cgemm operation returns wrong result Error in C Code?


i just started with cuda and I am very excited about all the stuff I can do. But right now I am stuck. Probably cause I am not a C - Pro. But perhaps one of you guys can help me:

I wanted to implement a complex matrix multiplication and although every cublasStatus return value is fine - i get a totally wrong result. Actually I don’t know what cublasCgemm is calculating there. The results are far away from being comprehensible and still confuses me…

thanks in advance,



int main(int argc, char** argv)


cuComplex* h_A;

cuComplex* h_B;

cuComplex* h_C;

cuComplex* d_A = 0;

cuComplex* d_B = 0;

cuComplex* d_C = 0;

cuComplex alpha = { 1, 0 };

cuComplex beta = { 0, 0 };

/* Initialize CUBLAS */


int pRowsA = 6;

int pColsA = 3;

int pRowsB = 3;

int pColsB = 2;

int pRowsC = pRowsA;

int pColsC = pColsB;

/* Allocate host memory for the matrices */

h_A = (cuComplex*)malloc(pRowsA * pColsA * sizeof(h_A[0]));

h_B = (cuComplex*)malloc(pRowsB * pColsB * sizeof(h_B[0]));

h_C = (cuComplex*)malloc(pRowsC * pColsC * sizeof(h_C[0]));

/* setting values */


int mA_lda = pRowsA;

int mA_ldb = pRowsA;

int mB_lda = pRowsB;

int mB_ldb = pRowsB;

int mC_lda = pRowsC;

int mC_ldb = pRowsC;

/* Allocate device memory for the matrices */

cublasAlloc(pRowsA * pColsA, sizeof(h_A[0]), (void**)&d_A);

cublasAlloc(pRowsB * pColsB, sizeof(h_B[0]), (void**)&d_B);

cublasAlloc(pRowsC * pColsC, sizeof(h_C[0]), (void**)&d_C);

/* Initialize the device matrices with the host matrices */

cublasStatus cpA = cublasSetMatrix(pRowsA, pColsA, sizeof(h_A[0]), h_A, mA_lda, d_A, mA_ldb);

cublasStatus cpB = cublasSetMatrix(pRowsB, pColsB, sizeof(h_B[0]), h_B, mB_lda, d_B, mB_ldb);

cublasStatus cpC = cublasSetMatrix(pRowsC, pColsC, sizeof(h_C[0]), h_C, mC_lda, d_C, mC_ldb);

cublasCgemm(‘n’, ‘n’, pRowsA, pColsA, pColsB, alpha, d_A, pRowsA, d_B, pColsB, beta, d_C, pRowsA);

cublasStatus cgRes = cublasGetError();

cublasStatus cgm = cublasGetMatrix(pRowsC, pColsC, sizeof(h_C[0]), d_C, mC_lda, h_C, mC_lda);

for(int i = 0; i < 12; i++) {

  cuComplex t = h_C[i];

  printf("\n re: %f o,: %f ", t.x, t.y );


/* Memory clean up */







/* Shutdown */




It looks like your arguments for the cublasCgemm are incorrect. You have

cublasCgemm(‘n’, ‘n’, pRowsA, pColsA, pColsB, alpha, d_A, pRowsA, d_B, pColsB, beta, d_C, pRowsA);

And it should be

cublasCgemm(‘n’, ‘n’, pRowsA, pColsB, pRowsB, alpha, d_A, pRowsA, d_B, pColsB, beta, d_C, pRowsA);

Here’s a handy way to remember what are m, n, and k - the first two integers represent the dimension of C. The third integer is the inner dimension between op(A) and op( B ). So, the way I’d write the statement would be this way:

int inner = pRowsB;
cublasCgemm(‘n’, ‘n’, pRowsC, pColsC, inner, alpha, d_A, pRowsA, d_B, pColsB, beta, d_C, pRowsA);


In what way is the result wrong? Does it not match the result from CGEMM on BLAS?

Thanks a lot - i will take another look on monday. Right now I have no access to my cuda-machine and unfortunately the emulation mode quits with an segmentation fault.

I took example3 from…ml/essl125.html, but w/o adding beta*C. The returning result is total different from what I expected. Unfortunately. I also programmed a ‘manual’ way of matrix multiplication (the old-fashioned way, I learned in school, years ago) by using cuCmul(), which worked perfectly. Therefore the input data seems to be also in the correct format. Hopefully it was only the small error JeremiahPalmer mentioned. Hopefully.


cublasCgemm(‘n’, ‘n’, pRowsC, pColsC, pRowsB, alpha, d_A, pRowsA, d_B, pRowsB, beta, d_C, pRowsC);

did it. But your hint was the clue! I should have asked more earlier. I still wonder why i haven’t figured out that simple error on my own. Anway - thanks a lot!

  • bjoern


Hello b-makes-cuda. Did you ever get the CUBLAS-Cgemm to work correctly. If so, would you please post what you did.



Yes I did. I only mixed up the parameters. After correcting it (see post #5) CUBLAS-Cgemm worked fine. I can send you the code, if you like to.

  • bjoern

Thanks for the offer. my e-mail address is

Thanks again.