Does fread64 and fwrite64 exist in Linux library?

jaepi · 11-06-2007, 12:53 AM

I was advised by a friend of mine to use fread64, fwrite64 instead of fread and fwrite. Does this exist? Been looking for it's documentation but I haven't found one.

matthewg42 · 11-07-2007, 10:21 AM

Yes there are 64 bit versions of fopen and friends. If you have already written your program using fopen, fread etc, you can simply define _FILE_OFFSET_BITS == 64 when you compile, and all fopen calls will be translated to fopen64 automatically.

For example, when building myprog.c, which would usually be built like this (where $ is your shell prompt):

Code:

$ gcc myprog.c -o myprog_32_bit_fh_version

For a 64 bit version you might do it like this:

Code:

$ gcc -D_FILE_OFFSET_BITS=64 myprog.c -o myprog_64_bit_fh_version

You can test to see if your program is using the large file option using strace. For example, if your program opens the file "myfile" for writing:

Code:

$ strace ./myprog_32_bit_fh_version 2>&1 |grep myfile
open("myfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
$ strace ./myprog_64_bit_fh_version 2>&1 |grep myfile
open("myfile", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3

Notice the addition of the O_LARGEFILE flag in the 64 bit version.

ta0kira · 11-09-2007, 11:07 AM

info libc has info on those functions. They are essentially interchangeable except that the 64 versions allow indexes and sizes larger than 2GB. I think you need open64 or fopen64 for them to work, though.
ta0kira