Thread: pipe

hi all

I am trying to be much familier with pipes

I tried to write a c program that generates the following output

child will print the 0th character and send it to parent
a : from child .. waiting for parent to print same character
a : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
b : from child .. waiting for parent to print same character
b : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
c : from child .. waiting for parent to print same character
c : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
d : from child .. waiting for parent to print same character
d : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
e : from child .. waiting for parent to print same character
e : from parent .. waiting for next character from child

but what I get was this
child will print the 0th character and send it to parent
a : from child .. waiting for parent to print same character
a : from parent .. waiting for next character from child

and here is my code

Code:

#include<stdio.h>
#include<string.h>
int main (void)
{
	int fd[2],nbytes,i;
	int childpid;
	char  *f1[5]={"a","b","c","d","e"};
	char f2[2];

	pipe(fd);
   
        childpid=fork();
	
	if(childpid==0)//start child process
	{

	     for(i=0;i<5;i++)
	     {
	    
		printf("child will print the %dth  character and send it to parent\n",i);
		printf("%s : from child .. waiting for parent to print same character\n",f1[i]);

  		
		
		close(fd[0]);

		write(fd[1],f1[i],(strlen(f1[i])+1));
		
		exit(0);
	      
	     }
	 }


	else
	{
		wait(NULL);

		close(fd[1]);

		nbytes=read(fd[0],f2,sizeof(f2));

		printf("%s : from parent .. waiting for next character from child\n",f2);
		
	}

	return 0;

}

I need your help to correct my code and thanx

Look at what your code is doing.. my comments in red:

Code:

#include<stdio.h>
#include<string.h>
int main (void)
{
	int fd[2],nbytes,i;
	int childpid;
	char  *f1[5]={"a","b","c","d","e"};
	char f2[2];

	pipe(fd);
   
        childpid=fork();
	
	if(childpid==0)//start child process
	{

	     for(i=0;i<5;i++)
	     {
	    
		printf("child will print the %dth  character and send it to parent\n",i);
		printf("%s : from child .. waiting for parent to print same character\n",f1[i]);

  		
		
		close(fd[0]);

		write(fd[1],f1[i],(strlen(f1[i])+1));
		/* You have a for loop above, but inside the loop you call exit below, so it will only write one character. */
		exit(0);
	      
	     }
             /* exit(0); should be here instead */
	 }


	else
	{
		wait(NULL);

		close(fd[1]);
                /* again here, you're only reading one character, there is no loop */
		nbytes=read(fd[0],f2,sizeof(f2));

		printf("%s : from parent .. waiting for next character from child\n",f2);
		
	}

	return 0;

}

I need your help to correct my code and thanx

If you understand what is wrong, you should be able to correct it yourself.

thanx for help

the code should be

Code:

#include<stdio.h>
#include<string.h>
int main (void)
{
	int fd[2],nbytes,i;
	int childpid;
	char  *f1[5]={"a","b","c","d","e"};
	char f2[2];

	pipe(fd);
   
        childpid=fork();
	
	if(childpid==0)//start child process
	{

	     for(i=0;i<5;i++)
	     {
	    
		printf("child will print the %dth  character and send it to parent\n",i);
		printf("%s : from child .. waiting for parent to print same character\n",f1[i]);

  		
		
		close(fd[0]);

		write(fd[1],f1[i],(strlen(f1[i])+1));
			      
	     }
             exit(0); 
	 }


	else
	{
		wait(NULL);

		close(fd[1]);
                /* again here, you're only reading one character, there is no loop */

for (i=0;i<5;i++){
		nbytes=read(fd[0],f2,sizeof(f2));

		printf("%s : from parent .. waiting for next character from child\n",f2);
		
	}}

	return 0;

}

thanx for explanning the mechanism

I have another question

if I have the following code

Code:

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

int main(void){
	int childpid,fd[2],nb,i,j;
	char line[BUFSIZ]="I want to print this line twice";
	char word[BUFSIZ] ;
	

	pipe(fd);

	childpid=fork();

	if(childpid==0)
		{
			printf("from child:\n");
			
			close(fd[0]);

			char *token=strtok(line," ");

			 while(token!=NULL)
			 {
				 printf(" %s\n",token);

				 write(fd[1],token,(strlen(token)+1));

				 token=strtok(NULL," ");

			}
	
		}

	else
	{

		wait(NULL);
		printf("from parent:\n");
		
		

		close(fd[1]);
		
	for( i=0;i<7;i++){
		nb=read(fd[0],word,sizeof(word));
		printf("%s\n",word);
	
	}

	}

	return 0;

}

I want the output to be:

from child:
I
want
to
print
this
line
twice

from parent:
I
want
to
print
this
line
twice


what I get is


from child:
I
want
to
print
this
line
twice

from parent:
I
I
I
I
I
I
I

I think I am doing as you said, but instead of the first for loop in child process I've used a while loop but I dont know why I did not get the correct results


thanx again for help

Your code is logically sound, but you need to realise that read and write have no concept of start/end. Example, if you write 5 bytes, then write 7 bytes, it may as well be the same as just writing 12 bytes in one go.

In the case of your code above, the first time the parent reads, it gets the entire buffer that the child wrote out, which is "I\0want\0\to\0print\0this\0line\0twice\0". So of course printing the output will just result in "I", since there is a null terminator straight after.

So, one work around to this would be to convert the \0's each time into \n's. You should only need to do one read, but you should check the return value. Something like:

Code:

while ( (n = read(fd, buf, size)) > 0)
{
    printf("read: %s\n", buf);
}

In general, you should check the return values of all system calls, even if you think they'll always work.

this is the way I was looking for


thank you very much