I am sending file from client to server using TCP. To mark the end of the file I like to send file size before the actual data. So I use stat
system call to find the size of the file. This is of type off_t
. I like to know how many bytes it occupies so that I can read it properly on the server side. It is defined in the <sys/types.h>
. But I do not understand the definition. It just defines __off_t or _off64_t
to be off_t
. Where to look for __off_t
? Also is it convention that __
is prefixed for most of the things in header files and scares me when I read header files to understand better. How to read a header file better?
#ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
#endif
This question is related to
c
sockets
networking
types
header-files
If you are having trouble tracing the definitions, you can use the preprocessed output of the compiler which will tell you all you need to know. E.g.
$ cat test.c
#include <stdio.h>
$ cc -E test.c | grep off_t
typedef long int __off_t;
typedef __off64_t __loff_t;
__off_t __pos;
__off_t _old_offset;
typedef __off_t off_t;
extern int fseeko (FILE *__stream, __off_t __off, int __whence);
extern __off_t ftello (FILE *__stream) ;
If you look at the complete output you can even see the exact header file location and line number where it was defined:
# 132 "/usr/include/bits/types.h" 2 3 4
typedef unsigned long int __dev_t;
typedef unsigned int __uid_t;
typedef unsigned int __gid_t;
typedef unsigned long int __ino_t;
typedef unsigned long int __ino64_t;
typedef unsigned int __mode_t;
typedef unsigned long int __nlink_t;
typedef long int __off_t;
typedef long int __off64_t;
...
# 91 "/usr/include/stdio.h" 3 4
typedef __off_t off_t;
If you are writing portable code, the answer is "you can't tell", the good news is that you don't need to. Your protocol should involve writing the size as (eg) "8 octets, big-endian format" (Ideally with a check that the actual size fits in 8 octets.)
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.
Source: Stackoverflow.com