How to link C against BLAS?

[jseabold]

All,

My knowledge of compiled languages is small. I spend most of my time in Python. Python is too slow for my recent needs, so I am trying to write a loop in C that calls BLAS's dgemm. I cribbed a C++ example from http://matrixprogramming.com/2008/01/matrixmultiply. The problem is that I can't get it to link against BLAS.

$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5) 4.4.3
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ls /usr/include/cblas.h
/usr/include/cblas.h

Here is my code. There may be other lurking errors still.

#include <stdio.h>
#include <stdlib.h>
#include <cblas.h>
#include <time.h>

//using namespace::std;

int main(int argc, char* argv[])
{
    if (argc < 2)
    {
        printf("Usage: mm dim1 dim2 dim3\n");
        printf("Please specify matrix dimensions\n");
        return 1;
    }
    int dim1, dim2, dim3;
    dim1 = atoi(argv[1]);
    if (argc < 3)
        dim2 = dim1;
    else
        dim2 = atoi(argv[2]);
    if (argc < 4)
        dim3 = dim1;
    else
        dim3 = atoi(argv[3]);
    double A[dim1][dim2], B[dim2][dim3], C[dim1][dim3];
    srand(12345);
    double maxr=RAND_MAX;
    int i,j;
    for (i = 0; i < dim1; i++)
        for (j = 0; j < dim2; j++)
            A[i][j] = rand()/maxr;
    for (i = 0; i < dim2; i++)
        for (j = 0; j < dim3; j++)
            B[i][j] = rand()/maxr;
    double alpha = 1.;
    double beta = 0.;
    double start;
    start = clock();
    cblas_dgemm(CblasColMajor, CblasNoTrans, CblasNoTrans, dim1, dim3, dim2, alpha, *A, dim1, *B, dim2, beta, *C, dim1);
    double finish = clock();
    printf("time for C(%d,%d) = A(%d,%d) B(%d,%d) is %f s\n", dim1, dim3, dim1, dim2, dim2, dim3, (finish - start)/CLOCKS_PER_SEC);
    return 0;
} 

I try to compile my code like so.

gcc -lcblas -Werror -Wall -pedantic -o usedgemm usedgemm.c -std=c99
/usr/bin/ld: cannot find -lcblas
collect2: ld returned 1 exit status

This is probably something simple as telling it where to look, but I haven't had any luck. Can someone help out in the code or the compile step?

Thanks.

[MadCow108]

do you have the blas library installed?
sudo apt-get install libblas-dev

you have to link against libblas not libcblas:
gcc -lblas ...

you can find this stuff out by e.g. looking at package contents (if installed over a package):
dpkg -c package.deb

in ubuntu the packages are in /var/cache/apt/archives/
you can also list the contents after installation in synaptic

or online e.g. here:
http://www.debian.org/distrib/packages

[jseabold]

Ha. Thanks! I have the blas headers, built my own with ATLAS a while back, and doing -lblas instead does the trick.