Can any one solve this Problem...!!! Post: 83484

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Top Forums Programming Can any one solve this Problem...!!! Post 83484 by Perderabo on Thursday 15th of September 2005 09:51:04 AM

09-15-2005

Administrator Emeritus

A solution involving portable C eludes me! I naturally rejected the idea of having change() locate its return address on the stack and alter it to step over the i=10 statement. The precise location of the return address and even whether to increment it or decrement it and certainly by how much can vary from system to system. There is really no guarantee that the return address will be stored on the stack or even that there is a stack. I admit that I don't know of any currently marketed unix systems without a stack or that move the PC backwards. But I also don't know of any standard prohibiting them. These days we are in transistion from 32 bit to 64 bit architectures. Most versions of unix currently support executables in either architecture. So the change() function would need to detect the length of the return address as well.

But the other problem that I see with this involves agressive optimization. There is no indication that i is accessible to change() or is otherwise aliased or volatile. So there is no reason that the compiler writer could not legally choose to emit code that operates like...
i=10;
change();
printf("%d",i);
Whether or not that would happen would probably depend on the optimization level selected and the compiler used, and whether or not the resulting executable is intended to interact with a symbolic debugger.

So if you have a portable solution, I would love to see it!

Perderabo

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LEARN ABOUT REDHAT

ctrsyl

CTRSYL(l)								 )								 CTRSYL(l)

NAME

       CTRSYL - solve the complex Sylvester matrix equation

SYNOPSIS

       SUBROUTINE CTRSYL( TRANA, TRANB, ISGN, M, N, A, LDA, B, LDB, C, LDC, SCALE, INFO )

	   CHARACTER	  TRANA, TRANB

	   INTEGER	  INFO, ISGN, LDA, LDB, LDC, M, N

	   REAL 	  SCALE

	   COMPLEX	  A( LDA, * ), B( LDB, * ), C( LDC, * )

PURPOSE

       CTRSYL solves the complex Sylvester matrix equation:
	  op(A)*X + X*op(B) = scale*C or
	  op(A)*X - X*op(B) = scale*C,

       where  op(A)  = A or A**H, and A and B are both upper triangular. A is M-by-M and B is N-by-N; the right hand side C and the solution X are
       M-by-N; and scale is an output scale factor, set <= 1 to avoid overflow in X.

ARGUMENTS

       TRANA   (input) CHARACTER*1
	       Specifies the option op(A):
	       = 'N': op(A) = A    (No transpose)
	       = 'C': op(A) = A**H (Conjugate transpose)

       TRANB   (input) CHARACTER*1
	       Specifies the option op(B):
	       = 'N': op(B) = B    (No transpose)
	       = 'C': op(B) = B**H (Conjugate transpose)

       ISGN    (input) INTEGER
	       Specifies the sign in the equation:
	       = +1: solve op(A)*X + X*op(B) = scale*C
	       = -1: solve op(A)*X - X*op(B) = scale*C

       M       (input) INTEGER
	       The order of the matrix A, and the number of rows in the matrices X and C. M >= 0.

       N       (input) INTEGER
	       The order of the matrix B, and the number of columns in the matrices X and C. N >= 0.

       A       (input) COMPLEX array, dimension (LDA,M)
	       The upper triangular matrix A.

       LDA     (input) INTEGER
	       The leading dimension of the array A. LDA >= max(1,M).

       B       (input) COMPLEX array, dimension (LDB,N)
	       The upper triangular matrix B.

       LDB     (input) INTEGER
	       The leading dimension of the array B. LDB >= max(1,N).

       C       (input/output) COMPLEX array, dimension (LDC,N)
	       On entry, the M-by-N right hand side matrix C.  On exit, C is overwritten by the solution matrix X.

       LDC     (input) INTEGER
	       The leading dimension of the array C. LDC >= max(1,M)

       SCALE   (output) REAL
	       The scale factor, scale, set <= 1 to avoid overflow in X.

       INFO    (output) INTEGER
	       = 0: successful exit
	       < 0: if INFO = -i, the i-th argument had an illegal value
	       = 1: A and B have common or very close eigenvalues; perturbed values were used to solve the equation (but the matrices A and B  are
	       unchanged).

LAPACK version 3.0						   15 June 2000 							 CTRSYL(l)