Intel(R) - Computational and Systems Biology at MIT

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7 Intel® Math Kernel Library User’s GuideCBLAS is a C-style interface to the BLAS routines. You can call CBLAS routines usingregular C-style calls. When using the CBLAS interface, the header file mkl.h will simplifythe program development as it specifies enumerated values as well as prototypes of all thefunctions. The header determines if the program is being compiled with a C++ compiler,and if it is, the included file will be correct for use with C++ compilation. Example 7-3illustrates the use of CBLAS interface.Calling BLAS Functions That Return the Complex Values inC/C++ CodeYou must be careful when handling a call from C to a BLAS function that returns complexvalues. The problem arises because these are Fortran functions and complex return valuesare handled quite differently for C and Fortran. However, in addition to normal functioncalls, Fortran enables calling functions as though they were subroutines, which provides amechanism for returning the complex value correctly when the function is called from a Cprogram. When a Fortran function is called as a subroutine, the return value shows up asthe first parameter in the calling sequence. This feature can be exploited by the Cprogrammer.The following example shows how this works.Normal Fortran function call: result = cdotc( n, x, 1, y, 1 ).A call to the function as asubroutine: call cdotc( result, n, x, 1, y, 1).A call to the function from C(notice that the hiddenparameter gets exposed): cdotc( &result, &n, x, &one, y, &one ).NOTE. Intel MKL has both upper-case and lower-case entry points inBLAS, with trailing underscore or not. So, all these names are acceptable:cdotc, CDOTC, cdotc_, CDOTC_.Using the above example, you can call from C, and thus, from C++, several level 1 BLASfunctions that return complex values. However, it is still easier to use the CBLAS interface.For instance, you can call the same function using the CBLAS interface as follows:cblas_cdotu( n, x, 1, y, 1, &result )7-6
Language-specific Usage Options 7NOTE. The complex value comes back expressly in this case.The following example illustrates a call from a C program to the complex BLAS Level 1function zdotc(). This function computes the dot product of two double-precision complexvectors.In this example, the complex dot product is returned in the structure c.Example 7-1 Calling a complex BLAS Level 1 function from C#include "mkl.h"#define N 5void main(){int n, inca = 1, incb = 1, i;typedef struct{ double re; double im; } complex16;complex16 a[N], b[N], c;void zdotc();n = N;for( i = 0; i < n; i++ ){a[i].re = (double)i; a[i].im = (double)i * 2.0;b[i].re = (double)(n - i); b[i].im = (double)i * 2.0;}zdotc( &c, &n, a, &inca, b, &incb );printf( "The complex dot product is: ( %6.2f, %6.2f)\n", c.re, c.im );}7-7
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