Re: curious about whether i can count on certain features of C

[Glynn Clements <glynn@gclements.plus.com>]

Robert P. J. Day wrote:

>   i just inherited a sizable C-based project and, perusing the code,
> i've come across a number of in-house defined constructs that would
> seem to already be supported in standard (C99?) C, and i'm wondering
> if there's a reason the previous developer felt he needed to define
> these features himself.
> 
>   first, there's the definition of "offsetof":
> 
> #ifndef offsetof
> # define offsetof(type, field) \
>     ( (char *) &( ((type *) 0)[0].field ) - (char *) &( ((type *) 0)[0] ) )
> #endif
> 
>   using any modern definition of C, can i safely assume that this
> function/macro is just part of the language (stddef.h)?

If you require C99, you can rely upon the presence of offsetof.
However, I wouldn't make code C99-only for this reason alone. Reasons
for not requiring C99 include:

1. You might want to compile your code on a system where there isn't a
stable C99 compiler.

2. You might want to use a third-party library whose headers aren't
compatible with C99 (C99 isn't entirely backward-compatible with C89).

> and, at the
> very least, is there a reason it's defined in such an obscure way
> rather than just
> 
>   #define offsetof(type,memb) ((size_t)&((type *)0) -> memb)   ???
> 
> that first definition might be technically correct but i'm really
> trying to simplify things and i don't see any obvious need to keep
> that local definition around.

1. Some compilers may complain about an explicit dereference of a null
pointer.

2. On some systems, casting the base address of the structure to
size_t may not yield zero.

>   next, booleans.  based on my copy of "harbison and steele" (5th
> ed.), can i reasonably assume the existence of a boolean data type
> (stdbool.h)?  i don't have much interest in supporting legacy
> compilers, and booleans appear to be part of the C99 definition, so
> i'd be really tempted to ditch the following enum type i found:
> 
>     enum TCS_bool_Type {
>         TCS_bool_FALSE = 0, /**< false/no state */
>         TCS_bool_TRUE /**< true/yes state */
>     } GCC_PACKED;                       /* enum TCS_bool_Type */
> 
>   next, there are a number of typedefs for fixed-width data types:
> 
>     typedef signed char TCS_int8_t; /**< signed 8-bit integer */
>     typedef unsigned char TCS_u_int8_t; /**< unsigned 8-bit integer */
>     typedef signed short TCS_int16_t; /**< signed 16-bit integer */
>     typedef unsigned short TCS_u_int16_t; /**< unsigned 16-bit integer */
>     typedef signed int TCS_int32_t; /**< signed 32-bit integer */
>     typedef unsigned int TCS_u_int32_t; /**< unsigned 32-bit integer */
>     typedef TCS_int8_t TCS_tiny_t; /**< signed tiny integer */
>     typedef TCS_u_int8_t TCS_u_tiny_t; /**< unsigned tiny integer
> 
> is there any compelling reason why i can't just use the types defined
> in /usr/include/stdint.h?  that is, int8_t, uint32_t, and so on?  is
> there any rationale for someone wanting to do this themselves, apart
> from perhaps legacy compiler support?

Just portability. In the real world, not everyone uses a bleeding-edge
compiler.

-- 
Glynn Clements <glynn@gclements.plus.com>