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/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
