Il 02/01/2021 01:07, Alan Bawden ha scritto:
jak <nos...@please.ty> writes:
Il 01/01/2021 06:23, Alan Bawden ha scritto:
> jak <nos...@please.ty> writes:
>
> Running the command:
>
> $ cat bible.txt > cmdpipe & cat bible.txt > cmdpipe & cat bible.txt >
cmdpipe
>
> the three texts do not mix....
>
> The three texts do not mix as long at the contents of bible.txt is short
> enough (and provided `cat' only calls `write' once). In the POSIX
> specification, the manual page for the `write' system call describes
> writing to a pipe or FIFO as follows:
>
> Write requests of {PIPE_BUF} bytes or less shall not be interleaved
> with data from other processes doing writes on the same pipe. Writes
> of greater than {PIPE_BUF} bytes may have data interleaved, on
> arbitrary boundaries, with writes by other processes, whether or not
> the O_NONBLOCK flag of the file status flags is set.
>
Ok. And...
...Running the command:
$ cat bible.txt > cmdpipe & cat bible.txt > cmdpipe & cat bible.txt >
cmdpipe
the three texts do not mix....
Saying it again doesn't make it any more true. If bible.txt is large
enough, they most definitely DO mix! Just to make sure I wasn't missing
something, I tested your exact command before I sent my previous reply.
They mixed.
You're right it's mixed. I tried this:
terminal 1:
$ python3 pmkfifo.py > output.txt
terminal 2:
cat bible.txt > mypipe & cat bible.txt > mypipe & cat bible.txt > mypipe
result:
-rw-rw-r-- 1 jak jak 4351187 gen 1 02:56 bible.txt
prw-rw-r-- 1 jak jak 0 gen 2 11:40 mypipe
-rw-rw-r-- 1 jak jak 13053561 gen 2 11:40 output.txt
-rw-rw-r-- 1 jak jak 237 gen 1 02:46 pmkfifo.py
then I split ouput.txt:
$ split -n 3 -d output.txt result
result:
-rw-rw-r-- 1 jak jak 4351187 gen 1 02:56 bible.txt
prw-rw-r-- 1 jak jak 0 gen 2 11:40 mypipe
-rw-rw-r-- 1 jak jak 13053561 gen 2 11:40 output.txt
-rw-rw-r-- 1 jak jak 237 gen 1 02:46 pmkfifo.py
-rw-rw-r-- 1 jak jak 4351187 gen 2 11:41 result00
-rw-rw-r-- 1 jak jak 4351187 gen 2 11:41 result01
-rw-rw-r-- 1 jak jak 4351187 gen 2 11:41 result02
but:
$ cmp result00 result01
result00 result01 differ: byte 1, line 1
$ cmp result01 result02
result01 result02 differ: byte 1, line 1
...but not happy I wanted to try sure that each process used a single
write and I wrote this program in C:
int main()
{
FILE *fp;
long len, readed;
char *buffer;
if((fp = fopen("bible.txt", "rt")) == NULL)
return -1;
// get file size
if(!fseek(fp, 0, SEEK_END))
{
len = ftell(fp);
}
else
return -1;
// build buffer
if((buffer = malloc(len)) == NULL)
return -1;
// reset position
if(fseek(fp, 0, SEEK_SET))
return -1;
if((readed = fread(buffer, len, 1, fp)) != 1)
return -1;
if((fwrite(buffer, len, 1, stdout)) < 1)
return -1;
fflush(stdout);
free(buffer);
fclose(fp);
return 0;
}
... and runned:
$ ./mycat > mypipe & ./mycat > mypipe & ./mycat > mypipe
I, however, got the same result.
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