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Hi all,

I want to get the size of a block device by ftell(). I found that I
can get
the size of a device by seek() and tell() in Python. But not in C.

What is difference between them? How can I get the size of a block
device by ftell()?


# whoami
root

# du -sh /etc/services
364K /etc/services

# df -h | grep hdb1
/dev/hdb1 111G 2.0G 103G 2% /mnt/data1

---------------------------------------------------------------------------
# cat ftell_test.c
#include <stdio.h>

long int ftell_test(const char *d);

int main(void) {
printf("%ld\n", ftell_test("/etc/services"));
printf("%ld\n", ftell_test("/dev/hdb1"));

return 0;
}

long int ftell_test(const char *d) {
FILE *fp;
long int l;

fp = fopen(d, "r");
fseek(fp, 0L, SEEK_END);
l = ftell(fp);
fclose(fp);

return l;
}

---------------------------------------------------------------------------
# cat ftell_test.py
#!/usr/bin/env python

def ftell_test(d):
f = open(d, "r")
f.seek(0, 2) # SEEK_END is 2
return f.tell()

if __name__ == '__main__':
print ftell_test("/etc/services")
print ftell_test("/dev/hdb1")

---------------------------------------------------------------------------
# gcc ftell_test.c -o ftell_test

---------------------------------------------------------------------------
# ./ftell_test
362031
-1

---------------------------------------------------------------------------
# ./ftell_test.py
362031
120034091520

["Followup-To:" header set to comp.os.linux.development.system.]
On 2007-11-20, Seongsu Lee <> wrote:
>
>
> Hi all,
>
> I want to get the size of a block device by ftell(). I found that I
> can get
> the size of a device by seek() and tell() in Python. But not in C.
>
> What is difference between them? How can I get the size of a block
> device by ftell()?
>
ftell() returns a long, which is 32 bits on a 32-bit system.
For files or devices larger than 2GB, use ftello() and compile with
#define _FILE_OFFSET_BITS 64

Seongsu Lee <> wrote in
news:4e573411-2b2a-444c-994e-:

> I want to get the size of a block device by ftell(). I found that I
> can get
> the size of a device by seek() and tell() in Python. But not in C.
>
> What is difference between them? How can I get the size of a block
> device by ftell()?

[snip]
> fp = fopen(d, "r");
> fseek(fp, 0L, SEEK_END);
> l = ftell(fp);
> fclose(fp);
>
> return l;

> ---- # ./ftell_test
> 362031
> -1

You need to check the return values. ftell is returning -1, which is an
indicator that it failed. If you were to print out errno, you could then
look up why it's failing. (Or, you could call perror(3) or strerror(3)
to get it translated into text for you.)

> -----------------------------------------------------------------------
----
> # ./ftell_test.py
> 362031
> 120034091520

Or, if you convert the python version's output to hexadecimal and count
the digits, you might figure out the proximate cause of your program's
failure.

GH