Re: Post to [email protected] denied: Re: how to interpret tell_off, and the right way to use sox_seek

Sorry for crossposting, but whoever's running the public-inbox.org mirror,
please get it fixed.

	Jan


On Nov 07 10:05:55, [email protected] wrote:
> Hi, this is the Mlmmj program managing the <[email protected]>
> mailing list.
> 
> The message from <[email protected]> with subject "Re: [SoX-users] how to
> interpret tell_off, and the right way to use sox_seek" was unable to be
> delivered to the list
> 
> (The denied message is below.)

> Date: Tue, 7 Nov 2017 09:50:36 +0100
> From: Jan Stary <[email protected]>
> To: [email protected]
> Subject: Re: [SoX-users] how to interpret tell_off, and the right way to
>  use sox_seek
> Reply-To: [email protected]
> 
> > First, it reads (which presumably moves the file pointer forwards),
> > then it seeks back to where it was.
> > I've verified in gdb that it actually does call sox_seek() (again and
> > again and again, but not forever).  I have also verified in gdb that
> > it is reading the same samples.
> 
> Ah, I see what your problem is now.
> 
> > So sox_seek() is definitely being called, and definitely working.
> > But nevertheless, the reading done after the seeking eventually fails.
> 
> Here is my slight rewrite of your example:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <err.h>
> #include <sox.h>
> 
> int 
> main(int argc, char **argv)
> {
> 	sox_format_t *s;
> 	int32_t buf[1024];
> 	ssize_t r;
> 	int i;
> 
> 	if (argc < 2)
> 		errx(1, "usage: ./soxseek input");
> 	if (sox_init() != SOX_SUCCESS)
> 		errx(1, "Cannot init libsox");
> 	if ((s = sox_open_read(*++argv, 0, 0, 0)) == NULL)
> 		errx(1, "Cannot open `%s'", *argv);
> 	for (i = 1; (r = sox_read(s, buf, 1024)) > 0; i++) {
> 		printf("[%04d] %zd samples, starting with %0x\n", i, r, *buf);
> 		if (sox_seek(s, 0, SOX_SEEK_SET) != SOX_SUCCESS)
> 			errx(1, "Cannot seek");
> 	}
> 	/* No way to test for sox_read() error */
> 	return 0;
> }
> 
> (Note how I don't use sox_site_t for the sox_read() return value,
> because it does not exist, eventhough that's what libsox(3) documents.)
> 
> $ sox -n /tmp/file.wav synth trim 0 $((1024 * 1000))s  
> $ cc -o soxseek soxseek.c -lsox -I/usr/local/include/ -L/usr/local/lib
> $ ./soxseek /tmp/file.wav 
> [0000] 1024 samples, starting with 0
> [0001] 1024 samples, starting with 0
> [....]
> [0999] 1024 samples, starting with 0
> [1000] 1024 samples, starting with 0
> 
> So I think you are right. It does seek back to the begining of the sine wave
> (thus reporting 0 as the first sample value in the buffer), but it gets
> exhausted at EOF anyway. I suspect now it is a bug in sox_seek()
> if we are calling it right.
> 
> > Certainly if you do analogous coding with calls to fread() and
> > fseek(), it would not terminate.
> 
> Yes, the following will read the same file forever:
> 
> #include <unistd.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <err.h>
> 
> int 
> main(int argc, char **argv)
> {
> 	int fd;
> 	int32_t buf[1024];
> 	ssize_t r;
> 	int i;
> 
> 	if (argc < 2)
> 		errx(1, "usage: ./seek input");
> 	if ((fd = open(*++argv, O_RDONLY)) == -1)
> 		err(1, NULL);
> 	for (i = 1; (r = read(fd, buf, 1024)) > 0; i++) {
> 		printf("[%04d] %zd samples, starting with %0x\n", i, r, *buf);
> 		if (lseek(fd, 0, SEEK_SET) == -1)
> 			err(1, NULL);
> 	}
> 	return (r != 0);
> }
> 
> 
> > What i'm trying to do now is to determine the correct way to use
> > sox_seek() if i am using it incorrectly.  If i'm using it correctly,
> > then i would like confirmation of my characterization (that it has
> > some internal counter which is decremented until it hits zero).
> 
> I share your suspition now.
> 
> > (I did have a signal processing problem that i was considering
> > earlier, but that's not relevant now because i avoided the sox_seek
> > issue by rearranging the computation.  So i can do my dsp, but i do
> > wonder about correct usage of sox_seek.)
> 
> Should we move this to sox-devel?
> 
> 	Jan
> 
> 
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