man 2 open has wrong size and unit for file triggering EOVERFLOW?

man 2 open has wrong size and unit for file triggering EOVERFLOW?

[Shriramana Sharma]

Hello. Please refer to
http://man7.org/linux/man-pages/man2/open.2.html -- the paragraph
under EOVERFLOW. It reads that -D_FILE_OFFSET_BITS=64 is required
while compiling to access files under size (2<<31)-1 *bits*. This
seems to be wrong on two counts: that the numerical value is (2<<30)-1
and not (2<<31)-1, and that the unit should be bytes and not bits.
Please see the following transcript from a Kubuntu Trusty 32 bit
system on an ext3 filesystem:

$ cat test.c
# include <stdio.h>
int main () {
        FILE * fp = fopen("toobigfile", "r");
        if (fp) printf("Success\n"); else printf("Failure\n");
}
$ clang -o test test.c

Creating file of size (2<<30)-1 *bytes*:

$ truncate -s 2147483647 toobigfile

It opens fine:

$ strace -f -eopen ./test
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("toobigfile", O_RDONLY)            = 3
Success
+++ exited with 0 +++

Increase it by one byte i.e. to (2<<30) bytes and it fails:

$ truncate -s 2147483648 toobigfile
$ strace -f -eopen ./test
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("toobigfile", O_RDONLY)            = -1 EOVERFLOW (Value too
large for defined data type)
Failure
+++ exited with 0 +++

Compile the prog with the prescribed def and it succeeds:

$ clang -o test test.c -D_FILE_OFFSET_BITS=64
$ strace -f -eopen ./test
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("toobigfile", O_RDONLY|O_LARGEFILE) = 3
Success
+++ exited with 0 +++

This shows that the threshold is actually 2 GiB which is (2<<30)
*bytes* and not (2<<31) *bits*. Note the difference in both the shift
length and units.

Note also that (2<<31) bits is (2<<28) bytes which is just 512 MiB and
certainly 32-bit apps can access 600 MiB files without special
compilation flags.

If you find this correct, please update the manpage. If you find this
incorrect, please inform me why. Thank you!

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा
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Re: man 2 open has wrong size and unit for file triggering EOVERFLOW?

[Michael Kerrisk (man-pages)]

On 09/25/2014 07:26 PM, Shriramana Sharma wrote:
> Hello. Please refer to
> http://man7.org/linux/man-pages/man2/open.2.html -- the paragraph
> under EOVERFLOW. It reads that -D_FILE_OFFSET_BITS=64 is required
> while compiling to access files under size (2<<31)-1 *bits*. This
> seems to be wrong on two counts: that the numerical value is (2<<30)-1
> and not (2<<31)-1, and that the unit should be bytes and not bits.
> Please see the following transcript from a Kubuntu Trusty 32 bit
> system on an ext3 filesystem:

Thanks, Shriramana,

You are of course quite right. The text should have said
"(1<<31) -1 bytes". I've fixed it now.

Thanks,

Michael



> $ cat test.c
> # include <stdio.h>
> int main () {
>         FILE * fp = fopen("toobigfile", "r");
>         if (fp) printf("Success\n"); else printf("Failure\n");
> }
> $ clang -o test test.c
> 
> Creating file of size (2<<30)-1 *bytes*:
> 
> $ truncate -s 2147483647 toobigfile
> 
> It opens fine:
> 
> $ strace -f -eopen ./test
> open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
> open("toobigfile", O_RDONLY)            = 3
> Success
> +++ exited with 0 +++
> 
> Increase it by one byte i.e. to (2<<30) bytes and it fails:
> 
> $ truncate -s 2147483648 toobigfile
> $ strace -f -eopen ./test
> open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
> open("toobigfile", O_RDONLY)            = -1 EOVERFLOW (Value too
> large for defined data type)
> Failure
> +++ exited with 0 +++
> 
> Compile the prog with the prescribed def and it succeeds:
> 
> $ clang -o test test.c -D_FILE_OFFSET_BITS=64
> $ strace -f -eopen ./test
> open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
> open("toobigfile", O_RDONLY|O_LARGEFILE) = 3
> Success
> +++ exited with 0 +++
> 
> This shows that the threshold is actually 2 GiB which is (2<<30)
> *bytes* and not (2<<31) *bits*. Note the difference in both the shift
> length and units.
> 
> Note also that (2<<31) bits is (2<<28) bytes which is just 512 MiB and
> certainly 32-bit apps can access 600 MiB files without special
> compilation flags.
> 
> If you find this correct, please update the manpage. If you find this
> incorrect, please inform me why. Thank you!
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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