Where to find the complete definition of off t type

Question

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   lt sys types h gt   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

User · Answer

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;

User · Answer

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    Use 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  As was mentioned before compile your code with -D FILE OFFSET BITS    64 and use usual off t and stat    Convert 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  lt stdint h gt           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

User · Answer

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

User · Answer

Since this answer still gets voted up  I want to point out that you should almost never need to look in the header files  If you want to write reliable code  you re much better served by looking in the standard  A better question than  how is off t defined on my machine   is  how is off t defined by the standard    Following the standard means that your code will work today and tomorrow  on any machine   In this case  off t isn t defined by the C standard  It s part of the POSIX standard  which you can browse here   Unfortunately  off t isn t very rigorously defined  All I could find to define it is on the page on sys types h      blkcnt t and off t shall be signed integer types    This means that you can t be sure how big it is  If you re using GNU C  you can use the instructions in the answer below to ensure that it s 64 bits  Or better  you can convert to a standards defined size before putting it on the wire  This is how projects like Google s Protocol Buffers work  although that is a C   project      So  I think  where do I find the definition in my header files  isn t the best question   But  for completeness here s the answer   On my machine  and most machines using glibc  you ll find the definition in bits types h  as a comment says at the top  never directly include this file    but it s obscured a bit in a bunch of macros  An alternative to trying to unravel them is to look at the preprocessor output    include  lt stdio h gt   include  lt sys types h gt   int main void      off t blah     return 0      And then         gcc -E sizes c    grep   off t typedef long int   off t         However  if you want to know the size of something  you can always use the sizeof   operator   Edit  Just saw the part of your question about the     This answer has a good discussion  The key point is that names starting with    are reserved for the implementation  so you shouldn t start your own definitions with

[c] Where to find the complete definition of off_t type?

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