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

[Skip Tavakkolian <skip.tavakkolian@gmail.com>]

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maybe it's a problem with eof detection.  I tried to reproduce it using a
threaded version (see attached), but it works correctly.

On Fri, Mar 31, 2017 at 5:29 PM arisawa <karisawa@gmail.com> wrote:

> Hello,
>
> I was playing with an experimental code on pipe and met with a problem
> which I don’t understand.
>
> the program reads a file and writes it 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 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 ===
>

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#include <u.h>
#include <libc.h>
#include <thread.h>

void
copyfunc(void *v)
{
	int *fd, n;
	char buf[256];
	int tot = 0;

	fd = v;
	while((n = read(fd[0], buf, sizeof buf)) > 0){
		if(write(fd[1], buf, n) != n)
			sysfatal("write: %r");
		tot += n;
		fprint(2, "copied %d byte chunk\n", n);
	}
	fprint(2, "total copied: %d\n", tot);
	close(fd[0]);
	close(fd[1]);
	threadexits(nil);
}

/* copy stdin to stdout through a pipe. run like ./pipetest </lib/constitution >/dev/null */
void
threadmain(int argc, char *argv[])
{
	int fd[4];
	int pfd[2];

	if(pipe(pfd) < 0)
		sysfatal("pipe error");

	fd[0] = 0;
	fd[1] = pfd[0];
	fd[2] = pfd[1];
	fd[3] = 1;

	proccreate(copyfunc, fd, 8192);
	copyfunc(fd+2);
}