Re: [SoX-users] how to interpret tell_off, and the right way to use sox_seek

[Jan Stary <hans@stare.cz>]

> 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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