What is timer_t in its base?
Are you talking about user space or kernel space? It matters even on one platform. It really boils down to the fact that the implementation of functions that should be treated as opaque.
The function implementation uses timer_t
can use an integer as an offset in an array of data structures, or dynamically allocate standby timer data structures.
This opacity of timer functions means that NULL may be valid on some systems, but not on others.
If you look at the Single Unix Specification for sys/types.h
, you will see that it mentions timer_t
and says "All types are defined as arithmetic types of the appropriate length", but it does not indicate what exactly is stored inside the type (since a pointer is actually just an integer, which turns out to be an address somewhere else in memory).
Linux kernel headers ( int
):
include/linux/types.h:typedef __kernel_timer_t timer_t;
include/uapi/asm-generic/posix_types.h:typedef int __kernel_timer_t;
Linux glibc sources / headers (pointer void*
):
time/bits/types/timer_t.h:typedef __timer_t timer_t;
bits/types.h:__STD_TYPE __TIMER_T_TYPE __timer_t;
sysdeps/unix/sysv/linux/x86/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/unix/sysv/linux/sparc/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/unix/sysv/linux/s390/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/unix/sysv/linux/generic/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/unix/sysv/linux/alpha/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/nacl/bits/typesizes.h:#define __TIMER_T_TYPE void *
sysdeps/mach/hurd/bits/typesizes.h:#define __TIMER_T_TYPE __S32_TYPE
bits/typesizes.h:#define __TIMER_T_TYPE void *
Solaris Headers (via GCC) ( int
):
#ifndef _TIMER_T
#define _TIMER_T
typedef int timer_t; /* timer identifier type */ <typedef:timer_t>
#endif /* ifndef _TIMER_T */
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The problem with how it's done is that it doesn't even define any invalid value, so you can't wrap timer_t without a second variable just to indicate if it was initialized successfully, instead of relying on 0. -1, NULL or just TIMER_T_NONE, for example. Whoever invented this API obviously hasn't approached programming for several years :-(
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