[Proj] libproj4 thread safety

Gerald Evenden gerald.evenden at verizon.net
Mon Feb 28 09:25:04 EST 2005

Thank-you very much.  Now I must retreat to my cave and assume a 
for deep meditation.  Some uses of pointers in C are designed by and for
Zen masters and *really* stain limited minds..  ;-)

On Feb 28, 2005, at 5:07 AM, Glynn Clements wrote:

> Gerald Evenden wrote:
> 	.....

> The way in which the GNU C library implements errno is to use a
> preprocessor macro to give each thread its own copy:
> 	/* Function to get address of global `errno' variable.  */
> 	extern int *__errno_location (void) __THROW __attribute__ 
> ((__const__));
> 	/* When using threads, errno is a per-thread value.  */
> 	#   define errno (*__errno_location ())
> The __errno_location() function returns a pointer to thread-specific
> memory (via pthread_getspecific()).
> The reason for the indirection is that errno must behave as a
> variable, in the sense that assignments must be supported, e.g.:
> 	errno = 0;
> 	somefunc();
> 	if (errno != 0)
> 	...
>> How are error flags handled for the math library in the threaded
>> environment? Close to the same issue. I do not do much checking here
>> as the code does so much manipulation to avoid math library
>> problems.
> The defined interface (in <fenv.h>) uses functions to test and modify
> the flags. The flags themselves are stored in the FPU's status
> registers, which are automatically saved and restored across a context
> switch, so each thread has its own set of flags.
> -- 
> Glynn Clements <glynn at gclements.plus.com>
> 	...

> _____________________________________
Jerry and the low riders: Daisy Mae and Joshua.
"The whole religious complexion of the modern world is due to the
absence from Jerusalem of a lunatic asylum." Havelock Ellis, 1914

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