pipe: bug or feature?

pipe: bug or feature?

[arisawa]

Hello,

I was playing with an experimental program on pipe and met with a problem which I don’t understand.

the program reads a file and writes it back to one end of pipe and then reads it from another end of pipe.
the buffer for writing pipe is named buf0, and for reading pipe is named buf.
and I found the program does not finish unless sizeof(buf) > sizeof(buf0).
is this a bug or a feature of pipe?

Kenji Arisawa

=== BEGIN a.c ===
#include <u.h>
#include <libc.h>

char *argv0;

void
usage(void)
{
	fprint(2,"usage: %s file\n",argv0);
	exits("usage");
}

void
main(int argc, char *argv[])
{
	int fd,pfd[2];
	char buf[256];
	char buf0[256];
	/* need to be sizeof(buf) > sizeof(buf0)
	 * but this condition is very curious to me */
	int n;
	char *file;
	argv0 = argv[0];
	argc--;argv++;
	USED(argc);
	if(argv[0] == nil)
		usage();
	file = argv[0];
	fd = open(file,OREAD);
	if(fd < 0)
		sysfatal("no such file");

	if(pipe(pfd) < 0)
		sysfatal("pipe error");
	print("pfd: %d %d\n",pfd[0],pfd[1]);

	while((n = read(fd,buf0,sizeof(buf0))) > 0){
		print("read: %d %s\n",n,file);
		n = write(pfd[1],buf0,n);
		print("write: %d\n",n);
	}
	close(pfd[1]);
	while((n = read(pfd[0],buf,sizeof(buf))) > 0){
		buf[n] = 0;
		print("%d %s\n",n,buf);
	}
	print("%d\n",n);
		
	exits(nil);
}
=== END a.c ===

Re: [9front] pipe: bug or feature?

[Alex Musolino]

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>
> is this a bug or a feature of pipe?
>

It is a bug in your code.  When you do buf[n] = 0 in the last while
loop, you are writing past the end of buf.  This causes pfd[0] to
become 0 and so the next read is actually from stdin.  Making buf
larger "fixes" the problem because n=256 becomes a valid index.

--
Cheers,
Alex Musolino

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Re: [9front] pipe: bug or feature?

[arisawa]

OK! you are right!

I was careless.
thanks.

Kenji Arisawa

> 2017/03/28 11:27、Alex Musolino <musolinoa@gmail.com> のメール：
> 
> is this a bug or a feature of pipe?
> 
> It is a bug in your code.  When you do buf[n] = 0 in the last while
> loop, you are writing past the end of buf.  This causes pfd[0] to
> become 0 and so the next read is actually from stdin.  Making buf
> larger "fixes" the problem because n=256 becomes a valid index.
> 
> --
> Cheers,
> Alex Musolino