thread: helping a m8 fifo/pipes

Yes. Maybe you haven't realized this yet (depending on your file browser, which may not show them) but the fifo is a real file in a real place. If you take the unlink() out, it will still be there when you are done.

If you use ls in the directory where your program runs, you should see an entry for "pipefile" (or whatever name you give it) while the program is running (unlink erases it at the end).

right i have put in the command in differnt places

i have messed around with the unlink() command and well still the same stuff

i get the spam mkfifo and sometimes i get segmentation fault sooo i dont know whats goin on. the pipe seems to be broken

Originally Posted by fortune2k

i get the spam mkfifo and sometimes i get segmentation fault sooo i dont know whats goin on. the pipe seems to be broken

The spam you refer to is because the file exists, right? You should be able to verify that with your file browser or ls, right? If "the writer" seg faults, the unlink() probably didn't happen.

I discovered a few minor complications with using the fifo that I imagine would be flabbergasting if you didn't have the C skills I possess
1) You must fflush() your transmissions.
2) You must fopen the pipe "r" (read-only) or "r+" (read and write). "w" or "a" cause it to hang.

The first one is important to you. Another problem is that it will be awkward to read and write to the fifo in both programs, so your prof probably does not intend that -- you want the first program to write, and the second one to read, as I think you've been thinking. So I wrote two little experiments that need to be run
1) in separate consoles
2) in the same directory, because the fifo will be there

THE WRITER

Code:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>

int getinput (char *ptr) {   /* simple multi-purpose keyboard input */
	int ch, i=0;
	printf("Message: ");
	while ((ch=fgetc(stdin))!='\n') { ptr[i]=ch; i++; }
	ptr[i]='\n';  /* need this */
	ptr[i+1]='\0';  /* null terminate */
	return i; /* we don't use the return value (length of string) in this program */
}


int main() {
	char buffer[256];
	FILE *fifo;
	if (mkfifo("pipefile", S_IRUSR | S_IWUSR)!=0) perror("mkfifo");
	
	fifo=fopen("pipefile", "r+"); /* "w" and "a" stall!! */
	if (!(fifo)) { perror("fifo"); return 0; }

	while (1) {   /* infinte loop */
		getinput(buffer);
		fprintf(fifo,buffer);  /* send message */
		fflush(fifo);  /* IMPORTANT! */
		if (strncmp(buffer,"end",3)==0) break;  /* quit */
	}

	fclose(fifo);
	unlink("pipefile");	/* erase it */
	return 0;
}

Start this first. You can type a message to send to
THE READER

Code:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>

int main() {
	int i;
	char buffer[256], byte;
	FILE *fifo=fopen("pipefile", "r");  /* read-only */
	if (!(fifo)) { perror("fopen"); return 0; }  /* error check */
	
	while (1) {  /* infinite loop */
		i=0;	/* reset */
		while ((fread(&byte,1,1,fifo))) {   /* our old loop */
			buffer[i]=byte;
			i++;
			if (byte=='\n') break;
		}
		printf("message is: %s", buffer); 
	}
	fclose(fifo);
	return 0;
}

If you type "end", the writer will erase the pipe, which causes the reader to exit cleanly.

Read your fflush() documentation, this turns out to be essential.

Originally Posted by fortune2k

writer.c: In function ‘main’:
writer.c:26: warning: format not a string literal and no format arguments

That is very correct (gcc -Wall missed this)! Line 26 should be:

Code:

                fprintf(fifo,"%s",buffer);

Also, the reader may not shut down cleanly after all, so you should use a strncmp line like the writer's that will "return 0" to close.

Originally Posted by MK27

That is very correct (gcc -Wall missed this)! Line 26 should be:

Code:

                fprintf(fifo,"%s",buffer);

Also, the reader may not shut down cleanly after all, so you should use a strncmp line like the writer's that will "return 0" to close.

pretty cool program i have managed to find out my problem i my 1.c didnt open up the pipe so nothing was being sent. i have made the changes and well slowly on my way to getting somewhere however i have hit another problem i have now split my program into 2 functions main and myfork (myfork does all the fork stuff makes the children and will put them into the pipe ) when i comiple i get this error:

Code:

1.c:(.text+0x7e): undefined reference to `myfork'
collect2: ld returned 1 exit status

i have looked all over the internet and messed around and it comes back to haunt me .

I just got the same error for trying to use a function that I was going to write in the future:

Code:

/tmp/ccKgrMsT.o: In function `pyramid':
scene.c:(.text+0x4e7): undefined reference to `setvect'
scene.c:(.text+0x507): undefined reference to `setvect'
scene.c:(.text+0x523): undefined reference to `setvect'
collect2: ld returned 1 exit status

But presumably that is not your problem.

Go ahead -- post some code.

yeaa i have been trying to fix it for the past hour or so

Code:

int myfork(int i);

int main(void)
{
int numofpro, i;
FILE *fifo;

printf("How many child processes? (max5) >");
scanf("%d", &numofpro);
printf("\n%d Child Process Being Created...please wait...\n\n",numofpro);
  

mkfifo("pipefile", S_IRUSR | S_IWUSR);
fifo=fopen("pipefile", "r+"); /* read to filehandle */

for(i=0;i<numofpro;i++)
  {
  myfork(i);
  }
  printf("Process 1 is sucsessfully ended\n\n ");
}


int mfork(int i)
{
int random;
int pipepid[10], pipeprior[10];
pid_t pid;
FILE *fifo;

 random = rand()%90;
 pid = fork();	// fork														
	if (pid == 0) /* child*/
	{
	pipepid[i] = getpid();
	pipeprior[i] = random;
	fprintf(fifo, "Prio : %d   PID : %d  \n",pipepid[i],pipeprior[i]); fflush(fifo);   /* notice the newline */					
	printf("Message sent ..\n");						
        }
        else
        {
        /*PARENT*/
        }
	fclose(fifo);
	unlink("pipefile");	/* erase it */
}

i dont know weather to have the forking in diff functions is this a good way will it work out

i want the user to input how many children for i i will then be passed into the fork function the fork function will then make sooo many children , put the pid and priority in order put them in the fifo then i will have a read function in 2.c to read it put the pid and prior back into arrays the other side and print them out soo i can then kill process n stuff . any ideas how i could achieve this ??

EDIT:
i have found the problem the myfork function had a typo although now when i run 2.c i get "Segmentation Fail" sooo now i have broken my fifo again.
This was my orignal code before i made the myfork function:

Code:

int main(void)
{
int numofpro, i=0,random;
int pipepid[10], pipeprior[10];
FILE *fifo;
pid_t pid;
random = rand()%90;



printf("How many child processes? (max5) > ");
scanf("%d", &numofpro);
printf("\n%d Child Process Being Created...please wait...\n\n",numofpro);

mkfifo("pipefile", S_IRUSR | S_IWUSR);
fifo=fopen("pipefile", "r+"); /* read to filehandle */

 for(i=0;i<numofpro;i++)
  {
	pid = fork();	// fork														
	if (pid == 0) /* child*/
	{
	pipepid[i] = getpid();
	pipeprior[i] = random;
	fprintf(fifo, "Prio : %d   PID : %d  \n",pipepid[i],pipeprior[i]); fflush(fifo);   /* notice the newline */					
	printf("Message sent ..\n");						
        }
        else
        {
        /*PARENT*/
        }

	fclose(fifo);
	return 0;

  }
	fclose(fifo);
	unlink("pipefile");	/* erase it */
	return 0;
}

Originally Posted by fortune2k

yeaa i have been trying to fix it for the past hour or so

Hack, man, hack!

I do not understand where do you open the pipe for writing? shouldn't your code be different for server and client?