As the "GNU C Library Reference Manual" says
off_t
This is a signed integer type used to represent file sizes.
In the GNU C Library, this type is no narrower than int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this
type is transparently replaced by off64_t.
and
off64_t
This type is used similar to off_t. The difference is that
even on 32 bit machines, where the off_t type would have 32 bits,
off64_t has 64 bits and so is able to address files up to 2^63 bytes
in length. When compiling with _FILE_OFFSET_BITS == 64 this type
is available under the name off_t.
Thus if you want reliable way of representing file size between client and server, you can:
off64_t type and stat64() function accordingly (as it fills structure stat64, which contains off64_t type itself). Type off64_t guaranties the same size on 32 and 64 bit machines.-D_FILE_OFFSET_BITS == 64 and use usual off_t and stat().off_t to type int64_t with fixed size (C99 standard).
Note: (my book 'C in a Nutshell' says that it is C99 standard, but optional in implementation). The newest C11 standard says:7.20.1.1 Exact-width integer types
1 The typedef name intN_t designates a signed integer type with width N ,
no padding bits, and a two’s complement representation. Thus, int8_t
denotes such a signed integer type with a width of exactly 8 bits.
without mentioning.
And about implementation:
7.20 Integer types <stdint.h>
... An implementation shall provide those types described as ‘‘required’’,
but need not provide any of the others (described as ‘‘optional’’).
...
The following types are required:
int_least8_t uint_least8_t
int_least16_t uint_least16_t
int_least32_t uint_least32_t
int_least64_t uint_least64_t
All other types of this form are optional.
Thus, in general, C standard can't guarantee types with fixed sizes. But most compilers (including gcc) support this feature.