Division in loop

Division in loop

[Randi Botse]

hi all,

....
int fd, fdp;
unsigned int size;
int progress = 0;
size_t bytes = 0;
void *buffer;
....

/* open fd from a device then create fdp, allocate buffer,, etc. */

while (bytes < size) {
        ret = read(fd, buffer + bytes, size - bytes);
        if (ret == -1) {
            perror("read");
            return -1;
        }
        ret = write(fdp, buffer + bytes, ret);
        if (ret == -1) {
            perror("write");
            return -1;
        }
        bytes += ret;

        progress = (int) ((bytes / size) * 100);
        printf("\rcompleted: %i%%", progress);
        fflush(stdout);
    }

....

I run this loop to receive some data, the printf() seem not updated,
the progress output is
always in it's initial value until the loop was finished. but when i do...

progress = (int) ((bytes / 2) * 100); /* change size to 2 (or random number) */

then it's works... what's wrong here???

Re: Division in loop

[Saurabh Sehgal]

Where do you initialize the variable size and to what value ?

On Tue, Sep 8, 2009 at 10:40 AM, Randi Botse <nightdecoder@gmail.com> wrote:
>
> hi all,
>
> ....
> int fd, fdp;
> unsigned int size;
> int progress = 0;
> size_t bytes = 0;
> void *buffer;
> ....
>
> /* open fd from a device then create fdp, allocate buffer,, etc. */
>
> while (bytes < size) {
>        ret = read(fd, buffer + bytes, size - bytes);
>        if (ret == -1) {
>            perror("read");
>            return -1;
>        }
>        ret = write(fdp, buffer + bytes, ret);
>        if (ret == -1) {
>            perror("write");
>            return -1;
>        }
>        bytes += ret;
>
>        progress = (int) ((bytes / size) * 100);
>        printf("\rcompleted: %i%%", progress);
>        fflush(stdout);
>    }
>
> ....
>
> I run this loop to receive some data, the printf() seem not updated,
> the progress output is
> always in it's initial value until the loop was finished. but when i do...
>
> progress = (int) ((bytes / 2) * 100); /* change size to 2 (or random number) */
>
> then it's works... what's wrong here???
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



--
Saurabh Sehgal
E-mail:     saurabh.r.s@gmail.com
Phone:     647-831-5621
LinkedIn: http://www.linkedin.com/pub/1/7a3/436
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Re: Division in loop

[Bert Wesarg]

On Tue, Sep 8, 2009 at 16:40, Randi Botse<nightdecoder@gmail.com> wrote:
> hi all,
Hi,

>
> while (bytes < size) {
:
>
>        progress = (int) ((bytes / size) * 100);
This is an integer division, and because bytes is always smaller than
size will result in 0. You should cast bytes to double, or multiply by
100 first and than divide by size.

Bert
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Re: Division in loop

[Tim Walberg]

What are the typical values of bytes and size? Suspect
"(bytes / size)", being integer division, probably truncates
to 0 if size is relatively large...

Maybe you want "(bytes * 100) / size"? Or as another
alternative (arithmetically approximately equivalent,
but not quite as clear, however, it saves on multiply
operations):


size = size / 100;
...
...
...
printf(... bytes / size);



On 09/08/2009 21:40 +0700, Randi Botse wrote:
>>	hi all,
>>	
>>	....
>>	int fd, fdp;
>>	unsigned int size;
>>	int progress = 0;
>>	size_t bytes = 0;
>>	void *buffer;
>>	....
>>	
>>	/* open fd from a device then create fdp, allocate buffer,, etc. */
>>	
>>	while (bytes < size) {
>>	        ret = read(fd, buffer + bytes, size - bytes);
>>	        if (ret == -1) {
>>	            perror("read");
>>	            return -1;
>>	        }
>>	        ret = write(fdp, buffer + bytes, ret);
>>	        if (ret == -1) {
>>	            perror("write");
>>	            return -1;
>>	        }
>>	        bytes += ret;
>>	
>>	        progress = (int) ((bytes / size) * 100);
>>	        printf("\rcompleted: %i%%", progress);
>>	        fflush(stdout);
>>	    }
>>	
>>	....
>>	
>>	I run this loop to receive some data, the printf() seem not updated,
>>	the progress output is
>>	always in it's initial value until the loop was finished. but when i do...
>>	
>>	progress = (int) ((bytes / 2) * 100); /* change size to 2 (or random number) */
>>	
>>	then it's works... what's wrong here???
>>	--
>>	To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
>>	the body of a message to majordomo@vger.kernel.org
>>	More majordomo info at  http://vger.kernel.org/majordomo-info.html
End of included message



-- 
+----------------------+
| Tim Walberg          |
| 830 Carriage Dr.     |
| Algonquin, IL 60102  |
| twalberg@comcast.net |
+----------------------+

Re: Division in loop

[Randi Botse]

Hi all,,,

i was too frustrate too ask this problem, actually this is percentage
output of received data, yes integer division ALWAYS have (bytes /
size) equal to 0, until bytes >= size, sorry all... :),..  my fool...
 THANKS SO MUCH!!!

Re: Division in loop

[Glynn Clements]

Tim Walberg wrote:

> Maybe you want "(bytes * 100) / size"?

However, if int and size_t are 32-bit, that will overflow at around
20 or 40 MB (20 for signed, 40 for unsigned), which isn't really all
that much. Converting to either double or long long might be wise,
i.e.

	percent = (int)(bytes * 100.0) / size;
or:
	percent = (int)(bytes * 100LL) / size;

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: Division in loop

[Randi Botse]

Hi again...

int fd; /* RS232 serial link */
int fdp; /* file descriptor for a regular file */
struct stat stat;
size_t bytes = 0;
void *buffer; /* buffer, will be allocated */
...
    while (bytes < stat.st_size) {
        ret = read(fdp, buffer + bytes, stat.st_size - bytes);
        if (ret == -1) {
            perror("read");
            return -1;
        }
        ret = write(fd, buffer + bytes, ret);
        if (ret == -1) {
            perror("write");
            return -1;
        }
        bytes += ret;
        progress = (int) (bytes * 100) / stat.st_size;
        printf("\rcompleted: %i%%", progress);
        fflush(stdout);
    }
...

Then i see printf() never output the message until the loop has ended...
'fdp' is a file descriptor to a local file, i send it through a serial
connection (RS232),
i use a serial connection so i can practice Linux's File I/O operation
in real world ;p

the receiver can output the progress's message, but the sender (the
above code) could not...
this is the receiver codes:

...
while (bytes < size) {
        ret = read(fd, buffer + bytes, size - bytes);
        if (ret == -1) {
            perror("read");
            return -1;
        }
        ret = write(fdp, buffer + bytes, ret);
        if (ret == -1) {
            perror("write");
            return -1;
        }
        bytes += ret;

        progress = (int) (bytes * 100) / size;
        printf("\rcompleted: %i%%", progress); /* THIS NEVER OUTPUT,
UNTIL LOOP END! */
        fflush(stdout);
    }
...



On Tue, Sep 8, 2009 at 8:49 PM, Glynn Clements<glynn@gclements.plus.com> wrote:
>
> Tim Walberg wrote:
>
>> Maybe you want "(bytes * 100) / size"?
>
> However, if int and size_t are 32-bit, that will overflow at around
> 20 or 40 MB (20 for signed, 40 for unsigned), which isn't really all
> that much. Converting to either double or long long might be wise,
> i.e.
>
>        percent = (int)(bytes * 100.0) / size;
> or:
>        percent = (int)(bytes * 100LL) / size;
>
> --
> Glynn Clements <glynn@gclements.plus.com>
>
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Re: Division in loop

[Randi Botse]

Hi again...

int fd; /* RS232 serial link */
int fdp; /* file descriptor for a regular file */
struct stat stat;
size_t bytes = 0;
void *buffer; /* buffer, will be allocated */
...
   while (bytes < stat.st_size) {
       ret = read(fdp, buffer + bytes, stat.st_size - bytes);
       if (ret == -1) {
           perror("read");
           return -1;
       }
       ret = write(fd, buffer + bytes, ret);
       if (ret == -1) {
           perror("write");
           return -1;
       }
       bytes += ret;
       progress = (int) (bytes * 100) / stat.st_size;
       printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP ENDED */
       fflush(stdout);
   }
...

Then i see printf() never output the message until the loop has ended...
'fdp' is a file descriptor to a local file, i send it through a serial
connection (RS232),
i use a serial connection so i can practice Linux's File I/O operation
in real world ;p

the receiver can output the progress's message, but the sender (the
above code) could not...
this is the receiver codes:

...
while (bytes < size) {
       ret = read(fd, buffer + bytes, size - bytes);
       if (ret == -1) {
           perror("read");
           return -1;
       }
       ret = write(fdp, buffer + bytes, ret);
       if (ret == -1) {
           perror("write");
           return -1;
       }
       bytes += ret;

       progress = (int) (bytes * 100) / size;
       printf("\rcompleted: %i%%", progress); /* HAS OUTPUT */
       fflush(stdout);
   }
...


On Tue, Sep 8, 2009 at 8:49 PM, Glynn Clements<glynn@gclements.plus.com> wrote:
>
> Tim Walberg wrote:
>
>> Maybe you want "(bytes * 100) / size"?
>
> However, if int and size_t are 32-bit, that will overflow at around
> 20 or 40 MB (20 for signed, 40 for unsigned), which isn't really all
> that much. Converting to either double or long long might be wise,
> i.e.
>
>        percent = (int)(bytes * 100.0) / size;
> or:
>        percent = (int)(bytes * 100LL) / size;
>
> --
> Glynn Clements <glynn@gclements.plus.com>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
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Re: Division in loop

[Eric Bambach]

On Tuesday 08 September 2009 09:39:40 pm you wrote:
> Hi again...
>
> int fd; /* RS232 serial link */
> int fdp; /* file descriptor for a regular file */
> struct stat stat;
> size_t bytes = 0;
> void *buffer; /* buffer, will be allocated */
> ...
>    while (bytes < stat.st_size) {
>        ret = read(fdp, buffer + bytes, stat.st_size - bytes);
>        if (ret == -1) {
>            perror("read");
>            return -1;
>        }
>        ret = write(fd, buffer + bytes, ret);
>        if (ret == -1) {
>            perror("write");
>            return -1;
>        }
>        bytes += ret;
>        progress = (int) (bytes * 100) / stat.st_size;
>        printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP
> ENDED */ fflush(stdout);
>    }
> ...
>
> Then i see printf() never output the message until the loop has ended...
> 'fdp' is a file descriptor to a local file, i send it through a serial
> connection (RS232),
> i use a serial connection so i can practice Linux's File I/O operation
> in real world ;p
>
> the receiver can output the progress's message, but the sender (the
> above code) could not...
> this is the receiver codes:
>
> ...
> while (bytes < size) {
>        ret = read(fd, buffer + bytes, size - bytes);
>        if (ret == -1) {
>            perror("read");
>            return -1;
>        }
>        ret = write(fdp, buffer + bytes, ret);
>        if (ret == -1) {
>            perror("write");
>            return -1;
>        }
>        bytes += ret;
>
>        progress = (int) (bytes * 100) / size;
>        printf("\rcompleted: %i%%", progress); /* HAS OUTPUT */
>        fflush(stdout);
>    }
> ...
>
> On Tue, Sep 8, 2009 at 8:49 PM, Glynn Clements<glynn@gclements.plus.com> 
wrote:
> > Tim Walberg wrote:
> >> Maybe you want "(bytes * 100) / size"?
> >
> > However, if int and size_t are 32-bit, that will overflow at around
> > 20 or 40 MB (20 for signed, 40 for unsigned), which isn't really all
> > that much. Converting to either double or long long might be wise,
> > i.e.
> >
> >        percent = (int)(bytes * 100.0) / size;
> > or:
> >        percent = (int)(bytes * 100LL) / size;
> >
> > --
> > Glynn Clements <glynn@gclements.plus.com>
>
> --
> To unsubscribe from this list: send the line "unsubscribe
> linux-c-programming" in the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Your printf statements are being buffered by the system. If enough of them 
accumulate you will see them being printed "inside the loop". You need to 
disable  buffering or print to STDERR.

Re: Division in loop

[Eric Bambach]

On Tuesday 08 September 2009 09:39:40 pm you wrote:
> Hi again...
>
> int fd; /* RS232 serial link */
> int fdp; /* file descriptor for a regular file */
> struct stat stat;
> size_t bytes = 0;
> void *buffer; /* buffer, will be allocated */
> ...
>    while (bytes < stat.st_size) {
>        ret = read(fdp, buffer + bytes, stat.st_size - bytes);
>        if (ret == -1) {
>            perror("read");
>            return -1;
>        }
>        ret = write(fd, buffer + bytes, ret);
>        if (ret == -1) {
>            perror("write");
>            return -1;
>        }
>        bytes += ret;
>        progress = (int) (bytes * 100) / stat.st_size;
>        printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP
> ENDED */ fflush(stdout);
>    }
> ...
>
> Then i see printf() never output the message until the loop has ended...
> 'fdp' is a file descriptor to a local file, i send it through a serial
> connection (RS232),
> i use a serial connection so i can practice Linux's File I/O operation
> in real world ;p
>
> the receiver can output the progress's message, but the sender (the
> above code) could not...
> this is the receiver codes:
>
> ...
> while (bytes < size) {
>        ret = read(fd, buffer + bytes, size - bytes);
>        if (ret == -1) {
>            perror("read");
>            return -1;
>        }
>        ret = write(fdp, buffer + bytes, ret);
>        if (ret == -1) {
>            perror("write");
>            return -1;
>        }
>        bytes += ret;
>
>        progress = (int) (bytes * 100) / size;
>        printf("\rcompleted: %i%%", progress); /* HAS OUTPUT */
>        fflush(stdout);
>    }
> ...
>
> On Tue, Sep 8, 2009 at 8:49 PM, Glynn Clements<glynn@gclements.plus.com> 
wrote:
> > Tim Walberg wrote:
> >> Maybe you want "(bytes * 100) / size"?
> >
> > However, if int and size_t are 32-bit, that will overflow at around
> > 20 or 40 MB (20 for signed, 40 for unsigned), which isn't really all
> > that much. Converting to either double or long long might be wise,
> > i.e.
> >
> >        percent = (int)(bytes * 100.0) / size;
> > or:
> >        percent = (int)(bytes * 100LL) / size;
> >
> > --
> > Glynn Clements <glynn@gclements.plus.com>
>
> --
> To unsubscribe from this list: send the line "unsubscribe
> linux-c-programming" in the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Wow. I feel silly. Ignore my previous message. It's late here and I missed the 
fflush().

Re: Division in loop

[Randi Botse]

No matter i print to unbuffered stderr or stdin, the message will not printed...
whether the loop is quite fast? so there are no chance to printf()
output the message?


On Wed, Sep 9, 2009 at 9:57 AM, Eric Bambach<bot403@cisu.net> wrote:
>
> Wow. I feel silly. Ignore my previous message. It's late here and I missed the
> fflush().
>

Re: Division in loop

[Glynn Clements]

Randi Botse wrote:

> void *buffer; /* buffer, will be allocated */
> ...

>        ret = read(fdp, buffer + bytes, stat.st_size - bytes);

Pointer arithmetic on a void* is undefined. gcc allows this as an
extension, treating void* like char*, i.e. p+i is treated as
(void*)((char*)p+i).

For portability, define buffer as a char*.

>        printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP ENDED */
>        fflush(stdout);

> Then i see printf() never output the message until the loop has ended...
> 'fdp' is a file descriptor to a local file, i send it through a serial
> connection (RS232),

Try checking the return value of printf() and fflush(), and errno upon
error.

Also, what is stdout connected to? A VT? An xterm? sshd?

-- 
Glynn Clements <glynn@gclements.plus.com>
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Re: Division in loop

[Randi Botse]

Thanks Glynn for let me know that the void* pointer arithmetic is
undefined, and gcc automatically cast it to char* pointer... is this
because void* doesn't have a size? im curious when i do sizeof(void*),
my machine tell me sizeof(void*) is 4kb... i thnk when i do (void* +
1) that's means "4 kb after void*"?....

i run the code in GNOME terminal, that's means stdout connected to xterm?


On Thu, Sep 10, 2009 at 7:46 AM, Glynn Clements
<glynn@gclements.plus.com> wrote:
>
> Randi Botse wrote:
>
>> void *buffer; /* buffer, will be allocated */
>> ...
>
>>        ret = read(fdp, buffer + bytes, stat.st_size - bytes);
>
> Pointer arithmetic on a void* is undefined. gcc allows this as an
> extension, treating void* like char*, i.e. p+i is treated as
> (void*)((char*)p+i).
>
> For portability, define buffer as a char*.
>
>>        printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP ENDED */
>>        fflush(stdout);
>
>> Then i see printf() never output the message until the loop has ended...
>> 'fdp' is a file descriptor to a local file, i send it through a serial
>> connection (RS232),
>
> Try checking the return value of printf() and fflush(), and errno upon
> error.
>
> Also, what is stdout connected to? A VT? An xterm? sshd?
>
> --
> Glynn Clements <glynn@gclements.plus.com>
>

Re: Division in loop

[Manish Katiyar]

On Thu, Sep 10, 2009 at 12:31 PM, Randi Botse <nightdecoder@gmail.com> wrote:
> Thanks Glynn for let me know that the void* pointer arithmetic is
> undefined, and gcc automatically cast it to char* pointer... is this
> because void* doesn't have a size?
void * or any pointer for that matter will have a size fixed based on
your platform ie... 4 for x86 and 8 for x86_64.
> im curious when i do sizeof(void*),
> my machine tell me sizeof(void*) is 4kb

That is 4 bytes and not 4KB.

> ... i thnk when i do (void* +
> 1) that's means "4 kb after void*"?....
>
> i run the code in GNOME terminal, that's means stdout connected to xterm?
>
>
> On Thu, Sep 10, 2009 at 7:46 AM, Glynn Clements
> <glynn@gclements.plus.com> wrote:
>>
>> Randi Botse wrote:
>>
>>> void *buffer; /* buffer, will be allocated */
>>> ...
>>
>>>        ret = read(fdp, buffer + bytes, stat.st_size - bytes);
>>
>> Pointer arithmetic on a void* is undefined. gcc allows this as an
>> extension, treating void* like char*, i.e. p+i is treated as
>> (void*)((char*)p+i).
>>
>> For portability, define buffer as a char*.
>>
>>>        printf("\rcompleted: %i%%", progress); /* NO OUTPUT, UNTIL LOOP ENDED */
>>>        fflush(stdout);
>>
>>> Then i see printf() never output the message until the loop has ended...
>>> 'fdp' is a file descriptor to a local file, i send it through a serial
>>> connection (RS232),
>>
>> Try checking the return value of printf() and fflush(), and errno upon
>> error.
>>
>> Also, what is stdout connected to? A VT? An xterm? sshd?
>>
>> --
>> Glynn Clements <glynn@gclements.plus.com>
>>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>



-- 
Thanks -
Manish
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Re: Division in loop

[Glynn Clements]

Randi Botse wrote:

> Thanks Glynn for let me know that the void* pointer arithmetic is
> undefined, and gcc automatically cast it to char* pointer... is this
> because void* doesn't have a size?

Technically, it's because the standard says that it's undefined.

Pointer arithmetic is defined in terms of pointers to array elements,
e.g. if you have:

	int foo[N];
	int *p = &foo[A];
	int *q = p+B;
	int *r = &foo[A+B];

then r==q. But you can't have an array of "void", so pointer
arithmetic on void* isn't meaningful.

> im curious when i do sizeof(void*),
> my machine tell me sizeof(void*) is 4kb... i thnk when i do (void* +
> 1) that's means "4 kb after void*"?....

The result of sizeof is in bytes (technically, "char"s; sizeof(char)
is guaranteed to be 1).

The result of "sizeof" applied to any pointer type will typically be 4
bytes on 32-bit systems or 8 bytes on 64-bit systems. Although it's
possible for pointers to different types to have different sizes, it's
rare.

> i run the code in GNOME terminal, that's means stdout connected to xterm?

No, to Gnome terminal; xterm is similar.

It's possible that the behaviour which you experience is a property of
the terminal emulator. You might try running the program on a
different terminal emulator, or on a VT, to see if the same problem
occurs there.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: Division in loop

[Zhenwen Xu]

On Tue, Sep 08, 2009 at 09:40:30PM +0700, Randi Botse wrote:
> hi all,
> 
> ....
> int fd, fdp;
> unsigned int size;
> int progress = 0;
> size_t bytes = 0;
> void *buffer;
> ....
> 
> /* open fd from a device then create fdp, allocate buffer,, etc. */
> 
> while (bytes < size) {
>         ret = read(fd, buffer + bytes, size - bytes);
>         if (ret == -1) {
>             perror("read");
>             return -1;
>         }
>         ret = write(fdp, buffer + bytes, ret);
>         if (ret == -1) {
>             perror("write");
>             return -1;
>         }
>         bytes += ret;
> 
>         progress = (int) ((bytes / size) * 100);
>         printf("\rcompleted: %i%%", progress);

printf them all and check them:

         printf("\rcompleted: %i%% bytes: %d size: %d \n", progress, bytes, size);

>         fflush(stdout);
>     }
> 
> ....
> 
> I run this loop to receive some data, the printf() seem not updated,
> the progress output is
> always in it's initial value until the loop was finished. but when i do...
> 
> progress = (int) ((bytes / 2) * 100); /* change size to 2 (or random number) */
> 
> then it's works... what's wrong here???
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