volatile problem.

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2 total messages Started by Joachim Huober Wed, 17 Apr 1996 15:44

#2609

Author: Joachim Huober
Date: Wed, 17 Apr 1996 15:44

51 lines
1834 bytes

Hi all,

Is volatile the oposite of const, and do the tricky things about const
also belong to volatile.
Exp:
	const char *foo;	/* foo points only to characters that
				 * will not be changed through foo 
				 */
	char *const foo;	/* foo is an unchanging pointer to a
				 * possibly changing char.
				 */
Does this correspond to:
	volatile char *foo;	/* foo points to characters that may
				 * be changed by other threads
				 */
	char *volatile foo;	/* foo is a pointer that may be changed
				 * by another thread.
				 */
My problem belongs to setjmp, longjmp.
I want to do a setjmp, then do some fancy mallocs. if a malloc fail,
I want to do a longjmp and I want to free everything allocated till then.

char *foo = NULL;
char *bar = NULL;
jmp_buf env;

if (setjmp(env) == 0) {
	foo = malloc(...);
	if (foo == NULL) {
		...
		longjmp(env,1);
	bar = malloc(...);
	if (bar == NULL) {
		...
		longjmp(env,1);
} else {
	if (foo != NULL)
		free(foo);
	if (bar != NULL)
		free(bar);
}
Where do I have to write the volatile qualifier to tell the compiler not 
to do anything clever.

Joachim
-- 
---------------------------------------------------------------------
Joachim Huober                                Phone: +49 731 502-6241
UDINE-Project,            URZ, University of Ulm, 89081 Ulm , Germany
E-mail: joachim.huober@rz.uni-ulm.de 
---------------------------------------------------------------------

Re: volatile problem.

#2626

Author: Jens M Andreasen
Date: Fri, 19 Apr 1996 14:36

48 lines
1638 bytes


On 17 Apr 1996, Joachim Huober wrote:

 > 	volatile char *foo;	/* foo points to characters that may
 > 				 * be changed by other threads
 > 				 */
 > 	char *volatile foo;	/* foo is a pointer that may be changed
 > 				 * by another thread.
 > 				 */

"May somehow change" will do. Dunno about the meaning of "thread" .

 > My problem belongs to setjmp, longjmp.
 > I want to do a setjmp, then do some fancy mallocs. if a malloc fail,
 > I want to do a longjmp and I want to free everything allocated till then.
 > 
 > char *foo = NULL;
 > char *bar = NULL;
 > jmp_buf env;
 > 
 > if (setjmp(env) == 0) {
 > 	foo = malloc(...);
 > 	if (foo == NULL) {
 > 		...
 > 		longjmp(env,1);}
 > 	bar = malloc(...);
 > 	if (bar == NULL) {
 > 		...
 > 		longjmp(env,1);}
 > } else {
 > 	if (foo != NULL)
 > 		free(foo);
 > 	if (bar != NULL)
 > 		free(bar);
 > }
 > Where do I have to write the volatile qualifier to tell the compiler not 
 > to do anything clever.

Since you are after the value of the pointer and not what it is pointing to, 
then write it like

char *volatile foo;
char *volatile bar;

// Jens M Andreasen

BTW: Are you sure that longjmp is the best way to do this ?

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