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From: bob 295 <icanprogram AT 295 DOT ca>
To: cygwin AT cygwin DOT com
Subject: Re: named pipe (fifo) question
Date: Wed, 20 Apr 2011 09:48:09 -0400
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(I'm on the list in digest mode so things can't thread easily)

On Tue, Apr 19, 2011 at 11:01:55AM -0400, bob 295 wrote:
>>I'm porting a library from Linux to Cygwin and I've encountered a problem 
with 
>>the behavior of named pipes (fifo's).
>>...
>>Is this the intended POSIX behavior?   Is the problem the RDWR open? 

>The problem is that Cygwin's implementation of fifos is very buggy.  I 
wouldn't
>recommend using them for anything but the simplest of applications.

>cgf

Here is some sample code I've been playing with which produces clean results 
on Linux but totally wonky results on Cygwin.

======= begin sender.c code ==========
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>

int main(int argc, char *argv[])
{
char command[80];
char fifoname[80];
int fd=-1;
int rfd;
int i;
int j;
int *r;
int bytesToGo;
int numBytes;
char buf[5];
char *p;
int rc;

printf("fifo sender starting\n");

sprintf(fifoname,"/tmp/fsender");

mkfifo(fifoname, 0666);
chmod(fifoname, 0666);


printf("starting receiver\n");

sprintf(command,"./receiver&");
rc=system(command); 

printf("receiver started\n");

sleep(2);

rfd=open("/tmp/freceiver", O_WRONLY);

for(j=1; j<10; j++)
	{
printf("j=%d\n",j);

write(rfd,&j,sizeof(int));

if(fd == -1) 
	{
//	fd=open(fifoname, O_RDONLY);
	fd=open(fifoname, O_RDWR);
	}

numBytes=0;
memset(buf,0,4);
p=buf;
for(i=0; i< 10; i++)
	{
	bytesToGo=sizeof(int) - numBytes;

printf("bytesToGo=%d numBytes=%d\n",bytesToGo,numBytes);

	if(bytesToGo <= 0) break;

	rc=read(fd,p,bytesToGo);
printf("rc[%d]=%d\n",i,rc);
	if(rc == -1)
		{
		printf("%s\n",strerror(errno));
		}
	else
		{
		numBytes+=rc;
		p+=rc;
		}
	}

printf("buf: 0x%X-%X-%X-%X\n",buf[0],buf[1],buf[2],buf[3]);
r=(int *)buf;

printf("reply[%d]=%d\n",j,*r);
	}

remove(fifoname);
remove("/tmp/freceiver");
	
exit(0);
}

======= end sender.c code ===========

======= begin receiver.c code ==========
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>

int main(int argc, char *argv[])
{
char command[80];
char fifoname[80];
int fd;
int rfd;
int i;
int j;
int k;
int *r;
int bytesToGo;
int numBytes;
char buf[4];
char *p;
int rc;

printf("fifo receiver starting\n");

sprintf(fifoname,"/tmp/freceiver");

mkfifo(fifoname, 0666);
chmod(fifoname, 0666);

fd=open(fifoname, O_RDWR);
//fd=open(fifoname, O_RDONLY);

for(k=0; k<10; k++)
	{

numBytes=0;
p=buf;
for(i=0; i< 10; i++)
	{
	bytesToGo=sizeof(int) - numBytes;

	if(bytesToGo <= 0) break;

	rc=read(fd,p,bytesToGo);
	if(rc == -1)
		{
		printf("%s\n",strerror(errno));
		}
	else
		{
		numBytes+=rc;
		p+=rc;
		}
	}

r=(int *)buf;
j=*r;
printf("received[%d]=%d\n",k,j);
j++;

sleep(1);
	
rfd=open("/tmp/fsender", O_WRONLY);
write(rfd,&j,sizeof(int));
close(rfd);
	}

remove(fifoname);

exit(0);
}

======= end receiver.c code ===========

Compile both sender and receiver and run as

./sender

Here are the results:
========== begin results Linux =========
fifo sender starting
starting receiver
receiver started
j=1
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x2-0-0-0
reply[1]=2
j=2
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x3-0-0-0
reply[2]=3
j=3
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x4-0-0-0
reply[3]=4
j=4
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x5-0-0-0
reply[4]=5
j=5
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x6-0-0-0
reply[5]=6
j=6
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x7-0-0-0
reply[6]=7
j=7
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x8-0-0-0
reply[7]=8
j=8
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x9-0-0-0
reply[8]=9
j=9
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0xA-0-0-0
reply[9]=10

========== end results Linux ==========

==========  begin results Cygwin ========
fifo sender starting
starting receiver
receiver started
j=1
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x2-0-0-0
reply[1]=2
j=2
bytesToGo=4 numBytes=0
rc[0]=0
bytesToGo=4 numBytes=0
rc[1]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[2]=0
j=3
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[3]=0
j=4
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[4]=0
j=5
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[5]=0
j=6
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[6]=0
j=7
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[7]=0
j=8
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[8]=0
j=9
bytesToGo=4 numBytes=0
rc[0]=4
bytesToGo=0 numBytes=4
buf: 0x0-0-0-0
reply[9]=0
fifo receiver starting
received[0]=1
received[1]=2
received[2]=3
received[3]=4
received[4]=5
received[5]=6
received[6]=7
received[7]=8
received[8]=9
received[9]=9

==========  end results Cygwin =========

In this run the Cygwin fifo /tmp/sender appears to get stuffed with 4 bytes of 
all zeros ... or at least that is what the sender read thinks it sees each 
time it loops back to that read.

Thanks in advance for any help with this.

bob


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