Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: pipe() and poll() problem in C
Top Forums Programming pipe() and poll() problem in C Post 302478772 by Corona688 on Thursday 9th of December 2010 12:21:25 AM
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
 

10 More Discussions You Might Find Interesting

1. Programming

Wierd pipe problem

I have encountered a strange problem dealing with pipes and forking. The program basicaly does this: cat file | tbl | eqn | groff Now, I have a parent process that forks children that that exec the stuff that they should. The pipes defined in the parent are the ones used. The chain goes... (1 Reply)
Discussion started by: denoir
1 Replies

2. Shell Programming and Scripting

read after pipe problem OSX10.4

I use read often in scripts to filter the right part into a variable like: $ print "abc cde efg" | read k l ; print "k=$k, l=$l" k=, l= This works on linux and unix versions I work with. On OSX 10.4 this doesn't work. I found a workaround but would like to know why the original line... (5 Replies)
Discussion started by: relyveld
5 Replies

3. Shell Programming and Scripting

Problem with pipe into sed

Basically I am trying to write a short script to report total space used on /u0? file systems. This is what I was trying to do:df -k /u0? | grep -v kbytes | awk '{ printf $2 "+" }' | sed s/.$// | bcBut it returns no output. This works however: > A=`df -k /u0? |grep -v kbytes | awk '{ printf $2... (2 Replies)
Discussion started by: 98_1LE
2 Replies

4. Programming

Pipe Problem

Is there a way to know whether is pipe is opened in read or write mode.I mean is there any signal that is generated when a pipe is opened in read or write mode. If you have some solution .please let me know ........ (2 Replies)
Discussion started by: vivivo2000
2 Replies

5. Programming

Problem in read() from a pipe

Hi, Can any one please help me with this. Am struggling hard to get a solution. I am doing telnet through a C program and getting the stdout file descriptor of the remote machine to pipe. read() function is getting data, But whenl it receives SOH character ie. ^A ( Start of heading = Console... (2 Replies)
Discussion started by: JDS
2 Replies

6. Programming

Pipe problem

Could anyone tell me whats wrong whit this piping? the commands that they execute are correct. the command I am trying is ls|wc. Both processes go to the right if statement. for(i=0;i<argc;i++){ if(i==0&&argc>1){//first command if(pipe(pipa1)==-1) ... (2 Replies)
Discussion started by: isato
2 Replies

7. Shell Programming and Scripting

problem using a pipe to grep

Hello ! I want to process a text file in order to extract desired data using sed and grep... Now I am facing a problem piping to grep... nothing happens.. The text consists of blocks of 3 lines that may (or not) contain the Desired data. the desired data is on each 2... (4 Replies)
Discussion started by: ShellBeginner
4 Replies

8. UNIX for Dummies Questions & Answers

problem with pipe operator

hi i am having issues with extra pipe. i have a data file and i need to remove the extra pipe in the(example 4th and 7thline) in datafile. there are many other line and filed like this which i need to remove from files. The sample data is below: 270 31|455004|24/03/2010|0001235|72 271... (3 Replies)
Discussion started by: abhi_n123
3 Replies

9. Shell Programming and Scripting

The problem of pipe

Hi,guys: I want to use c to implement a pipe. For example: ps auxwww | grep fred | more I forked three child processes. Each is responsible for each command, and pipe to next one. for(i=0;i<2;i++) pipe(fd) if(child==1) // child 1 { close(1) dup2(fd,1) close(fd) }... (3 Replies)
Discussion started by: tomlee
3 Replies

10. Programming

Problem with Pipes => Only works first pipe

Hi! I'm having problems with pipes... I need comunnications with childs processes and parents, but only one child can comunnicate with parent (first child), others childs can't. A brief of code: if(pipe(client1r)<0){ perror("pipe"); } ... (1 Reply)
Discussion started by: serpens11
1 Replies

LEARN ABOUT DEBIAN

stcall

stcall(3)						    ShapeTools Toolkit Library							 stcall(3)

NAME

       stCallEditor, stCallCmd, stCallCmdErrno, stFindProgram - call command processor with command string

SYNOPSIS

       #include <config.h>
       #include <sttk.h.h>

       int stCallEditor (char *editor, char *file, char *contents, char **newcontents);

       int stCallCmd (char *commandProcessor, char *commandString);

       int stCallCmdErrno;

       char*stFindProgram (char *fileName)

DESCRIPTION

       stCallEditor  calls  editor  editor  with file file and returns its  contents after the editor session in newcontents.  Return value is the
       length of the new text. On failure, 0 is returned to indicate the error. Newcontents is not updated and points to  nowhere.  On	error  the
       file  will  be  removed.  If contents points to a valid text, this text is put (not appended) into the temporary file before editor starts.
       Contents must be NULL terminated, otherwise strange things will happen.

       stCallCmd invokes commandProcessor as a child process and writes commandString to its standard input.  The current process waits for termi-
       nation  of  the child process.  stCallCmd returns the exit status of the child process reported by wait(2). The commandProcessor string may
       contain command line arguments to the command processor, separated by whitespace.  (This is necessary for some programs to make	them  read
       commands  from  standard  input.)  The command processor program is searched for in the directories given in the environment variable PATH.
       If commandString does not end with a newline, a newline is added.

       stFindProgram returns the full pathname of programName if program is found and executable. Otherwise NULL.

ENVIRONMENT

       PATH   List of colon-separated directoriy names where execvp(3) searches for the program to execute (default /bin:/usr/bin:/usr/ucb).

SEE ALSO

       wait(2)

DIAGNOSTICS

       On a successful call stCallCmd returns the exit status of the child process. If an error occured, stCallCmd returns a  negative	number	as
       defined in sttk.h:

       CMDPROC_EMPTY
		     An empty or NULL string has been supplied for commandProcessor.

       NO_MORE_CORE  A call to malloc(3) or calloc(3) returned a NULL pointer.

       FORK_FAILED   fork(2) could not create a child process.

       PIPE_FAILED   A call to pipe(2) failed.

       WAIT_ERROR    A call to wait(2) failed.

       EXEC_FAILED   execvp(3) could not execute commandProcessor.

       CHILD_KILLED  The child process was killed by an uncaught signal.

       WRITE_FAILED  write(2)  could  not  write  commandString to the pipe. This usually happens when commandProcessor does not read its standard
		     input and terminates before commandString is written.

       NO_PROGRAM    commandProcessor could not be found.

       For the most error conditions the integer variable stCallCmdErrno (declared in sttk.h) contains additional information about the error con-
       dition, usually the contents of errno(3) after a failed system call.
       In the case of CHILD_KILLED, stCallCmdErrno contains the statBus of the child process as reported by wait(2).

BUGS

       On systems where no usable vfork(2) is available, the value of stCallCmdErrno does not make sense in case of EXEC_FAILED.

       Under IRIX stCallCmd sometimes (or always?) returns WAIT_ERROR where it should be EXEC_FAILED, NO_PROGRAM, or WRITE_FAILED.

AUTHORS

       Jurgen Nickelsen <nickel@cs.tu-berlin.de> and Andreas.Lampen@cs.tu-berlin.de

sttk-1.7						     Thu Jun 24 17:43:29 1993							 stcall(3)
All times are GMT -4. The time now is 10:46 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy