* Re: how to reading 0 byte files properly
2022-11-17 10:15 ` Andreas Schwab
@ 2022-11-18 1:04 ` Binbin YE
2022-11-18 2:17 ` Binbin YE
1 sibling, 0 replies; 4+ messages in thread
From: Binbin YE @ 2022-11-18 1:04 UTC (permalink / raw)
To: Andreas Schwab; +Cc: emacs-devel
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Thank you Andreas for pointing that out
at this moment "total" is READ_BUF_SIZE
```c
/* emacs/src/fileio.c:4613 */
/* In the following loop, HOW_MUCH contains the total bytes read so
far for a regular file, and not changed for a special file. But,
before exiting the loop, it is set to a negative value if I/O
error occurs. */
how_much = 0;
```
I have confirmed the file is not seekable on my side using, which is
different from /proc files
```c
/* test.c:11 */
int fd = open("/proc/2051/arch_status", O_RDONLY);
int sek = lseek(fd, 0, SEEK_CUR);
printf("proc file is seekable %d\n", sek); // returns 0
fd = open("/run/test.json", O_RDONLY);
sek = lseek(fd, 0, SEEK_CUR);
printf("fuse file is seekable %d\n", sek); // returns -1
```
I think it hits this block. But I don't see anything special to increase
the count. Could that mean emacs only reads "READ_BUF_SIZE" amount of data?
```c
/* emacs/src/fileio.c:4627 */
while (how_much < total)
{
/* `try' is reserved in some compilers (Microsoft C). */
ptrdiff_t trytry = min (total - how_much, READ_BUF_SIZE);
ptrdiff_t this;
if (!seekable && NILP (end))
```
should the fix be quitting at actual io?
Best,
Binbin
On Thu, Nov 17, 2022 at 7:15 PM Andreas Schwab <schwab@linux-m68k.org>
wrote:
> On Nov 17 2022, Binbin YE wrote:
>
> > /* emacs/src/fileio.c:4587 */
> >
> > if (seekable || !NILP (end))
> > total = end_offset - beg_offset;
> > else
> > /* For a special file, all we can do is guess. */
> > total = READ_BUF_SIZE;
> > ```
> > Judging from the code, it assume the total size would be READ_BUF_SIZE
>
> For a non-seekable file this is just a buffer size, see the read loop
> later in the function (how_much stays zero then).
>
> If the file is seekable, the important part is this:
>
> /* The file size returned from fstat may be zero, but data
> may be readable nonetheless, for example when this is a
> file in the /proc filesystem. */
> if (end_offset == 0)
> end_offset = READ_BUF_SIZE;
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
> "And now for something completely different."
>
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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: how to reading 0 byte files properly
2022-11-17 10:15 ` Andreas Schwab
2022-11-18 1:04 ` Binbin YE
@ 2022-11-18 2:17 ` Binbin YE
1 sibling, 0 replies; 4+ messages in thread
From: Binbin YE @ 2022-11-18 2:17 UTC (permalink / raw)
To: emacs-devel
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Sorry I put mailing list in "cc", but not "to"
Andreas
Thank you for pointing that out
at this moment "total" is READ_BUF_SIZE
```c
/* emacs/src/fileio.c:4613 */
/* In the following loop, HOW_MUCH contains the total bytes read so
far for a regular file, and not changed for a special file. But,
before exiting the loop, it is set to a negative value if I/O
error occurs. */
how_much = 0;
```
I have confirmed the file is not seekable on my side using, which is
different from /proc files
```c
/* test.c:11 */
int fd = open("/proc/2051/arch_status", O_RDONLY);
int sek = lseek(fd, 0, SEEK_CUR);
printf("proc file is seekable %d\n", sek); // returns 0
fd = open("/run/test.json", O_RDONLY);
sek = lseek(fd, 0, SEEK_CUR);
printf("fuse file is seekable %d\n", sek); // returns -1
```
I think it hits this block. But I don't see anything special to increase
the count. Could that mean emacs only reads "READ_BUF_SIZE" amount of data?
```c
/* emacs/src/fileio.c:4627 */
while (how_much < total)
{
/* `try' is reserved in some compilers (Microsoft C). */
ptrdiff_t trytry = min (total - how_much, READ_BUF_SIZE);
ptrdiff_t this;
if (!seekable && NILP (end))
```
Should the fix be quitting at actual io?
Best,
Binbin
On Thu, Nov 17, 2022 at 7:15 PM Andreas Schwab <schwab@linux-m68k.org>
wrote:
> On Nov 17 2022, Binbin YE wrote:
>
> > /* emacs/src/fileio.c:4587 */
> >
> > if (seekable || !NILP (end))
> > total = end_offset - beg_offset;
> > else
> > /* For a special file, all we can do is guess. */
> > total = READ_BUF_SIZE;
> > ```
> > Judging from the code, it assume the total size would be READ_BUF_SIZE
>
> For a non-seekable file this is just a buffer size, see the read loop
> later in the function (how_much stays zero then).
>
> If the file is seekable, the important part is this:
>
> /* The file size returned from fstat may be zero, but data
> may be readable nonetheless, for example when this is a
> file in the /proc filesystem. */
> if (end_offset == 0)
> end_offset = READ_BUF_SIZE;
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
> "And now for something completely different."
>
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