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pipe() and poll() problem in C

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Top Forums Programming pipe() and poll() problem in C
# 8  
Old 12-08-2010
Quote:
Originally Posted by blackmamba21
thanks alot, I can't try your solution cuz im stuck with the dup2 error. Its weird cuz it was working when i didnt check the return value
Replace fileno(stdout) with STDOUT_FILENO and it should work. If not, print the value it actually DOES return. I think it's succeeding and your error checking code is wrong. Have you copied it literally?
# 9  
Old 12-09-2010
thanks alot corona i change the fileno(stdout) to STD_FILENO and its working however i dont know how you manage to get tr to work i did the following change:
Code:
void executerCommande( char *commande[], int descripteur, char* option )
{
	int tubeEntree[2];
	int tubeSortie[2];
	int erreur;
	
	erreur = pipe( tubeEntree );
	if ( erreur == -1 )
	{
		printf( "Erreur lors de la création du tube %s\n", strerror( errno ) );
		exit( 1 );
	}
	erreur = pipe( tubeSortie );
	if ( erreur == -1 )
	{
		printf( "Erreur lors de la création du tube %s\n", strerror( errno ) );
		exit( 1 );
	}

	pid_t pid = fork();
	if ( pid == -1 )	// erreur
	{
		perror( "fork a échoué" );
		exit( 1 );
	}
	else if ( pid == 0 )	// child
	{
		erreur = close( tubeEntree[1] );		// closing writing end of tube from parent to child
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		erreur = close( tubeSortie[0] );		// closing reading end of tube from child to parent
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		erreur = dup2 ( tubeSortie[1], STDOUT_FILENO );		
		if ( erreur == -1 )	// redirecting stdout to writing end of tube from child to parent
		{
			printf( "Erreur lors du dup2 stdout: %s\n", strerror( errno ) );
			exit( 1 );
		}
		erreur = close( tubeSortie[1] );		// fermeture du doublon
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		
		erreur = dup2( tubeEntree[0], STDIN_FILENO );		
		if ( erreur == -1 )	// redirecting stdin to reading end of tube from parent to child
		{
			printf( "Erreur lors du dup stdin: %s\n", strerror( errno ) );
			exit( 1 );
		}
		erreur = close( tubeEntree[0] );		// fermeture du doublon
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		execvp( commande[0], commande );
	}
	else		// parent
	{
		int evenement;
		int commandeTerminee = 0;
		int length;
		char tampon [256];
		struct pollfd entrees[2];
		entrees[0].fd = 0;		// check stdin
		entrees[0].events = POLLIN | POLLPRI;		// surveiller pour des données normales ou prioritaires
		entrees[0].revents = 0;
		entrees[1].fd = tubeSortie[0];		// check reading end of tube from child to parent
		entrees[1].events = POLLIN | POLLPRI;		// surveiller pour des données normales ou prioritaires
		entrees[1].revents = 0;

		erreur = close( tubeEntree[0] );		// closing reading end of tube from parent to child
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		erreur = close( tubeSortie[1] );		// closing writing end of tube from child to parent
		if ( erreur == -1 )
		{
			printf( "Erreur lors du close %s\n", strerror( errno ) );
			exit( 1 );
		}
		
		while ( commandeTerminee == 0 )
		{
			entrees[0].revents = 0;
			entrees[1].revents = 0;
			evenement = poll( entrees, 2, 0 );
			if ( evenement == -1 )
			{
				printf( "Erreur lors du poll %s\n", strerror( errno ) );
				exit( 1 );
			}
			if ( ( entrees[0].revents & POLLIN ) || ( entrees[0].revents & POLLPRI ) )
			{
				memset(tampon, '\0', 256);
				length = read( entrees[0].fd, tampon, 256 );
				if ( ( strcmp( option, "-i" ) == 0 ) || ( strcmp( option, "-io" ) == 0 ) )
				{
					erreur = write ( descripteur, tampon, strlen( tampon ) );
					if ( erreur == -1 )
					{
						printf( "Erreur lors de l'écriture sur le fichier %s\n", strerror( errno ) );
						exit( 1 );
					}
				}
				erreur = write ( tubeEntree[1], tampon, length );
				if ( erreur == -1 )
				{
					printf( "Erreur lors de l'écriture dans le tube d'entrée %s\n", strerror( errno ) );
					exit( 1 );
				}
			}
			if ( entrees[0].revents & POLLHUP )
			{
				commandeTerminee = 1;
			}
			if ( ( entrees[1].revents & POLLIN ) || ( entrees[1].revents & POLLPRI ) )
			{
				do {
					memset(tampon, '\0', 256);
					length = read( entrees[1].fd, tampon, 256 );
					if ( erreur == -1 )	
					{
						printf( "Erreur lors de la lecture du tube de sortie %s\n", strerror ( errno ) );
						exit( 1 );
					}
					if ( ( strcmp( option, "-o" ) == 0 ) || ( strcmp( option, "-io" ) == 0 ) )
					{
						erreur = write( descripteur, tampon, strlen( tampon ) );
						if ( erreur == -1 )	
						{
							printf( "Erreur lors de l'écriture sur le fichier %s\n", strerror ( errno ) );
							exit( 1 );
						}
					}
					printf( "%s", tampon );
				} while (erreur != 0);
			}
			if ( entrees[1].revents & POLLHUP )
			{
				commandeTerminee = 1;
			}
		}	
		pid = wait( NULL );
	}
}

# 10  
Old 12-09-2010
This works:
Code:
#include <stdio.h>
#include <unistd.h>
#include <poll.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
	char *args[]={"/bin/tr", "a-z", "A-Z", NULL};
	pid_t pid;
	int run=1;

	int wrpipe[2], rdpipe[2];
	pipe(wrpipe);	pipe(rdpipe);

	pid=fork();
	if(pid < 0)
		return(1);
	else if(pid == 0) // child
	{
		dup2(wrpipe[0], STDIN_FILENO);
		dup2(rdpipe[1], STDOUT_FILENO);

		close(wrpipe[0]);	close(wrpipe[1]);
		close(rdpipe[0]);	close(rdpipe[1]);

		execvp(args[0], args);
		fprintf(stderr, "exec failed\n");
		exit(1);
	}
	else
	{
		struct pollfd p[3];
		char buf_in[512];
		size_t pos_in=0;
		
		// watch stdin
		p[0].fd=STDIN_FILENO;
		p[0].events=POLLIN|POLLHUP;
		p[0].revents=0;

		// watch output pipe
		p[1].fd=wrpipe[1];
		p[1].events=POLLOUT|POLLHUP;
		p[1].revents=0;

		// watch input pipe
		p[2].fd=rdpipe[0];
		p[2].events=POLLIN|POLLPRI|POLLHUP;
		p[2].revents=0;

		close(wrpipe[0]);	close(rdpipe[1]);

		while(run)
		{
			int err=poll(p, 3, 0);

			if(err < 1)
				continue;

			// Events on stdin
			if(p[0].revents & (POLLIN|POLLHUP))
			{
				size_t max=512-pos_in;
				size_t len=read(p[0].fd, buf_in+pos_in, max);

				fprintf(stderr, "stdin read %d bytes\n", len);

				if(len >= 0) pos_in += len;

				// EOF.
				if(len == 0)
				{
					fprintf(stderr, "stdin EOF\n");
					// Write the last data
					write(wrpipe[1], buf_in, pos_in);
					close(wrpipe[1]);
					run=0;	// break the loop
					continue;
				}
			}

			if((p[1].revents & (POLLOUT | POLLHUP)) && pos_in )
			{
				size_t left;
				size_t len=write(p[1].fd, buf_in, pos_in);
				left=pos_in-len;

				fprintf(stderr, "pipe out wrote %d bytes\n", len);

				// Move anything unwritten to the start
				if(left > 0)
					memmove(buf_in, buf_in+len, left);
				pos_in=left;
			}

			if(p[2].revents & (POLLIN | POLLHUP))
			{
				char buf[512];
				size_t len=read(p[2].fd, buf, 512);

				fprintf(stderr, "pipe in read %d bytes\n", len);

				if(len == 0) // EOF
				{
					fprintf(stderr, "pipe in EOF\n");
					// Write the last data
					write(wrpipe[1], buf_in, pos_in);
					close(wrpipe[1]);
					run=0;	// break the loop
					continue;
				}

				write(STDOUT_FILENO, buf, len);
			}
		}

		// Keep reading until eof
		run=1;
		while(run)
		{
			char buf[512];
			size_t len=read(p[2].fd, buf, 512);

			if(len <= 0)
				break;

			fprintf(stderr, "read %d bytes\n", len);

			write(STDOUT_FILENO, buf, len);
		}
	}
}

Code:
$ echo asdf | ./a.out 
stdin read 5 bytes
pipe out wrote 5 bytes
stdin read 0 bytes
stdin EOF
read 5 bytes
ASDF
$

...and the dup2 calls return 0 and 1 respectively.

---------- Post updated at 11:21 PM ---------- Previous update was at 11:06 PM ----------

For OSX, replace /bin/tr with /usr/bin/tr

Otherwise it works fine on OSX
# 11  
Old 12-09-2010
This is really interesting however it print everything uppercase at the end, I want to do something interactive where the user type and the system print back uppercase

Im gonna study this code because theres alot of verification. If you check the man for poll pollhup on the event bitmask is ignored (only use for the revents).

Also I dont get why poll for p[1]. Excellent job tough im impress
# 12  
Old 12-09-2010
After a lot of hair pull and experimentation on a real OSX machine I've discovered an insurmountable roadblock to doing this with poll() in OSX. From man poll:

Code:
...
BUGS
     The poll() system call currently does not support devices.

Devices includes terminals, real and virtual.

select() might still support them. You can see code here on how to use select(). linux code, but should be applicable to OSX.
Quote:
This is really interesting however it print everything uppercase at the end, I want to do something interactive where the user type and the system print back uppercase
Well, that's how pipes work. They buffer until they're full. To use them in a nonblocking manner you'll need to set them nonblocking, I think:

Code:
#include <sys/ioctl.h>

void nonblock(int fd)
{
        int opt=1;
        ioctl(fd, FIONBIO, &opt);
}


...

pipe(array);
nonblock(array[0]);
nonblock(array[1]);

...though this behavior may vary from system to system.

Using a seperate process and a pipe to convert lowercase to uppercase is overkill, but I bet you know that and this is just an example...

---------- Post updated at 12:41 AM ---------- Previous update was at 12:31 AM ----------

Quote:
Also I dont get why poll for p[1].
To make sure write(p[1].fd, data, size) doesn't block. Imagine that the child and the parent are writing at the same time, waiting for each other. They'll both wait forever. Instead, we write only when the pipe can hold it.
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